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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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Appendix A
Line Comments

Line comments are provided for the Executive Summary and all chapters contained in the draft CSSR. For each comment, committee members indicated how important they thought addressing the comment was by providing one of three letter designations, ranked in order of highest to lowest priority: V indicates a strong (or vigorous) recommendation, R indicates a recommendation, and S indicates a suggestion.

EXECUTIVE SUMMARY

#	page/line	V/R/S
1	General	R	Climate models or Earth System models? In the early days of USGCRP the models were primarily atmosphere models with radiative forcing and some feedback loops. Today’s models have become more fully coupled system models and hence the term “Earth System models” is more appropriate.
2	P11/L10-18	S	Narrowly defined, climate may be the statistics of weather, but this discussion could be improved by considering the context of the climate system—notably the role of the oceans, which make the climate change problem so different from weather prediction.
3	P11/L18	R	This statement implies monotonic change which is certainly not true of all weather patterns. This should be rewritten to be scientifically accurate.
4	P11/L19	S	Augment the statement about 150 years with one about more recent changes, e.g. since the big increase in slope of radiative forcing (Figure 2.6) around 1960.
5	P11/L20	R	This sentence is unclear. The text implies that the non-uniformity caused the changes, when it should state that the warming caused the changes, with modulation by the non-uniformity.
6	P11/L29-33	R	Chapters 2 and 10 should better reflect how ecosystem responses are feeding back to climate (especially for ocean CO₂ uptake).
7	P11/L29	R	A statement that the number of extremes in recent years exceeds that expected by chance is needed here.
8	P13-31	S	Throughout this chapter, and probably the entire document, temperature refers to surface temperature, yet it is rarely stated this way. Somewhere, it would be helpful to state explicitly that ‘temperature’ refers to surface temperature and also to remind readers that temperature does not just change at the surface.
9	P13/L32-P14/L7	R	A key point here: for most of the United States, the observed warming is consistent with anthropogenic forcing (Figure 6.5).

Page 64 Cite

Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
10	P13/L6	R	The sentence “Fifteen of the last 16 years…” is unclear. Could rephrase as “All 15 of the 15 warmest years in the instrumental record have occurred in the last 16 years.”
11	P13/L6	S	Entire box: it is possible that the next couple of years will be cooler than 2015 and 2016 due to the large El Niño event of 2015-2016. It is worth pointing this out somewhere, though not necessarily here. Page 13, line 28 might be a good place to put a statement about the 2015-2016 El Niño event.
12	P13/L12	V	Why state “more than 1.6° F”? It may be more appropriate to put confidence intervals on the change.
13	P13/L16-27	S	Bullets beginning on lines 16 and 21 could be combined.
14	P13/L17	R	Human activities are described as “primarily responsible”—does this mean that > 50% of the change is being ascribed to human activity? Does it mean something else? Should be specific.
15	P13/L28-31	S	Variability might also be changing, but is hard to measure and quantify, and also complicates the detection and attribution (see previous paragraph).
16	P13/L26	R	Need to provide some quantification for “small”. Possible wording, “…over that period is not more than a small fraction of the total trend.” It would be even clearer if authors could provide a real quantification, along the lines of “over that period is not more than a small 25% of the total global trend.”
17	P13/L30	S	The comment about “limited” influence of El Niño needs some level of quantification. Even something like “its influence is limited to a small fraction of global and regional climate trends…” would be beneficial.
18	P13/L32-P14/L2	S	It might be appropriate to compare the speed of the warming to previous paleo-temperature changes.
19	P14/L2	R	Figure 6.2 (on which this statement is based) is not convincing, certainly not with high confidence.
20	P14/L4	R	The phrase “early 1900s” is too general and inaccurate shorthand for the 1901-1960 average. See also main text remark about using slope-based statements.
21	P14/L5	R	For western United States temperatures, it would be wise to add a terse qualifier from the discussion in the chapter, e.g., that changes in circulation might be partly responsible for the enhanced warming in the West and suppressed warming in the Southeast, lest an unsuspecting reader surmise that one area is more susceptible to GHG increases and the other less so. See also the main text comment about treatment of observed trends using a slope-based approach.
22	P14/Fig. ES.1	S	Perhaps it would be worthwhile to point the reader back to the Front Matter for an explanation of reference time periods or other approach to describing observed trends. See review

Page 65 Cite

Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			comments on Chapter 2 about computing trends and about using hatching.
23	P14/L11	V	This is the 4th temperature baseline in 3 pages. Reduce variations if at all possible.
24	P14/L16-22	R	The key point, largely missing from the ES, is that warming from CO₂ is essentially permanent. That is the point that should be made here. The modest warming from inertia is not irrelevant, but it is not a big deal.
25	P14/L16-19	R	The statement here about committed warming seems to be at odds with some papers, including Mathews and Weaver, 2010 (and others cited in main text for Section III.14), which show that there would be no additional warming if human GHG emissions were to immediately cease. There’s an important difference between freezing concentrations and eliminating anthropogenic GHG emissions. Recommend revising to better reflect breadth of literature on this topic. Matthews, H. D., and A. J. Weaver. 2010. Committed climate warming. Nature Geoscience 3(3):142-143. DOI: 10.1038/ngeo813.
26	P14/L16-22	R	Consider adding a statement regarding remaining uncertainty in estimates of climate sensitivity. This seems too important not be stressed in the ES. Need to define or describe what sensitivity is, and be careful in caption of Figure ES.2. As written, warming commitment and climate sensitivity, which are most relevant at the low and high end of future emissions, are a little bit tangled up in this bullet point.
27	P15/L1-6	S	Several edits will give Figure ES.2 more impact and better grounding in the chapters. Near-term and “few decades” (30-50 years?) need to be clearly specified. Since the “lower scenario” in many figures is RCP4.5 instead of (apparently, here) RCP2.6, the qualitative descriptions of the scenarios should be clarified by labeling them also with RCPs. Some reference to Chapter 14 and the steps needed to achieve RCP2.6 would be appropriate. The two panels would benefit from tick marks on the right hand side or horizontal lines accompanying the tick marks. It might be helpful to explain why there is a broader range for the temperature response in the higher scenario. Finally, note that with RCP4.5 and RCP2.6, temperature is nearly stabilized by 2100 while with RCP8.5 is still rapidly warming: the world beyond 2100 also matters.
28	P15/L9	R	This sentence should note the emissions are global. An uninformed reader might misinterpret this to mean U.S. emissions only.
29	P16/Fig. ES.3	V	Figure ES.3 (and especially the source figure, Figure 6.7) should explicitly state which RCPs are used. The caption for Figure ES.3 says “See Figure 6.7...for more details” but there are no more

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			details in Figure 6.7. It is identical, except for the addition of “Figure source: NOAA/NCEI.” Also “near-present” is ambiguous, as elsewhere “near-present” and “present-day” are both used to represent 1986-2015. Use the date range throughout for clarity.
30	P17-19	R	The evidence is a lot stronger for increases in temperature-related extremes than for precipitation, where the changes are barely more than could be attributable to chance (see comments on Chapter 7 in particular). Language should be added that indicates this distinction.
31	P17-18	S	There is no home in the ES for seasonal precipitation changes, either observed or modeled. Page 13 begins a section “Global and U.S. temperatures will continue to rise” and page 17 pivots to extremes. Although the results may seem uninteresting, this might be considered a gap. Even just a short statement about the ambiguities of precipitation projections would suffice.
32	P17/L3	S	The terminology “extreme weather,” “extreme climate,” and “extreme event” or some combination appears many times in the report, but no definition is provided.
33	P17/L7-9	S	For balance, this sentence should also note that cold extremes are becoming less frequent, and perhaps also that flooding is (contrary to popular view) changing in complicated ways with no clear national trend. As written, this feeds the inaccurate blanket statement “all kinds of extremes are getting worse/increasing” even though lines 17ff clarify.
34	P17/L6-9	S	Could note that extremes also present challenges for businesses and national security.
35	P17/L10-12		There is, at best, scant evidence that tornadoes are exhibiting changes linked to climate change. What does seem to be missing in this list are heat waves, storm surges, intense precipitation events.
36	P17/L17-20	S	Is there a geographic pattern for the observed changes in cold/heat waves, as there is for heavy precipitation (next paragraph)?
37	P17/L26-P18/L4	R	The findings that (1) the frequency and severity of ARs, which account for 30-40% of western snowpack (a California-centric view that doesn’t apply to the rest of the West), is projected to increase and (2) reductions are projected in western United States winter and spring snowpack are not consistent. Some explanation is needed, e.g., that the first refers to increased precipitation and the second refers to a reduction in the proportion of snowfall to total precipitation due to warming. As defined, ARs almost always leave behind less snow in Oregon and Washington because they raise the freezing level usually to the 5000-6000 foot level and the combination of high temperatures, high dew points, heavy rain, and strong winds melt a lot of snow (in other words, they do not end a drought,

Page 67 Cite

Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			they set one up). Of more interest would be whether rain-on-snow events will increase, or heavy precipitation, regardless of whether they qualify as atmospheric rivers (and researchers do not agree on a definition of ARs).
38	P17/L33	R	The term “benchmark” is unclear. The passage seems to be saying that the Dust Bowl era is the period of worst drought and highest temperatures in the historical record for the U.S., which may only be true for certain regions and certain ways of measuring drought and extreme temperatures. For many regions of the U.S., recent temperatures are warmer and/or drought is more severe than in the 1930s. Recommend rewording to better reflect the state of knowledge.
39	P18/L1-4	V	This bullet should be revised. There is strong evidence that western United States snowpacks have already been decreasing, so the sentence should say “continued reductions.” The phrase “assuming no change in current water-resources management” strays into impacts, policy, and adaptation. It is sufficient to say that temperature changes will overwhelm any increases in precipitation in many places, leading to reductions in snowpack (and summer soil moisture) and to changes in unregulated streamflow in rivers where snowmelt is a significant component, or something similar. The West already experiences summer low flows as part of its natural hydroclimate. And it would read better to add an article: “end of the century.”
40	P18/L5-9	S	This Bullet could mention expansion of area where tropical cyclones can occur (Figure 9.2).
41	P18/L11-16	S	The wording in this figure caption is awkward.
42	P19/Fig.ES.5	R	Would it be possible to include Alaska in this figure? Also, the final version of the CSSR should use a higher quality image, because the legend in the top panels is barely legible. The definitions used in the bottom panels also need to be explained somewhere in the report.
43	P19/L6	R	The 1901-1960 average is not the average for the first half of the 20th century.
44	P19/L16	S	The word “chaotic” has a specific mathematical meaning to atmospheric dynamicists, and a rather different meaning for the public. Suggest clarifying which is meant—if the former, a bit of explanation would be needed.
45	P19/L22-P20/L3	R	This statement slightly oversteps the evidence presented on the topic of NPO in Chapter 5, page 191, lines 11-13, where NPO is briefly mentioned and “effects on U.S. hydroclimate...have been reported.” Absent strong evidence (e.g., reasonably impressive correlation coefficients), the use of the word “important” is a stretch. Similar concerns apply to NAM.
46	P20/L5-7	R	This last sentence is either a weak allusion to attribution studies of which the Committee is unaware, or a speculation. If the

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			former, evidence should be presented (perhaps in Chapter 5) and a stronger statement written. If it is speculation, it should be removed.
47	P20/L10	S	Connected, yes; “strongly” is debatable.
48	P20/L20-25	S	This might be an appropriate place to note also the effects of a poleward expansion of the Hadley circulation on tropical cyclone ranges (Chapter 9, page 309, lines 29-35). Some clarification is also needed because, as shown in Chapter 1, these purported shifts in subtropical dry zones have not been clearly observed over land.
49	P21/Fig. ES.6	R	Figure ES.6 does not illustrate “natural variability now being influenced by human activities,” and the report does not present any evidence to support the claim. Chapter 5 says merely that “only low confidence is indicated for specific projected changes in ENSO variability” (Page 191, lines 32-33, emphasis added). Recommend deleting the figure, or if retained, revising the caption to accurately reflect the state of science. Also see ES comment on P19/L22.
50	P21/L8-11	S	The ocean is an integral part of a coupled system not just a planetary component that has feedback. In other words, the “climate” is not just the atmosphere. “Ocean” should be singular, not plural.
51	P21/14-15	S	Consider listing GMSL rise in both inches and SI units (cm?) to be consistent with use of both ºF and ºC, and usage of both units in Chapter 12.
52	P22/L4-7	S	The findings on SLR could start with a conclusion about risks of long-term commitment to several feet/meters and then address 21st century.
53	P22/L14	R	“In most projections…” Really? Are there any projections in which GMSL does not continue to rise after 2100? None are shown or discussed in Chapter 12. Recommend changing to “all” or explaining.
54	P22/L19-26	R	It would be helpful if all of the conclusions on differences between local and global sea level rise were quantified (e.g., “0.2 m more or less than the global average”). It would also be very useful to indicate that the regional differences look a lot less important if SLR is at the high end of the range, especially after 2100.
55	P22/L33-35	S	Reference to impacts strays from draft CSSR intended focus; if retained, this statement could be revised to note that some of these impacts are already observed.
56	P23/L4-13	S	These two paragraphs should follow the same structure and need not both mention effects of changes in oxygen. Suggest starting the first paragraph with observations of change, then mention potential consequences (not well understood), and then

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			removing potential consequences from second paragraph.
57	P24/L4	S	There is no need to use “include” if the list that follows is complete. “Examples are shown above for...”
58	P24/L9-14	S	This paragraph is stylistically inconsistent with the rest of the ES.
59	P24/L18-21	S	The potential consequences of thawing permafrost for the carbon cycle are more nuanced than presented here. Recommend adding “but the magnitude of carbon release is currently uncertain” to the end of this statement. See comments in Section III.11 of main text.
60	P25/L3-4	S	“Human activities…” This is an awkward and unnecessary statement here. It almost hints that activities other than (or in addition to) emissions are to blame. Could reword this to more directly link the ice loss to human-induced warming, rather than ambiguous “activities.”
61	P25/L5-6	R	It does not add information to conclude with high confidence that earlier models were wrong. The statement would be much more powerful if framed in terms of a rate of ice loss (with confidence) and a parenthetical statement that the new estimates are substantially higher than older ones.
62	P25/L7	V	The basis for this very important statement is a single sentence in Chapter 11 (page 373, lines 32-34) and it does not appear in the Chapter 11 Key Findings. For the prominence in the ES, it deserves more prominence in Chapter 11.
63	P25/L9-12	R	See comments in Section III.11 of main text regarding confidence levels. It would also be useful to add some indication of which weather patterns show some evidence of Arctic influence, if any; otherwise the last clause is too vague to include.
64	P26/L7-8	R	The section heading regarding a 2ºC temperature limit requires some explanation of why a 2ºC limit is important is needed. Consider using the Box on page 27 as the heading for this section instead
65	P26/L7-P28/L4	R	There is almost no mention in this section of non-CO₂ GHGs and some discussion of them is warranted.
66	P27/L10-16	R	This bullet is confusing. Consider revising the first portion to something like: “Significant actions taken today to reduce CO₂ emissions would take a decade or longer to influence atmospheric CO₂ concentrations. This delayed response—the result of the long lifetime…” The key conclusion is that warming from CO₂ is essentially permanent, unless the CO₂ is removed by carbon capture and storage. The draft report will be clearer with a stronger focus on the climate issue as one related to cumulative emissions of CO₂. The statement “reductions in atmospheric CO₂ concentrations” is not absolute reductions, but reductions relative to a high-emissions scenario. This should be

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			phrased clearly so that readers do not infer that aggressive climate policies would cause atmospheric CO₂ concentrations to drop any time soon.
67	P27/L15	S	The phrase “modified by natural variability” muddles the message. Perhaps the clause is meant as a concession to the point made in the cited Hawkins and Sutton papers (2009, 2011) that in the near-term, internal (natural) variability dominates over greenhouse warming?
68	P27/L17-24		If the figures cited here are meant to somehow include the effects of human non-CO₂ forcers (e.g. methane) then that should be clearly stated and the text should say how one establishes equivalence between cumulative emissions of CO₂ and other forcers which have much shorter lifetimes.
69	P27/L29	S	If stating “unproven at scale,” what would the authors consider to be the correct scale for proof?
70	P27/L26-27	R	Without adequate background and discussion here of what solar radiation management is, this sends a dangerous message. It is also important to say more about drawbacks (e.g. solar radiation management does not address other concerns like ocean acidification, and could lead to other problems). Recommend deleting the first clause. Also, the National Academies reports on climate intervention stress the differences between solar radiation management and CO₂ removal and to reflect this, discussion of the two topics should be in separate bullets (see also main text).
71	P27/L31-P28/L4	R	These two bullet points need revision. Both cover similar ground and should either be separated cleanly into one on past CO₂ analogs and one on sea level, or combined artfully. Remove the word ‘precise’ (page 28, line 3)—there is no precise paleo analog at any point in Earth’s history, and ascribing the differences to CO₂ (by mentioning only CO₂) neglects important other factors in driving the differences. As written, a reader could infer that if atmospheric CO₂ concentrations are not reduced, then Earth will eventually experience the conditions mentioned here (+3.6 C global mean temperature, and +66 feet GMSL). If that is not the case, then those figures may not be appropriate. If it is the case, then say so. If this is uncertain, then the text could say something like “if CO₂ concentrations are sustained at Pliocene levels long enough, the risk of reaching Pliocene sea levels is unknown.” May also be worth mentioning the Paleocene-Eocene Thermal Maximum.
72	P28	R	The green box at top of the page is misplaced.
73	P28/L5-P29/L14	V	This section could be better framed by invoking the concern about low probability, high impact events. Replace the fuzzy “cannot be ruled out” by something like “important enough to merit serious consideration.” It would also be worth mentioning explicitly the worrisome fact that the processes and/or feedbacks

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			that contributed to vastly different states in the past seem to be missing from climate models, and therefore they are not suited to predict at what CO₂ levels those processes and/or feedbacks may kick in and push the planet to a different state. Finally, the issues could be linked to the concept of Paris Agreement temperature limits (previous section) and avoiding unknown but potentially catastrophic risks.
74	P29/L20-P30/L33	R	See Section II.1. Since this list appears in almost the same form in Chapter 1, it could be trimmed here to focus on the newest and/or most important developments.
75	P29/L26	S	Perhaps change “changing extreme-climate” to “changing regional texture of extreme-climate.”
76	P29/L31-P30/L2	V	This statement should be stronger. It should start with a clear statement that warming has continued and that there was not a pause or hiatus. If the hiatus is still mentioned in the revised version, replace 2000 with 1998 because the red herring of the hiatus only worked if the trend analysis started in 2000.
77	P30/L12	S	Might be good here to say “seasonal regrowth.” Otherwise referring to regrowth of sea ice might be puzzling.
78	P30/1	S	This statement should be “as predicted by basic atmospheric and ocean physics...” since ocean heat uptake is a very important part of story.

1: OUR GLOBALLY CHANGING CLIMATE

#	page/line	V/R/S
79	P32/L13-15	S	This sentence is vague and does not add useful information to the Key Finding.
80	P32/L16-20	V	Key Finding 3 is appropriate, but steals some thunder from Chapter 3 as it is currently written. See Chapter 3 comments for recommended suggestions to address this.
81	P33/L10-19	R	This paragraph could be a lot clearer with a bit more explanation. Now, it reads like a series of ungrounded assertions.
82	P33/L26	S	“quite unpredictably” should be replaced with a more appropriate word or phrase
83	P34/L9-13	R	For most readers, the contrast between increasing Antarctic sea ice and a shrinking Antarctic ice sheet will be unclear. This is important enough to explain clearly.
84	P34/L9-11	S	Text should indicate over what period the small increase in Antarctic sea ice occurred. Recent reports indicate that Antarctic sea ice declined unexpectedly in 2016. See http://nsidc.org/

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
85	P34/L26	S	The use of “compelling” in this sentence adds little value. Recommend deleting it.
86	P34/L28	V	Use of a different length of the averaging period for the start and the end of the interval is confusing. See Section II.2 of main text regarding better statistical approaches for reporting observed changes.
87	P35/L4	R	“the previous”—the previous very strong El Niño but not the previous El Niño as the text implies
88	P35/L5	R	Quantify how much lower the global temperature was during the last El Niño (1998) relative to 2015.
89	P35/L9	S	It is generally preferable to work on the basis of evidence rather than assumptions. The phrase “we must assume” could be replaced by “it is possible” or something similarly circumspect.
90	P36/L24-26	R	The wording gives the impression that RCP2.6 is likely to be less than 1.5, which is not correct.
91	P37/L8-10	R	It’s not clear what is meant here about 13-year and 18-year intervals. Are these running means?
92	P37/L9	R	It is important to note that satellite data and surface data are not measuring the same things. (This point can, with effort, be deduced from the next sentence or with less effort from Figure 1.5.)
93	P37/L28-29	R	Emphasize that the hiatus was revealed as a slow-down in surface warming. As described on page 38, excess heat may have been transferred to the deep oceans. Benestad et al. 2016 also shows that other measures of climate change indicate continued warming of the planet during the hiatus.
93	P37/L28-29	R	Benestad, R. E. 2016. A mental picture of the greenhouse effect: A pedagogic explanation. Theoretical and Applied Climatology:1-10. DOI: 10.1007/s00704-016-1732-y.
94	P37/L34-35	R	A citation needed for this statement.
95	P38/L18-19	R	This statement might appear to contradict Key Finding 5
96	P38/L28-31	R	Discussion of the comparison between CMIP5 models and observations seems to let the models off the hook. Acknowledge that the capability of models to capture the internal variability of the oceans is probably flawed.
97	P38-39	S	The emphasis on PDO in this section could be lessened (see Chapter 5).
98	P38/L34	S	A word appears to be missing after “new”
99	P39/L2-3	S	Reader may think that looking only at 17-year intervals obscures the true signals of climate change. Why 17?
100	P39/L6-	S	The use of “attributed” would benefit from referring to the

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
	P40/L13		explanation of detection and attribution in Chapter 3.
101	P39/L15-23	R	An explanation of why wet areas are getting wetter, and dry areas drier, would improve this paragraph.
102	P39/L15-23	R	The changes described in this paragraph should be better quantified, with uncertainty or statistical significance noted. See Section II.2 of main text.
103	P39/L21-23	R	Sentence on changes in Arctic precipitation needs to be clarified. As written, it is unclear whether increases or decreases have been detected, and with what magnitude.
104	P39/L29-30	S	The placement of the reference to Figure 1.7 implies that it also shows moisture levels instead of just precipitation. Recommend moving the reference to follow “century” page 39, line 29.
105	P39/L29-30	R	Is the slight increase in precipitation statistically significant? Even if it is, is it appropriate to discuss global changes in precipitation when the responses to climate change are so regionally diverse?
106	P39/L32-34	R	Citations are needed for the ENSO statement; the references at the end of the sentence seem to refer to the operational updating by NCEI, not the ENSO attribution.
107	P40/L1-14	R	Quantify the changes described in this section. How much and over what interval?
108	P40/L25-29	R	This sentence gives the incorrect impression that there needs to be a change in the shape of the probability distribution for a small shift in the mean to lead to a large change in extremes.
109	P41/L33	R	As written, this sentence conveys a very limited amount of information. Are the low confidence trends up or down? Are they low confidence because there are trends in opposite directions across regions or because regional signals are weak?
110	P42/L9-10	S	Quantify the shift in storm tracks.
111	P42/L15-22	S	This sentence overstates the position of Barnes and Polvani (2015). They emphasize that Arctic amplification may modulate certain aspects of mid-latitude circulation response to climate change (emphasis is theirs, page 5526 in citation).
112	P42/L20-22	S	This sentence requires clarification. Is this mainly about the strengths of ETCs or about the locations? Is the key point that weakening of meridional gradients will lead to less intense ETCs overall?
113	P42/L24-26	R	This statement conflates the statistical problems of detection and of attribution—they are not the same. Recommend clarifying the language.
114	P43/L5-7	S	The sentence beginning, “However, the same study demonstrated…” is unclear.
115	P43/L9-10	R	Clarify that the carbon emissions from deforestation come mainly from biomass burning.

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
116	P43/L26-27	R	Quantify the change in snow cover extent and change in albedo.
117	P44/L8	R	It is important to make the point that, early in the anthropogenic era, deforestation was mainly temperate. The dominance of tropical deforestation is a post-1950 phenomenon.
118	P44/L18	R	Parmesan and Yohe 2003 is not the right reference for this statement.
119	P44/L19	R	The Reyes-Fox paper talks about CO₂ extending the growing season length in places where the length is water limited. As written, the text seems misleading in suggesting that a longer frost-free season and a possible growing season extension due to water conservation are additive, or even potentially additive.
120	P44/L20-26	R	Some of this material is revisited in Chapter 10; there should be a tighter linkage.
121	P45/L24-25	R	Over what period of record? The previous two sentences suggest it could either be 1979-2014 or “since 1988,” or something else since the statement refers to IPCC 2013.
122	P46/L9-10	R	Stating that IPO controls tropical SSTs is not an accurate reflection of the current understanding of this topic.
123	P46/L19-P47/L20	R	Much of this material parallels Chapter 12 and the two chapters should be better linked.
124	P47/L14	S	Clarify that these are mountain glaciers.
125	P47/L21-38	S	Much of this parallels material in Chapter 11 and should be better coordinated.
126	P48/L1-29	S	Much of this parallels material in Chapter 12 and should be better coordinated
127	P50/L1-4	S	Statistical downscaling is hardly new; could add a sentence or two explaining how the LOCA method differs from earlier methods.
128	P52/L23-28	R	This paragraph places too much emphasis on the importance of improving climate models.
129	P55/L5-6	R	Consider providing a range or upper limit to projected changes in climate over the next 100 years.
130	P56/L1-4	R	Defining the role of ENSO and other natural cycles as “limited” is too imprecise. It would be much more useful to give a quantitative range or to say something like “no more than a small fraction of anthropogenic changes.”
131	P56/L2-4	R	As written, a reader may wonder about natural variability in the past, for example, paleoclimates. Make clear that the “limited influence” of natural variability refers to this influence in the recent past and present-day.
132	P58/L3-5	R	Figure 1.1 The different curves should be identified as well as the time resolution of the data.

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133	P62/L2-14	S	Figure 1.5 Describe what the different curves represent.
134	P65/L2-7	S	Figure 1.8: Mann et al. 2008 was unwilling to say much about the southern hemisphere temperature trends due to paucity of proxies in that hemisphere. Recommend revising the caption to reflect this uncertainty in the southern hemisphere, and therefore global temperatures over the past 1700 years. Or just change “global” to “northern hemispheric.”

2: PHYSICAL DRIVERS OF CLIMATE CHANGE

#	page/line	V/R/S
135	General	R	Many acronyms are unnecessary. For example, SSI, TSI, RFari, RFaci, SWCRE, and LWCRE are used only in 1-2 paragraphs.
136	P86/L6	R	Text needs a citation for the 33°C calculation.
137	P86/23-25	R	Figure 2.1 includes more factors than those listed here in the text, some of which have larger fluxes than solar radiation reflected by the surface. Clarify the caption by stating that many of the fluxes pictured are feedbacks.
138	P87/L26-27	R	The text should make clear that the equilibrium surface temperature response for the equation given here is global.
139	P87/L23	R	Text should define the “top” of the atmosphere.
140	P87/L27-P100	V	Discussion of radiative forcing could begin with definition of instantaneous radiative forcing.
141	P88/L17-27	V	Text on aerosol forcings should be saved for later in the chapter, as the reader has not yet been introduced to the different aerosol effects.
142	P88/L3	R	Text states: “A change that results in a net increase in the downward flux at the tropopause constitutes a positive RF…” Depending on the definition of RF, the increase could be at the surface or top of atmosphere.
143	P88/L35	R	Text should emphasize evidence for the relatively small effects of cosmic rays on climate. See: Krissansen-Totton, J., and R. Davies. 2013. Investigation of cosmic ray-cloud connections using MISR. Geophysical Research Letters 40(19):5240-5245. DOI: 10.1002/grl.50996.
144	P88/L38-P89/L1	R	Text should mention changes in snow and ocean-ice as examples of changing albedo.
145	P89/L11-12	R	This paragraph is overly complex even for a scientifically literate audience and should be simplified, even for a scientifically literate audience. For example, the reader may know little about stratospheric vs. tropospheric ozone, and cannot be expected to

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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			follow the line of reasoning here. Section could be shortened considerably.
146	P90/L6-7	R	Text should clarify that only the most explosive volcanic eruptions lead to aerosol reaching the stratosphere, where they can have global climate effects. Most volcanoes affect only regional climate due to short lifetime of aerosols in the troposphere.
147	P90/L2-4	V	“On millennial timescales, changes in solar output are expected to have influenced climate.” Text should be made more specific or deleted.
148	P91/L5-7	R	Text should explain that the long lifetimes of these gases account for their relatively homogeneous distributions.
149	P91/L6-7	V	The text remarks that seasonal variations in CO₂ occur in response to changing “transpiration.” While carbon uptake is to some degree controlled by stomatal opening, the main reason for the seasonal variation in CO₂ is photosynthesis.
150	P92/L12-13	V	“Over the last 50 years or more, CO₂ has shown the largest annual concentration and RF increases among all GHGs (Figures 2.4 and 2.5).” Methane has the largest relative increase in concentration. Recommend just stating CO₂ RF increase is largest.
151	P93/L9-11	V	Information on methane trends should be updated. Global methane has increased by 5.7 ppb per year over 2007-2013, with extreme increase in 2014. See Nisbet et al., 2016, and references therein. Nisbet, E. G., E. J. Dlugokencky, M. R. Manning, D. Lowry, R. E. Fisher, J. L. France, S. E. Michel, J. B. Miller, J. W. C. White, B. Vaughn, P. Bousquet, J. A. Pyle, N. J. Warwick, M. Cain, R. Brownlow, G. Zazzeri, M. Lanoisellé, A. C. Manning, E. Gloor, D. E. J. Worthy, E. G. Brunke, C. Labuschagne, E. W. Wolff, and A. L. Ganesan. 2016. Rising atmospheric methane: 2007–2014 growth and isotopic shift. Global Biogeochemical Cycles 30(9):1356-1370. DOI: 10.1002/2016GB005406.
152	P93/L2-11	R	Paragraph about methane should provide information on relative magnitudes (with uncertainty ranges) of sources and sinks.
153	P94/L7	V	CO₂-eq needs to be defined.
154	P95/L28	V	Sentence on “improving” aerosol uncertainties needs clarification.
155	P96/L35-36	V	Sentence should state in at least the past 800,000 years…
156	P97/L20-22	V	Sentence confuses emissions with secondary aerosol formation, which is not considered an “emission.”
157	P97/L3-8	R	Text should define synthetic GHG emissions.
158	P97/L20	V	Text should clarify that aerosols have short lifetimes and are

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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			relatively quickly rained out or deposited on timescales of days to weeks. It is the short lifetimes that leads to the inhomogeneous distributions. Both meteorological factors (such as temperature and clouds) and chemical transformations influence the production and lifetime of aerosols.
159	P97/L28	R	Sentence should be clear that responses are climate responses.
160	P98/L7-9	V	ERFs drive cloud and surface temperature changes, not the other way around. See Myhre et al. 2013, cited in main text.
161	P98/L4-11	V	List of feedbacks should include the ocean response e.g., changes to ocean circulation
162	P99/L13-15	R	The sentence would benefit from explanation of why the climate effect of clouds varies with altitude.
163	P100/L17-18	S	“However, there is evidence that the presence of a polar surface-albedo feedback influences the tropical climate as well…” Mention the climate effect of soot deposition on glaciers at low latitudes e.g., see: Wang, M., B. Xu, J. Cao, X. Tie, H. Wang, R. Zhang, Y. Qian, P. J. Rasch, S. Zhao, G. Wu, H. Zhao, D. R. Joswiak, J. Li, and Y. Xie. 2015. Carbonaceous aerosols recorded in a southeastern Tibetan glacier: analysis of temporal variations and model estimates of sources and radiative forcing. Atmos. Chem. Phys. 15(3):1191-1204. DOI: 10.5194/acp-15-1191-2015. Yang, S., B. Xu, J. Cao, C. S. Zender, and M. Wang. 2015. Climate effect of black carbon aerosol in a Tibetan Plateau glacier. Atmospheric Environment 111:71-78. DOI: 10.1016/j.atmosenv.2015.03.016.
164	P100/L1-5	S	Suggest mentioning the interaction of warming oceans with sea ice and the subsequent acceleration of ice sheet loss.
165	P100/L16	V	Text neglects to mention that snow is present in mid-latitudes where it makes a big difference in absorbed solar in springtime.
166	P100/L2-5	V	Text should cite new paper on AMOC: Liu, W., S.-P. Xie, Z. Liu, and J. Zhu. 2017. Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate. Science Advances 3(1). DOI: 10.1126/sciadv.1601666.
167	P100/L29-32	R	There are more recent papers examining climate feedbacks of land cover change on ozone that could be cited. For example, Tai et al., 2013, and papers examining the effects of climate on wildfires: Yue et al., 2013; 2014; 2015. Tai, A. P. K., L. J. Mickley, C. L. Heald, and S. Wu. 2013. Effect of CO₂ inhibition on biogenic isoprene emission: Implications for air quality under 2000 to 2050 changes in climate, vegetation, and land use. Geophysical Research Letters

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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			40(13):3479-3483. DOI: 10.1002/grl.50650. Yue, X., L. J. Mickley, and J. A. Logan. 2014. Projection of wildfire activity in southern California in the mid-twenty-first century. Climate Dynamics 43(7):1973-1991. DOI: 10.1007/s00382-013-2022-3. Yue, X., L. J. Mickley, J. A. Logan, R. C. Hudman, M. V. Martin, and R. M. Yantosca. 2015. Impact of 2050 climate change on North American wildfire: Consequences for ozone air quality. Atmospheric Chemistry and Physics 15(17):10033-10055. DOI: 10.5194/acp-15-10033-2015. Yue, X., L. J. Mickley, J. A. Logan, and J. O. Kaplan. 2013. Ensemble projections of wildfire activity and carbonaceous aerosol concentrations over the western United States in the mid-21st century. Atmospheric Environment 77:767-780. DOI: 10.1016/j.atmosenv.2013.06.003.
168	P102/L31-36	V	Text should describe lifetimes and subsequent distribution of heat in the ocean.
169	P103/L8-21	V	Paragraph on trends in phytoplankton NPP is confusing. Why would climate change affect phytoplankton? Can the observed trends in phytoplankton be reconciled? If not, then the text should at least acknowledge that.
170	P103/L26-31	R	Text requires clarification as to why intensification of hydrological cycle leads to changes in salinity.
171	P104/L23	S	The flat trend in atmospheric methane shown in Figure. 2.5 suggests that thawing permafrost has not lead to increases in methane.
172	P104/L14-17	V	“...the strength of MOC will significantly decrease…” The word “will” should be “may.”
173	P104/L20-23	V	“Permafrost and methane hydrates contain large stores of carbon in the form of organic materials, mostly at northern high latitudes...” Permafrost contains organic materials, and methane hydrates do not. Text should more clearly distinguish between these two potential sources of greenhouse gases.
174	P106/L24-25	R	Only large, very explosive volcanoes can lead to climate impacts of years to decades. See: Raible, C. C., S. Bronnimann, R. Auchmann, P. Brohan, T. L. Frolicher, H. F. Graf, P. Jones, J. Luterbacher, S. Muthers, R. Neukom, A. Robock, S. Self, A. Sudrajat, C. Timmreck, and M. Wegmann. 2016. Tambora 1815 as a test case for high impact volcanic eruptions: Earth system effects. Wiley Interdisciplinary Reviews-Climate Change 7(4):569-589. DOI: 10.1002/wcc.407. Robock, A. 2000. Volcanic eruptions and climate. Reviews of Geophysics 38(2):191-219. DOI: 10.1029/1998RG000054.
175	P107/L32-33	V	The text should acknowledge that aerosols are increasing over Asia and possibly Arabian peninsula. See: Hsu et al., 2012; Chin

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			et al., 2014; Lynch et al., 2016. Given that the climate impacts from aerosol are regional, such regional increases could be very important. Reader is also curious why the trends in aerosol are inhomogeneous, and the text should mention that aerosol sources are being reduced in the developed world due to air quality concerns. Hsu, N. C., R. Gautam, A. M. Sayer, C. Bettenhausen, C. Li, M. J. Jeong, S. C. Tsay, and B. N. Holben. 2012. Global and regional trends of aerosol optical depth over land and ocean using SeaWiFS measurements from 1997 to 2010. Atmos. Chem. Phys. 12(17):8037-8053. DOI: 10.5194/acp-12-8037-2012. Chin, M., T. Diehl, Q. Tan, J. M. Prospero, R. A. Kahn, L. A. Remer, H. Yu, A. M. Sayer, H. Bian, I. V. Geogdzhayev, B. N. Holben, S. G. Howell, B. J. Huebert, N. C. Hsu, D. Kim, T. L. Kucsera, R. C. Levy, M. I. Mishchenko, X. Pan, P. K. Quinn, G. L. Schuster, D. G. Streets, S. A. Strode, O. Torres, and X. P. Zhao. 2014. Multi-decadal aerosol variations from 1980 to 2009: a perspective from observations and a global model. Atmos. Chem. Phys. 14(7):3657-3690. DOI: 10.5194/acp-14-3657-2014. Lynch, P., J. S. Reid, D. L. Westphal, J. L. Zhang, T. F. Hogan, E. J. Hyer, C. A. Curtis, D. A. Hegg, Y. X. Shi, J. R. Campbell, J. I. Rubin, W. R. Sessions, F. J. Turk, and A. L. Walker. 2016. An 11-year global gridded aerosol optical thickness reanalysis (v1.0) for atmospheric and climate sciences. Geoscientific Model Development 9(4):1489-1522. DOI: 10.5194/gmd-9-1489-2016.
176	P108/L4-6	V	Text should make clear that aerosols both scatter and absorb incoming sunlight.
177	P108/L16-17	V	“… only a few very specific types of aerosols (for example, from diesel engines) are sufficiently dark that they have a positive radiative forcing.” This sentence should be deleted as it appears to minimize the impact of absorbing aerosols. Black carbon and brown carbon aerosols from many different sources absorb sunlight.
178	P109/L24-39	V	Much of this section repeats what should be in the “Description of evidence base” section. Text should stick with the terms aerosol-radiation interactions and aerosol-cloud interactions throughout. The terms “indirect” and “semi-direct” should be retired.
179	P110/L17-18	V	Regional effects of aerosols can be quite large, which is not surprising given that the regional forcing of aerosols can be equal to or greater than the magnitude of global forcing from GHGs. Recommend taking this under consideration in this description of evidence. See for example: Philipona, R., K. Behrens, and C. Ruckstuhl. 2009. How declining aerosols and rising greenhouse gases forced rapid

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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			warming in Europe since the 1980s. Geophysical Research Letters 36:5. DOI: 10.1029/2008gl036350. Ruckstuhl, C., R. Philipona, K. Behrens, M. C. Coen, B. Dürr, A. Heimo, C. Mätzler, S. Nyeki, A. Ohmura, L. Vuilleumier, M. Weller, C. Wehrli, and A. Zelenka. 2008. Aerosol and cloud effects on solar brightening and the recent rapid warming. Geophysical Research Letters 35(12). DOI: 10.1029/2008GL034228. Wild, M. 2016. Decadal changes in radiative fluxes at land and ocean surfaces and their relevance for global warming. Wiley Interdisciplinary Reviews: Climate Change 7(1):91-107. DOI: 10.1002/wcc.372. Leibensperger, E. M., L. J. Mickley, D. J. Jacob, W. T. Chen, J. H. Seinfeld, A. Nenes, P. J. Adams, D. G. Streets, N. Kumar, and D. Rind. 2012. Climatic effects of 1950-2050 changes in US anthropogenic aerosols-Part 2: Climate response. Atmospheric Chemistry and Physics 12(7):3349-3362. DOI: 10.5194/acp-12-3349-2012.
180	P110/L28-32	R	Other major uncertainties to note include ocean uptake of CO₂ and the biological and physical response of the ocean to climate change. Another large unknown is the response the Brewer Dobson circulation and the subsequent impact for stratosphere-troposphere coupling.
181	P110/L3	R	Text should say that the largest positive feedback is water vapor.
182	P113	V	Figure 2.1: Caption should state what year, or range of years, is/are represented and whether these values are annual averages. Caption should also make clear that some of these fluxes represent feedbacks.

3: DETECTION AND ATTRIBUTION OF CLIMATE CHANGE

#	page/line	V/R/S
183	P139-143	S	The efforts of the International Detection and Attribution Group (IDAG) should be mentioned (http://www.clivar.org/clivarpanels/etccdi/idag/international-detection-attribution-group-idag; http://www.image.ucar.edu/idag/).
184	P141/L18-21	R	The “relevant chapters” are mentioned but not referred to. Chapters where attribution statements are made should be specified.
185	P141/L34-P142/L4	R	“this topic cannot be comprehensively reviewed here”—while the highlights from NAS (2016) are helpful, they do not convey the full impact of that report. Please elaborate a bit, e.g., how much less confidence is there in attributing drought than heat waves?

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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4: CLIMATE MODELS, SCENARIOS, AND PROJECTIONS

#	page/line	V/R/S
186	P152/L7	R	The phrase “depends primarily on future emissions” could be misleading. If the intent of the sentence is to say that uncertainty in future warming beyond mid-century is due to uncertainty in future emissions, it should be noted that the amount of warming will depend on emissions up until that point. Perhaps “depends primarily on prior emissions and ...”
187	P152/L9	S	It is worth mentioning that it is very unlikely that the atmospheric concentration of CO₂ will be below 400 ppm in this century.
188	P152/L30	S	Chapter 2 is referred to as Scientific Basis, but the title of Chapter 2 is Physical Drivers of Climate Change. The same discrepancy is on page 153, lines 6 and 12, page 159, line 36-37, and page 317, line 22.
189	P153/L20	S	“led by China and the United States” might be an overstatement, as certain European countries might validly claim to have taken aggressive measures sooner
190	P154/L4	S	“earth system” should be “Earth system”
191	P154/L26-33	R	This paragraph should also mention that RCP8.5 is a scenario in which the concentration of atmospheric aerosols is anticipated to be greatly reduced, making the combined radiative effect of increased CO₂ and reduced aerosols even greater than expected for CO₂ increase alone.
192	P155/L16	S	Sanderson et al., 2016 should be listed as Sanderson et al., 2016a., since there are two Sanderson et al., 2016 papers cited.
193	P156/L24-31	R	This paragraph is not clear and the purpose of the calculation is not described. The paragraph should be rewritten for clarity and motivation, and the purpose of the calculation should be described. Swain and Hayhoe, 2015 should also be referenced.
194	P156/L10	R	Section 4.2.3: Is this pattern scaling / GMT scenario used in the rest of the report? If not, it should be deleted from this chapter.
195	P156/L15-19	S	If Section 4.2.3 is retained, terms should be better defined or explained, including time-slice, scenario uncertainty, and climate sensitivity, to make more understandable for the intended audience.
196	P157/L22-P158/L4	R	The paragraphs illustrate a lack of organization found throughout this chapter. The first two lines restate, without reference, the point made (unclearly) on P156/L16-21, then abruptly introduce Key Finding 1, with no elaboration, and with the confusing clause about an unlikely scenario in which sequestration suddenly increases, all with no references. The next paragraph introduces the Paris Agreement, and links RCP scenarios to

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			cumulative emissions to temperature targets, again with no references. The paragraph should be rewritten for clarity.
197	P157/L30	R	The statement, that only 150 Gt more carbon can be emitted globally in order to meet the 1.5C target in the Paris Agreement, should have a reference.
198	P158/L14-P159/L30	R	This brief foray into paleoclimate is more appropriate for the paleoclimate discussion in Chapter 6 and should be integrated there.
199	P159/32-P161/L7	R	This section should discuss why models differ in their calculation of climate sensitivity.
200	P160/L25	S	CMIP6 is unlikely to be much farther along by the time this report is issued. Suggest omitting reference to CMIP6.
201	P160/L32	S	Also refer to Sanderson et al. 2016b.
202	P162/L30	R	The phrase “bias correction will remove the physical interdependence between variables” is imprecise, because the latter is not necessarily a consequence of bias correction. Recommend rewording to “statistical downscaling can alter some of the physical interdependence between variables.”
203	P163/L15-P164/L18	R	A statement should be added here that the intent of weighting models is to increase confidence in a particular response but that in doing so, there is a danger of underestimating the range of uncertainty, and hence missing possible climatic outcomes.
204	P164/L37	S	The IPCC AR5 uses 1.5-4.5C as a range. Why the difference?
205	P165/L7-10	S	The two sentences: For precipitation … entire century. states that precipitation is necessarily a sub-grid-scale process. But, precipitation is constrained by large-scale moisture convergence, so there are large-scale constraints. Recommend focusing the statement on reference to extreme precipitation, or individual precipitation events.
206	P165/L12-13	R	Insert a reference for the statement that natural variability is mostly related to uncertainty in ocean initial conditions.
207	P166/L37-P167/L5	S	Unclear what “the second” refers to. The second statement in the key finding would appear to be “projections...differ modestly” but the traceable account statement invokes “radiative properties...” which are not obviously related to available candidates for the second statement. The summary is also inaccurate, because the notion of “committed warming” was not introduced until about the IPCC Third Assessment Report.
208	P167/L4-5	R	This response sounds reactionary and as such, dismissive. Recommend rewording to focus on “basic physical principles of radiative transfer” or something more specific.
209	P168/L15-18	S	These statements should include references.

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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210	P169	V	The table of emissions rates for RCP8.5 and actual values include some values with 10 significant digits. At most, the values are known to 2 or 3 significant digits. The table should be reformatted to include no more than 3 significant digits for all values shown.
211	P174/Fig. 4.4	R	The history portrayed here is not entirely consistent with the IPCC equivalent (Figure 1.13 of Cubasch et al., 2013)—aerosols are included in SAR (1996), carbon cycle in TAR (2001). Also, the main story is not just increasing amounts of physical science as some of these could fall into other kinds of natural science (as line 29 of page 160 notes). The Committee has recommended deleting this figure, as noted in the main text. If the CSSR authors choose to retain it, consider the suggestions provided in this comment.

5: LARGE-SCALE CIRCULATION AND CLIMATE VARIABILITY

#	page/line	V/R/S
212	P186/L33	R	“teleconnections” should be defined, or a different phrase should be used.
213	P186/L35	S	Change “Principle” to “Principal”
214	P187/L5-19	V	Removal of Figure 5.1 and instead a brief explanation in the text is recommended, as stated in main text. The text could convey these key points: the general circulation transports heat poleward in complex, time-varying circulations. In the tropics, the overturning direct Hadley cell is made up of several more zonally confined circulations and large east-west overturning cells (e.g., the Walker circulation). The mid-latitudes are characterized by zonal jets that become dynamically or baroclinically unstable, and by extratropical cyclones and large planetary scale waves, the latter two responsible for the bulk of the poleward atmospheric heat and moisture transport. The polar latitudes are similarly asymmetric with the principal activity in the form of cyclones and anticyclones.
215	P187/L19	R	NWS 2016 is cited as the Figure 5.1 source, but there is no listing in the references section.
216	P187/L20-28	R	Add references for statements linking U.S. climate to NAO, PDO, ENSO, etc.
217	P187/L31-34	R	Recommend referencing Palmer, T. N., F. J. Doblas-Reyes, A. Weisheimer, and M. J. Rodwell. 2008. Toward seamless prediction: Calibration of climate change projections using seasonal forecasts. Bulletin of the American Meteorological Society 89(4):459-+. DOI: 10.1175/bams-89-4-459.

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218	P187/L32-34	R	The second part of this sentence could be better articulated. Suggest something like: “The climatic response to external forcing may be altered by the forced response of these existing, recurring modes of variability. Further, the structure and strength of regional temperature and precipitation impacts of these recurring modes of variability may be modified due to a change in the background climate.”
219	P188/L5	S	This sounds more like attribution than detection in the usual formulation.
220	P188/L3-37	S	Can any of these changes be quantified, even in a relative sense?
221	P189/L17-30	R	The relatively small sample of ENSO events that have been observed in either the EP or CP categories should be mentioned. The differences between these “flavors of ENSO” are described in the peer-reviewed literature, but care is usually taken to note that the number of events in each category is < 10, so statistical significance is marginal.
222	P189/L31-38	R	The first part of this paragraph indicates that models don’t agree on the projected changes in El Niño intensity or on changes in the zonal SST gradient, and then the paragraph ends by saying that studies suggest a near doubling in frequency of extreme ENSO events. It sounds contradictory. Also, the studies cited use a very strange metric for extreme ENSO variability, which may not be appropriate for comparing 20th century and 21st century ENSO events. Removing the last sentence wouldn’t change the overall intent of the paragraph.
223	P190/L1	V	“Robust evidence” is mentioned twice (also page 288, line 34). Reference to specific kinds of evidence should be provided... For example, “Model studies (cite) and observational analyses (cite) show a …”
224	P191/L11	R	The NPO is not the dominant pattern of variability, but usually is the second most dominant pattern of variability (this is true of Linkin and Nigam as well, the study cited here). Recommend rewording to “a recurring mode.”
225	P191/L34-P193/L2	R	Readers without a strong background in atmospheric sciences/dynamic meteorology will have trouble following this subsection, and its contribution to the messages of the chapter is unclear. Recommend either rewriting or removing the subsection.
226	P193/L7	V	Delete “with a characteristic time scale of 40-60 years.” See the cited Newman et al., 2016, Section 5, for a discussion of the lack of a characteristic time scale for the PDO. Christensen et al. (2013) says 20-30 years, Gedalof et al. (2002) says it behaved quite differently in the 19th century (as indeed is also the case in the past ~15 years). Christensen, J. H., K. Krishna Kumar, E. Aldrian, S.-I. An, I. F.

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			A. Cavalcanti, M. de Castro, W. Dong, P. Goswami, A. Hall, J. K. Kanyanga, A. Kitoh, J. Kossin, N.-C. Lau, J. Renwick, D. B. Stephenson, S.-P. Xie, and T. Zhou. 2013. Climate Phenomena and their Relevance for Future Regional Climate Change. In Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P. M. Midgley, eds. Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press. Gedalof, Z. e., N. J. Mantua, and D. L. Peterson. 2002. A multi-century perspective of variability in the Pacific Decadal Oscillation: new insights from tree rings and coral. Geophysical Research Letters 29(24):57-51-57-54. DOI: 10.1029/2002GL015824.
227	P193/L11-13	V	Suggest rewording the sentence to: “Consequently, PDO-related variations in temperature and precipitation in the United States are very similar to (and indeed may be caused by) variations associated with ENSO and the Aleutian Low strength (North Pacific Index, NPI), as shown in Figure 5.3. A PDO-related temperature variation in Alaska is also apparent ...”
228	P193/L21-24	V	Similar to previous comment, suggest rewording to: “United States temperature and precipitation variations related to the Pacific Decadal Oscillation (PDO) are very similar to (and indeed may be caused by) variations associated with ENSO and the Aleutian Low strength (North Pacific Index, NPI).
229	P193/L35-P194/L10	R	Additional comments should be added about the AMO: 1) Some authors refer to AMO as AMV, i.e., Atlantic Multidecadal Variability to acknowledge the fact that the instrumental record is insufficient to detect an oscillation with 50-70 year period. 2) The oscillatory nature of AMO is further called into question by the possibility that it is arises in response to inter-decadal fluctuations in atmospheric aerosols, so there is nothing intrinsically oscillatory about it. 3) The fact that an AMO signal only emerges from SST time series after detrending should be mentioned, i.e., the “warm” and “cold” phases described in the text are w.r.t. a background upward trend.
230	P195/L6	S	This might be a natural point to include a short digression on Hawkins and Sutton 2009 (move from Section 4.4). It would make more sense complementing Figure 5.4.
231	P199/L12	S	Suggest: … lack of climate models’ ability to properly simulate …

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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6: TEMPERATURE CHANGES IN THE UNITED STATES

#	page/line	V/R/S
232	P218/L12-13	S	“between 1901 and 2015” is an ambiguous way of describing how the trends are calculated. Recommend using phrasing more similar to Table 6.1.
233	P218/L21	R	“Each NCA region” (et seq.)—according to the first figure in the report, there are 10 regions. There is no mention in this chapter of changes in the Caribbean.
234	P218/L24-25	S	“driven by a combination of natural variations and human influence” needs a reference.
235	P219/L9-10	R	This statement would be strengthened considerably with a time series plot to back it up. Such a figure could then be revisited in a subsequent figure with the GCM-simulated past and future temperatures.
236	P219/L34	R	Should be “Figure 6.2”
237	P219/L30-38	V	This conclusion requires a few logical steps: (a) the pollen-based reconstruction indicates temperatures about 0.2°C lower for the last data point compared to the warmest data point around AD 850; (b) during the period of overlap, the instrumental curve is exactly accurate with respect to the pollen-based graph; and (c) the last data point on the instrumental curve is higher than the high data point around AD 850. The problems with this set of logical steps include (1) it is not clear exactly how close the relationship between the instrumental and pollen curves really is and (2) the uncertainties for the instrumental curve are not computed. None of this is covered in the traceable account, and the key finding is therefore unsupported by the figure and the text.
238	P220	R	There is no mention of changes in extremes for Hawai’i, Alaska, and the Caribbean in Tables and only mention of Alaska in the text (across subsections covering extremes). Extremes should be included, or omission should be explained.
239	P221/L13	R	Please clarify whether the 90th percentile is over the entire record or defined for 1901-1960 or 1986-2015.
240	P221/L17-20	R	A reference is needed for this statement.
241	P221/L20-21	S	Extremely, extremely slight.
242	P221/L22-24	V	The metrics, “brief period” and “intense cold waves” need to be explained more fully.
243	P221/L26	V	The definition of “extreme cold waves” is clearer, but still needs explanation. Is “extreme cold wave” the 10th percentile for the coldest 5-day stretch of each year?
244	P221/L34-	R	Similar to previous comments, metrics in this paragraph should

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	P222/L9		be better defined, including heat waves.
245	P221/L35	R	“somewhat less common” seems to be an understatement?
246	P222/L7-9	R	This statement may be true but it is not supported by Figure 6.4 or by any citations.
247	P222/L8-9	S	“as evidenced by”—a single event is not evidence, but could be an example. Suggest rewording: “… than those in the 1930s; one example is the multi-month heat waves …”
248	P222/L16-17	R	Presumably this is a different definition of 1901-2015 temperature trends from that used up to now in this chapter? Clarification is needed.
249	P222/L22-26	R	See Abatzoglou et al., 2007 that suggested that the lower warming in the southeast and higher warming in the west were both connected to atmospheric circulation. Abatzoglou, J. T., and K. T. Redmond. 2007. Asymmetry between trends in spring and autumn temperature and circulation regimes over western North America. Geophysical Research Letters 34(18). DOI: 10.1029/2007GL030891.
250	P224/L6-P225/L6	S	Provide a date range for “near term.”
251	P225/L27-37	R	It is difficult to interpret these results without further understanding of what the “cold spells, extreme cold waves, etc.” metrics mean. As noted previously, recommend providing definitions.
252	P225/L37-P226/L3	R	While this statement seems obvious, a reference is needed. Also, it seems like it does not fit here. The entire chapter has been listing statistics of how temperature is changing, and then it ends with two sentences describing the physical relationship between heat waves and land surface conditions. This might fit better earlier in the section.
253	P228/L20-22	V	This statement does not help trace anything, since some of the specific indices used here are not defined sufficiently to match them to indices in Zhang et al., 2014. Recommend deleting this sentence, and providing details of all calculations used to support this key finding.
254	P231-235	R	Tables that list the regions in this chapter should indicate they are NCA4 regions.
255	P237/Fig. 6.2	R	Y-axis and caption say that the anomalies are calculated with respect to 1904-1980 average. The average for the blue curve over that period looks to be about -0.25°C, not zero. Is this correct? The caption also says that the instrumental data shown are only for the period 2000-2006. Is this a typo?
256	P238-239	R	The methods for generating the time series in the lower panels of Figure 6.3 and all of Figure 6.4 should be described.

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257	P240/Fig. 6.5	V	Gray boxes in Figure 6.5 presumably are where insufficient observations exist, and the CMIP5 data have been masked in the same places. This should be explained.
258	P240/Fig. 6.5	R	It is difficult to understand the green boxes with white hatching, notably the one near Oklahoma (?): the observed trend is 0.5-1 F/100 yrs and the modeled trend is 1.5-2°F per 100 yrs, and somehow that’s not a detectable trend but is consistent with models? Clarify.
259	P243/Fig. 6.8	R	The patchy texture of Figure 6.8 likely arises from statistical oddities in the downscaling technique rather than from physical processes. Does the ESD add any information or would it be just as defensible to plot the CMIP5 output directly?

7: PRECIPITATION CHANGE IN THE UNITED STATES

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260	P253/L3-6	R	In Chapter 6, a similar reduction was attributed to the lengthening of the period averaged for “recent” times—note previous comments regarding averaged time periods. The southwest drought since 2011 does not pop out in Figure 7.1 as claimed here.
261	P253/L8-15	S	This discussion of interannual variability and individual regional droughts is slightly out of place in a paragraph that references a map (Figure 7.1). Recommend starting with a description of the spatial patterns (and conceding that the splotchiness may be an artifact of the gridding process). Are any of the trends statistically significant?
262	P253/L34-36	S	These seasonal changes are not equivalent. Isn’t it the case that the changes in fall are small and not significant, whereas the changes in spring are very large (especially June)?
263	P253/L33-P254/L6	S	Recommend providing additional, more authoritative citations for this information, perhaps IPCC.
264	P254/L6-17	V	“increase”...”decrease”...”trend”...”decreased”... for a lot of these comparisons, the period of record and possibly method are important in determining the sign of the result and should be specified.
265	P254/L22-P267/Fig. 7.2	V	Page 254 says 5-day but page 267 (Figure 7.2) says daily. Page 254 discusses individual stations (implying that they are visible in the figure) but page 267 shows regional averages. These discrepancies should be reconciled or explained more clearly.
266	P254/L33-34	R	Methods for calculating 5-year return value should be detailed here or in an appendix. Are any of the changes statistically significant?

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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267	P255/L9-10	V	How were station data combined into a spatial average? Was it CONUS? Greater detail is needed.
268	P255/L27-29	R	This passage seems to be a vehicle to discuss a single study. ETCs are surely more important in winter, and ETCs are surely a less important factor in summer in many NCA regions than other factors (e.g., tropical cyclones, southwest monsoon, other summertime convection). The link to the cited Pfahl et al., 2015 study is not clear- what season? Were they so idealized as to be irrelevant? What do GCMs say?
269	P257/L14-26	R	Since there’s only one example of a U.S. storm, perhaps in addition to the lessons drawn from the two studies on the Colorado event, some more general lessons about detection and attribution of individual storms can be drawn from other parts of the world—the UK folks have done several studies of heavy precipitation events there.
270	P257/L15	S	“fewer extreme storms”—fewer than what? Fewer than observed?
271	P257/L21	R	Given this result, why show projections of snowpack change from a GCM (Figure 8.3)?
272	P259/L31-33	R	“large compared to natural variations”—as computed from observations or from the models’ respective 20th century or NAT simulations? Larger, meaning what exactly (> 1 sigma?) and why distinguish between “small compared to natural variations” and “inconclusive”—why not just reduce the load on the reader and use stippling only?
273	P260/L2	S	Recommend referring back to Chapter 5 where changes in Hadley circulation are discussed.
274	P260/L7-17	S	This is also covered in Chapter 8 and better coordination across chapters would improve this section.
275	P260/L34	V	How were the standard deviations calculated? Across (how many) participating GCMs?
276	P261/L10-16	S	Recommend reorganizing this section to discuss the landfalling portion first, since that is more relevant to the U.S.
277	P261/L18-21	R	Should “CM3” be “CMIP3”? If not, what does it mean?
278	P262/L16	S	Recommend also noting that encroachment or removal of vegetation can contribute to uncertainties in observed precipitation trends.

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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8: DROUGHTS, FLOODS, AND HYDROLOGY

#	page/line	V/R/S
279	P281/L7	R	Opening this key finding with “…is complicated” weakens the impact of what follows by suggesting that our understanding must be poor or limited. Recommend deleting this sentence here and where repeated in traceable accounts.
280	P281/L3-6	R	Important to note here that the “dust bowl” was not a purely natural phenomenon—it was exacerbated by human land management practices.
281	P281/L29	R	This section should note that the three characterizations of drought also have a varying range of timescales and are implicitly defined as deficits relative to some notion of what constitutes sufficient water (precipitation, soil moisture, stream flow).
282	P281/L29	R	“scarcity” has economic connotations. “deficit” may be more appropriate.
283	P281/L35-P282/L2	S	Stating “no region” seems perhaps oversimplified and inconsistent with Figure8.1, where it looks as if parts of northern Canada may see increased moisture during almost all seasons.
284	P281-285	S	Key Findings 1 and 2 have no figures associated with them. Inclusion of a time series for Key Finding 1 could nicely illustrate the message.
285	P282/L3-4	S	both increase and decrease should be either increase or decrease.
286	P282/L10	V	It is important to mention that precipitation deficits occur on a range of timescales, not just seasonal and annual. Some researchers maintain that “flash droughts” can result from just a few weeks of dry weather, and it is also clear from the paleo record that the long end of the timescale for droughts may be measured in decades, as indeed is mentioned later in this section.
287	P282/L11	S	It is unclear what is meant by “effect of these natural variations.” Consider reframing this to ask how rising temperatures change the hydrologic balance, and how human-induced changes in atmospheric circulation might change the magnitude or frequency of precipitation deficits.
288	P283/L5-6	R	Reference is needed.
289	P283/L10-12	S	“Attribution statements…are without associated detection.” This is a very indirect way of noting that Swain et al. (2014) found positive attribution of the ridiculously resilient ridge to human-caused climate change. This should be stated directly (while also noting the lack of associated detection).
290	P283/L22-23	R	“The Great Plains/Midwest drought of 2012 was the most severe summer meteorological drought in the observational record for

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			that region (see cited Hoerling et al., 2014 paper).” Is this consistent with earlier statements about the ‘30s Dust Bowl being the worst drought ever? If so, please explain how.
291	P283/30-34	R	Clarify whether this is intended to indicate that human influences intensified the drought by increasing temperatures and reducing soil moisture.
292	P284/L1-15	S	This paragraph seems to imply positive attribution of “the blob” and associated precipitation deficit to human influences. Is that correct? If so, it should be stated more clearly.
293	P284/L5-8	R	Bond et al. actually say the opposite—the ridge caused the blob. Also see Mote et al. (2016), who suggested that the blob influenced the likelihood of drought in 2015, mainly in the Northwest. Mote, P. W., D. E. Rupp, S. Li, D. J. Sharp, F. Otto, P. F. Uhe, M. Xiao, D. P. Lettenmaier, H. Cullen, and M. R. Allen. 2016. Perspectives on the causes of exceptionally low 2015 snowpack in the western United States. Geophysical Research Letters 43(20):10,980-910,988. DOI: 10.1002/2016GL069965.
294	P284	R	In discussion of “the blob” and “ridiculously resilient ridge,” recommend mention that it has been hypothesized that persistent phenomena like these are associated with arctic amplification, and link to Chapter 11, where this is already stated.
295	P284/L33-34	V	In the statement, “less sensitivity to temperature increases than to precipitation variations, which have increased over the 20th century,” the juxtaposition of a directional temperature change and an increase in magnitude of precipitation variations is confusing. The quantity of soil moisture should be sensitive to total moisture input, not to the interannual variability.
296	P284/L38-P285/L2	S	This sentence, although correct, is potentially confusing/misleading. The reader could conclude simply that there has been no human influence on meteorological drought in the United States, when the authors may be intending to convey is that such an influence may exist, but studies based on precipitation trends do not show one. If this is intended to be an “absence of evidence” statement, rather than an “evidence of absence” statement, it should be rephrased for clarity and moved to follow the next sentence, which states a positive finding.
297	P284	S	The 2012 Central United States drought has been classified as a “heat wave flash drought” (see Mo and Lettenmaier, 2015). But the frequency of such droughts has been going down over about the last 100 years, and 2012 represented what appears to be an isolated uptick in a type of event that is becoming increasingly rare. This trend should be noted in this section. Mo, K. C., and D. P. Lettenmaier. 2015. Heat wave flash

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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			droughts in decline. Geophysical Research Letters 42(8):2823-2829. DOI: 10.1002/2015GL064018.
298	P284	S	Recommend incorporating discussion of the challenges in interpreting of P-E from climate models over the western U.S. because of their inability to resolve topography properly at a coarse resolution.
299	P285/L22-26	R	If the PET formulation in the cited Walsh et al., 2014 is the standard Thornthwaite temperature index method, it will likely lead to an over-estimation of droughts, as the cited Sheffield et al. (2012) paper shows.
300	P285/L14-P286/L2	S	Recommend splitting this long paragraph into two for clarity, with one focused on precipitation deficits and one on soil moisture.
301	P285/L7-12	R	Are other basins that have received attention in the literature that could also be included here?
302	P286/L3-6	S	This statement is very similar to one made on the previous page.
303	P286/L11-15	R	If available, consider addition citations for more robust simulations with offline hydrologic or other land surface models.
304	P286/L6-9	R	The statement “a direct CMIP5 multimodel projection.... total depth of the soil” is incorrect. Soil moisture percentiles based on total column soil moisture (from multiple land surface models) are already used in NOAA’s input to the U.S. Drought Monitor. Generally, the estimated soil moisture percentiles are more, rather than less, consistent than the models’ surface soil moisture.
305	P287	V	As shown in Lins and Slack, 1999 and 2005, runoff has been increasing across most of the United States. at percentiles up to about the median, therefore model projections that indicate decreases seem questionable. What do the models show over the historic period? Aside from the western U.S., where snowpack-related changes clearly are related to warming, conclusions regarding runoff should be given low confidence. Lins, H. F., and J. R. Slack. 1999. Streamflow trends in the United States. Geophysical Research Letters 26(2):227-230. Lins, H. F., and J. R. Slack. 2005. Seasonal and regional characteristics of U.S. streamflow trends in the United States from 1940-1999. Physical Geography 26:489-501.
306	P287	R	It may be inaccurate to refer to runoff changes associated with shifts in runoff timing in the western United States (related to reduced snowpack) as increased drought. Instead, this is a permanent shift in runoff timing.
307	P287/L29-P289/L23	R	This section would be strengthened with inclusion of some discussion of changes in the risk of floods associated with ARs. The discussion of ARs in Chapter 9 suggests that such changes

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			might be expected. Even if no studies have been done and there is little one can say, it would be good to mention the issue.
308	P288/L4-16	V	Recommend specifying the time period. Using “trends” without reporting the time period (and perhaps method, if not the default least-squares linear fit) is vague and comparisons across studies can lead to contradictions if the time periods do not match.
309	P288/L21-26	S	This paragraph shifts abruptly from observed to projected changes. Was this inserted to reinforce the point in the previous three sentences that precipitation and runoff extremes happen in different seasons? The study mentioned points toward increases in fall (as well as winter), which do not support the point very cleanly. Recommend revising this text.
310	P288/L31-P289/L2	V	The discussion of attribution of flooding should recognize—and the text should state—that changes in non-climatic factors like channel structure, basin land use, etc., can be significant factors complicating such attribution.
311	P289/L24-P290/L5	R	This section is very out of place in Chapter 8. Recommend moving to Chapter 10.
312	P290/L3-4	R	The information presented here seems to suggest medium confidence, based on the definition provided in the draft CSSR. Recommend citing Westerling et al., 2007, who argue that there is a strong anthropogenic signal, and reviewing the NASA fuel fire index (see e.g., doi:10.5194/nhess-15-1407-2015). Beyond the studies cited, authors might also consider mentioning the well-established indirect effects of human activities on wildfire activity in the western United States: warmer temperatures, earlier snowmelt and runoff, and in many areas and times of year, lower soil moisture. These effects would suggest that there are not “competing schools of thought” on this issue, but instead a question of the relative importance of anthropogenic climate change versus other factors.
313	P291/L20-21	R	This statement appears to be based on comparison between recent droughts and the dust bowl, but the latter probably has a human-induced component, therefore is not an example of “Earth’s hydrologic natural variation.” This should be revised.
314	P291	S	Recommend stating that while soil moisture is not well observed over long periods, land surface models do a pretty good job of reproducing it, and have allowed reconstructions for ~100 years.
315	P292/L31	S	Uncertainty is “not low” is awkward. Recommend stating high or moderate as appropriate.
316	P293/L17-18	R	It is important to note the long-standing nature of our understanding of effects on climate change on western United States hydrology here. While these changes are described in the cited Barnett et al., 2008 paper, they were also described well before then and this should be noted. Also Barnett et al. and others attributed changes to human-induced climate change,

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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			which should be noted here.
317	P294/L11-17	R	The summary statement should address water scarcity, since the key finding does.

9: EXTREME STORMS

#	page/line	V/R/S
318	P308/L18	S	“this increase” should be “this projected increase”
319	P308/L28-32	R	This statement pertains to projected changes in ARs. Can anything be said about observed historical trends in ARs?
320	P308/L34	S	Recommends opening this section with a brief reminder about why we care about this subject: severe storms cause disruption, financial losses, and loss of life.
321	P308/L34-P309/L8	R	Is it really the uncertainty in sign or trends that makes detection and attribution relatively difficult for severe storms? Is the relative rarity of these events, which reduces the statistics significance of observed trends, not a more important factor?
322	P308/L34-P309/L8	S	This introductory paragraph suggests that the scope of the chapter will be limited to analysis of past trends, when in fact future projections are also discussed. Some commentary about the difficulty of projecting changes in severe storms would therefore also seem appropriate to include here.
323	P309/L10-19	R	Is it worth noting here that Hartmann et al., 2013 found increases in tropical cyclone activity to be “Virtually certain in North Atlantic since 1970?”
324	P309/L32-35	R	The statement “particularly robust” does not seem well supported by the trend shown in Figure 9.1. In what sense is 0.2°/decade or about 1.5° latitude in total robust? Is the trend statistically significant? Does the evidence really support the statement that the observed rate of movement can “substantially change patterns of tropical cyclone hazard exposure”?
325	P310/L15-21	R	Were these formal attribution studies? Some of the citations listed here predate the application of detection and attribution methodology to questions other than global mean surface temperature. Possibly a word other than “attribution” (with accompanying reference to Chapter 3) would be appropriate, perhaps “ascribed”?
326	P310/L26	S	Could eliminate “in the literature.”
327	P310/L33-P311/L14	S	In this passage, IPCC statements are referred to as “consensus”. This does not occur elsewhere in the report and “assessment” would be a more accurate and common term for IPCC statements.

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328	P310/L33-P311/L13	R	Do these modeling studies reproduce any of the observed variations in response to the mechanisms described on page 310, lines 15-20?
329	P311/L11	S	Recommend stating “the increase [decrease] in tropical cyclone frequency” instead of referring to “the sign of the change in tropical cyclone frequency.”
330	P311/L19-20	S	What is the difference between consistency and consensus?
331	P312/L9-11	S	This sentence should be posed as a statement and not a question.
332	P313/L5	S	Is it helpful to describe the hurricane drought as “anomalous” (meaning deviating from what is standard, normal, or expected)? It seems that that is the premise here. The question is, what is the explanation for this anomaly? The evidence presented suggests a large random element, with a possible contribution from climate change of uncertain magnitude.
333	P313/L23	S	Are post-storm damage assessments also used to determine the occurrence of a tornado? If so, indicate it here.
334	P315/L15-22	S	This passage is confusing and would be improved with clarification of the “climate conditions” and their relationship to CAPE and supercell strength.
335	P315/L30	S	It is unclear which “part of the United States” is being referred to.
336	P315/L33	S	Arctic should be arctic, since it is an adjective here.
337	P315/L37-P316/L2	R	The explanation of “anomalously strong Pacific trade winds,” even if correct, is not very informative without identifying the cause of the anomaly. Can anything more fundamental be said about alternative possible causes of the “weather patterns of recent years?”
338	P316/L3-7	R	There seems to be broad agreement here between observed and projected (increasing) trends, suggesting that we should have some confidence in those increases. Yet, the corresponding section in Key Finding 4 (page 308 lines 20-24) seems to convey much less certainty and confidence. This should be reconciled.
339	P316/L4	S	Recommend this poleward shift should also be mentioned in Chapter 5.
340	P316/L13-P317/L3	R	The story is quite different for the Northwest states than for California—AR events are so warm that they lead to net removal of snow and therefore do not “end” droughts there. Some of the literature cited has a California bias in that it does not acknowledge ways in which ARs, and their effects, differ in other parts of the West. Some discussion of their role in rain-on-snow floods (like those in February 1996 in Oregon and December 2007 in NW Oregon-SW Washington) would be an appropriate

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			balance to the overly California-centric flavor of this paragraph.
341	P317/L3-7	R	Framing the question about how total distribution of precipitation (means and extremes) will change by discussing ARs is a popular approach, but it is unclear how this framing adds to the question. At minimum, this discussion should be put into the context of extreme precipitation discussed in Chapter 7 (and to some extent Chapter 8). The IVT approach discussed briefly at the bottom of page 317 may be a more useful way to cover this topic.
342	P317/L30	S	Are the “studies that show qualitatively similar increases” noted here observational studies? Please clarify.
343	P321/L35-37	V	This ‘summary’ discusses methodologies, not conclusions, and should be revised to reflect the key messages of this finding.

10: CHANGES IN LAND COVER AND TERRESTRIAL BIOGEOCHEMISTRY

#	page/line	V/R/S
344	P337/L9-11	R	This is a non-informative use of the confidence language: with a literal interpretation, this is just saying you are pretty sure the probability is not zero. It would be much more useful to be explicit that the land could become a net source, with a probability that is not known but might be on the order of something between 10% and 50%, especially with continued high emissions.
345	P337/L26-27	S	It is probably better not to try to provide a mechanistic explanation of the urban heat island effect in this brief statement.
346	P338/L2-3	V	This is a misleading opening, implying that all LUC effects are via albedo.
347	P338/L33	R	“Earth browning” and “global greening” need definitions or to be replaced with self-explanatory terms.
348	P338/L34-36	S	Update to include Girardin et al. (2016), who found no overall growth stimulation for continental boreal forest. Girardin, M. P., O. Bouriaud, E. H. Hogg, W. Kurz, N. E. Zimmermann, J. M. Metsaranta, R. De Jong, D. C. Frank, J. Esper, U. Büntgen, X. J. Guo, and J. Bhatti. 2016. No growth stimulation of Canada’s boreal forest under half-century of combined warming and CO₂ fertilization. Proceedings of the National Academy of Sciences of the United States of America 113(52):E8406-E8414. DOI: 10.1073/pnas.1610156113.
349	P339/L31-33	S	Possible citations to add here include: and Bond-Lamberty, B. and Thomson, A. 2010. Temperature-associated increases in the global soil respiration record.

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			Bond-Lamberty, B., and A. Thomson. 2010. Temperature-associated increases in the global soil respiration record. Nature 464(7288):579-582. DOI: 10.1038/nature08930. Crowther, T. W., K. E. O. Todd-Brown, C. W. Rowe, W. R. Wieder, J. C. Carey, M. B. MacHmuller, B. L. Snoek, S. Fang, G. Zhou, S. D. Allison, J. M. Blair, S. D. Bridgham, A. J. Burton, Y. Carrillo, P. B. Reich, J. S. Clark, A. T. Classen, F. A. Dijkstra, B. Elberling, B. A. Emmett, M. Estiarte, S. D. Frey, J. Guo, J. Harte, L. Jiang, B. R. Johnson, G. Kroël-Dulay, K. S. Larsen, H. Laudon, J. M. Lavallee, Y. Luo, M. Lupascu, L. N. Ma, S. Marhan, A. Michelsen, J. Mohan, S. Niu, E. Pendall, J. Peñuelas, L. Pfeifer-Meister, C. Poll, S. Reinsch, L. L. Reynolds, I. K. Schmidt, S. Sistla, N. W. Sokol, P. H. Templer, K. K. Treseder, J. M. Welker, and M. A. Bradford. 2016. Quantifying global soil carbon losses in response to warming. Nature 540(7631):104-108. DOI: 10.1038/nature20150.
350	P340/L20-27	S	Similarly compelling statistics have been calculated for California’s drought, and could be included here.
351	P340/L27-30	S	This sentence requires clarification, it’s confusing as written.
352	P341/L4-17	S	This paragraph needs a sense of scale. Are these generally small or large effects, especially relative to other impacts of climate change and human activity?
353	P341/L4-38	V	Since SOCCR-2 is a draft report that will not be finalized until after CSSR, it should not be cited. Instead, the primary literature underlying the statements should be referenced.
354	P341/L18-29	R	This paragraph would benefit from a little more detail on the relationship between N availability and plant growth. For instance, line 24 should state that N mineralization transforms the N into a form that can then be taken up by plants, which results in the shift in N from the soil to vegetation.
355	P341/L27-29	S	Sentence on CMIP5 models seems out of place; remove?
356	P341/L33-35	R	This sentence requires the mechanistic context to explain why CO₂ losses from soils would decrease with N deposition.
357	P342/L1-15	S	Paragraph is too long, relative to importance of VOCs for climate change and vis-à-vis main chapter points.
358	P342/L1-15		Nearly all the references regarding VOCs are outdated. The chemical mechanisms involved in the oxidation of VOCs in the atmosphere have been much revised in recent years, and current understanding of the effects of VOCs on regional climate has changed. See for example Tai et al. (2013), Achakulwisut et al. (2015), and Heald and Ridley (2016). Tai, A. P. K., L. J. Mickley, C. L. Heald, and S. Wu. 2013. Effect of CO₂ inhibition on biogenic isoprene emission:

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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			Implications for air quality under 2000 to 2050 changes in climate, vegetation, and land use. Geophysical Research Letters 40(13):3479-3483. DOI: 10.1002/grl.50650. Achakulwisut, P., L. J. Mickley, L. T. Murray, A. P. K. Tai, J. O. Kaplan, and B. Alexander. 2015. Uncertainties in isoprene photochemistry and emissions: implications for the oxidative capacity of past and present atmospheres and for climate forcing agents. Atmos. Chem. Phys. 15(14):7977-7998. DOI: 10.5194/acp-15-7977-2015. Heald, C. L., and J. A. Geddes. 2016. The impact of historical land use change from 1850 to 2000 on secondary particulate matter and ozone. Atmos. Chem. Phys. 16(23):14997-15010. DOI: 10.5194/acp-16-14997-2016.
359	P342/L22-26	S	These sentences should ideally address issues beyond just fire.
360	P342/L28-P343/L17	S	This paragraph is not well defined and is a mixture of too many topics. Recommend breaking it apart.
361	P342/31-32	V	Chapter 5 did not present compelling evidence that such changes are underway or even expected.
362	P343/L18-30	R	This is a confusing paragraph that ranges from storms to fires, with many puzzling comments. It is poorly structured and out of place; remove and/or break up to put elsewhere, or significantly rewrite to improve logical flow and emphasize important points.
363	P343/L25-29	S	This sentence is unclear. Does this mean flows will be lower than the historic extreme lows?
364	P343/L31-38	V	This would be a natural place for the Wildfires Section 8.3.
365	P344/L2-5	V	Almost every indicator of human activity has increased since about 1950, making statements about the correlation between CO₂ uptake and emissions unhelpful without additional context. Also, emissions could mean either industrial or ecosystem.
366	P345/L1-3	V	The description of the trend seems over-precise. If this interpretation is not supported by a robust statistical analysis, it should not be presented, and it should certainly not be presented as clearly understood.
367	P345/L1	R	Stating the growing season changes are “more variable” using referenced figures is not an apt comparison, since Figure 10.3 is a map and Figure 10.4 is a time series.
368	P345/L16-17	S	Are not plant hardiness zones based on temperate and growing season length? So, does this sentence add anything?
369	P345/L28	S	The cited EPA (2016) report is a peer reviewed document, but it seems to not be the most appropriate reference to adequately support this statement.
370	P346/L6	S	“exacerbated” has the wrong implication. Recommend

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			“amplified.”
371	P346/L6-7	S	The Reyes-Fox et al. (2014) paper cited here makes it clear that their conclusion is intended for settings where the season end is set by drought and not by cold. Zhu et al. (2016) provide an example where this does not appear to be the case. Zhu, K., N. R. Chiariello, T. Tobeck, T. Fukami, and C. B. Field. 2016. Nonlinear, interacting responses to climate limit grassland production under global change. Proceedings of the National Academy of Sciences of the United States of America 113(38):10589-10594. DOI: 10.1073/pnas.1606734113.
372	P346/L16-18	S	This sentence is difficult to understand.
373	P346/L27-32	S	This sudden shift to CMIP5 model projections is unexpected and out of place.
374	P347/L13-17	S	Groundwater depletion is one of the major themes in recent years. It deserves more than this cursory treatment.
375	P347/L25	R	The chapter starts with lots of statements about the role of climate change in increasing sinks and then states that the general effect is to decrease forest sinks. These two elements of the interpretation need to be reconciled.
376	P348/L31-32	S	Were any of those cities in the United States?
377	P349/L1-11	S	This discussion is not useful without some information on direction and magnitude of the effects.

11: ARCTIC CHANGES AND THEIR EFFECTS ON ALASKA AND THE REST OF THE UNITED STATES

#	page/line	V/R/S
378	General	R	There could be more discussion on the relative importance of the main drivers of sea-ice change (air vs ocean temperature, prevailing wind-driven export, etc.) in this chapter.
379	P370/L25-27	S	Sea level rise should also be mentioned.
380	P370/L25-33	S	Lines 31-33 seem to serve the same purpose as 25-27, but state things in less obscure terms. Consider consolidating.
381	P370/L23-P371/L15	S	A number of statements in the introduction are very obvious and are not necessary, given the “scientifically literate” target audience.
382	P370/L27	R	Statement on “high sensitivity” should include source or evidence.

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
383	P370/L24-P371/2	S	This paragraph contains a lot of useful information but does not flow well. Recommend revising to make the sentence order more logical.
384	P371/L1	R	Much of Alaska is within the Arctic, so the statement that Alaska’s climate is “connected to the Arctic” does not seem necessary.
385	P371/L3-13	V	As written, this section could leave a read with the impression that not enough is known about Arctic climate change to inform policy, which is not the case. Recommend adding some statement to the effect that despite these uncertainties, we certainly know enough to formulate effective policies.
386	P371/L11-15	R	The concepts of “stunted scientific progress” and “significant scientific progress” are at odds. This section of the introduction needs editing.
387	P371/L20	R	“Vertical profiles of temperature.” Where? In the boundary layer? Free atmosphere, upper ocean? Wording needs to be more direct and explicit.
388	P371/L26-35	S	The post-1979 temperature changes are impressive! It would be nice to see a map of them.
389	P372/L4	S	Should “however” be “moreover?”
390	P372/L9	S	“will continue”
391	P372/L22	R	Where does the “New Arctic” era come from? A reference is needed.
392	P373/L5-7	R	It is unclear how the statistics cited support the statement about “The age distribution....” If the decrease in multi-year ice were greater than the decrease in first-year ice, that would support the statement, but instead, the decreases are the same, within uncertainties. Furthermore, looking at Figure 11.1, the decrease in extent of multi-year ice appears to be much greater than 13%. Even considering the different baseline years (1988 vs 1984), this seems to be an inconsistency.
393	P373/L17-19	R	What is the definition of melt season (also in the caption for Figure 11.2)? Also, from Figure 11.2, it looks like part of Alaska’s west coast has seen an increase in melt season.
394	P373/L29-32	V	“very likely” a human contribution to sea ice loss? Is this implying that there is up to a 10% chance that there is no human contribution at all to loss of Arctic sea ice? This seems surprising and inconsistent with the subsequent statement that future sea ice loss is virtually certai. If future human forcing is so certain, how can past human forcing be less certain?
395	P373/L30-31	R	“Internal climate variability alone could not have caused recently observed record low Arctic sea ice extents (Zhang and Knutson 2013).” A probability associated with this statement should be provided, if possible.

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
396	P373/L39-P374/L2	R	The section on sea ice is long on observational trends but short on projections of the future. In particular there is no quantitative discussion of how future sea ice extents depend on emissions scenarios. Recommend adding material on future projections.
397	P373/L3	R	“The thickness…” Presumably you mean the mean thickness?
398	P374/L17-18	R	“AW” is an abbreviation used only here and only twice. Recommend just writing out.
399	P374/L18	R	Is the “observed AW warming unprecedented in the last 1150 years” referring to rates of warming, total warming since 1970, or maximum temperatures? This is too vague as written.
400	P374/L28-31	R	Projections of SLR should be left to Chapter 12, which is very different from Church et al. 2013 cited here.
401	P375/L18-P376/L11	R	It would be appropriate to tie this passage to the equivalent in Chapter 10. But, it is not clear that it fits within the purview of this report, viz, physical aspects of climate change.
402	P375/L33	S	It is unclear what “Thresholds in temperature and precipitation shape Arctic fire regimes…” means. Please clarify.
403	P375/L14	S	Modeling studies (projections) and observations are being awkwardly blended in this statement.
404	P375/L18-P376/L5	S	This paragraph is OK but completely Alaska-focused. It could be improved by inclusion of at least a few sentences of context with respect to other parts of the North American and global Arctic.
405	P375/L22	S	“Shortened snow cover and higher temperatures…” compared to what?
406	P375/L27-30	S	This sentence could be broken up and re-written for improved clarity.
407	P375/L37	R	The basis for the stated projections is not given. Is it based on fire-weather analysis calculated from GCM climate projections? Some basic information should be provided, rather than just citations.
408	P376/L6-7	S	This sentence is confusing. Recommend restructuring to something like, “Approximately 50% of the total global soil carbon is found in boreal forest and tundra ecosystems.” Also, please clarify whether this value contains carbon contained in permafrost.
409	P376/L16-17	R	The math here is unclear here—50% decline between 1967 and 2012 (45 years) = 11% per decade not 19.8% per decade? The citation is a broken URL.
410	P376/L20-22	R	Please explain why May is chosen for comparison instead of another month.
411	P376/L32	R	Why “since 2000”? Every time series in Figure 11.3 goes back to the 1980s.

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
412	P377/L12	R	“Mass loss from ice sheets and glaciers influences sea level rise” is too much of an understatement. It is important to explain that the relative contribution of mass loss to SLR continues to increase, now exceeds thermal expansion, and has the potential to eventually alter the landscape.
413	P377/L36	R	The sentence: “Ice mass loss… has steadily declined” is confusing, as it seems to indicate that the rate of mass loss is decreasing around the Gulf of Alaska. That would be surprising given that the Arctic is warming rapidly, where mass loss from the biggest single ice sheet (Greenland) is accelerating, and where the Pan Arctic rate of mass loss seems roughly constant since around 2000 (Figure 11.4). Is this intended to state that ice mass [not mass loss] has steadily declined? If not, an explanation of why mass loss is decelerating, i.e., why glaciers near Gulf of Alaska are behaving differently from those in the rest of the Arctic, is needed.
414	P378/L14	S	The meaning of this sentence is unclear. What is meant by “factor”?
415	P378/L32-33	S	It would be useful to mention the recent California drought as an example of “persistent circulation phenomena like blocking and planetary wave amplitude.”
416	P379/L12-24	R	The statement that “these simulations do not support” a dominant role for loss of sea ice is followed by the argument that the models do not adequately represent the processes relevant to this question. If that is the case, then the “these simulations do not support” statement seems misleading. Clarify what is meant and why the results of these models are worth reporting.
417	P379/L36-P380/L2	S	Since AMOC has been covered in other chapters, cross-reference should be included.
418	P380/L2	S	Refer to Chapter 15.
419	P380/L5-6	S	Might want to weaken this statement; Alaska’s “carbon rich” permafrost is in a narrow band on North Slope and doesn’t compare to e.g., Hudson Bay Lowlands (see Figure 1a in the cited Schuur et al., 2015 paper).
420	P380/L7-8	V	The statement that “warming Alaska permafrost …is a concern…for the global carbon cycle” is too tepid and obscurely worded. Warming is a concern for the global climate, and the possibility of significant and uncontrollable releases of carbon threaten to undermine global efforts to control climate change.
421	P380/L12-29		For balance, consider citing Oh et al. (2016) who suggest that much of the Arctic can act as a sink for methane, even when permafrost thaws. Oh, Y., B. Stackhouse, M. C. Y. Lau, X. Xu, A. T. Trugman, J. Moch, T. C. Onstott, C. J. Jørgensen, L. D’Imperio, B. Elberling, C. A. Emmerton, V. L. St. Louis, and D. Medvigy. 2016. A

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			scalable model for methane consumption in arctic mineral soils. Geophysical Research Letters 43(10):5143-5150. DOI: 10.1002/2016GL069049.
422	P380/L15	S	Consider citing Treat et al. synthesis papers. For example: Treat, C. C., S. M. Natali, J. Ernakovich, C. M. Iversen, M. Lupascu, A. D. McGuire, R. J. Norby, T. Roy Chowdhury, A. Richter, H. Šantrůčková, C. Schädel, E. A. G. Schuur, V. L. Sloan, M. R. Turetsky, and M. P. Waldrop. 2015. A pan-Arctic synthesis of CH₄ and CO₂ production from anoxic soil incubations. Global Change Biology 21(7):2787-2803. DOI: 10.1111/gcb.12875.
423	P380/L15-18	R	This significantly misstates the central finding of the cited Schädel et al., 2016 paper, which was that emissions from thawed permafrost soils are likely to be overwhelmingly dominated by CO₂, not CH₄.
424	P380/L17	R	“Schädel”
425	P380/L17	R	How much permafrost-sourced CH₄ production oxidizes to CO₂? The statement that CH₄ is 20 times stronger a greenhouse gas than CO₂ is misused here.
426	P380/L18-22	R	How does the estimate of this feedback square with the fact of little change in CH₄ (Figure 2.5) during a period of rapid Arctic warming?
427	P380/L19-20	R	Explain why are there negative signs in front of 14 and 19?
428	P380/L21	V	The global temperature rise quoted here is from the permafrost-carbon feedback alone. Clarify.
429	P380/L30-P381/L9	R	There is some overlap here with Chapter 15. Note also that much of the CH₄ released is likely to oxidize to longer-lived CO₂.
430	P382/L10-11	S	“Climate models have been predicting… for more than 40 years.” To make the meaning completely clear, it would be better to say “For more than 40 years, climate models have been predicting…”
431	P383/L12	S	Summary sentence simply reiterates previous text and does not integrate the key finding, evidence base, and key uncertainties in a concise way. The key finding and summary sentences almost seem reversed.
432	P383/L16	S	Typo. Check grammar.
433	P384/L21	R	Why is the likelihood of impacts only 2/3 when finding states that “crumbling buildings, roads, and bridges are being observed.”? It seems like it should be 100%, since impacts are already occurring.
434	P383/L28-	R	Recommend citing Schädel et al., 2016, and mentioning

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
	32		dominance of CO₂
435	P383/L32	S	Perhaps add in-situ gas flux measurements to the list? Schuur et al. 2009, among many others. Schuur, E.A.G., Vogel, J.G., Crummer, K.G., Lee, H., Sickman, J.O., Osterkamp, T.E. 2009. The effect of permafrost thaw on old carbon release and net carbon exchange from tundra. Nature, 459(7246): 556-559. doi:10.1038/nature08031
436	P384/L1	S	Recommend adding microbial activity (warming) to the list. For example, see: Hollesen, J., H. Matthiesen, A. B. Møller, and B. Elberling. 2015. Permafrost thawing in organic Arctic soils accelerated by ground heat production. Nature Climate Change 5(6):574-578. DOI: 10.1038/nclimate2590.
437	P384/L30	R	Are estimates of permafrost soil carbon content generally based on just the upper 1m of the soil column (e.g., see Tarnocai, 2009)? Is there still high uncertainty and possibly much greater potential losses than current estimates? Tarnocai, C., J. G. Canadell, E. A. G. Schuur, P. Kuhry, G. Mazhitova, and S. Zimov. 2009. Soil organic carbon pools in the northern circumpolar permafrost region. Global Biogeochemical Cycles 23(2):n/a-n/a. DOI: 10.1029/2008GB003327.
438	P385/L38	S	Recommend revising “…is affecting coastal erosion” to “is increasing coastal erosion.”
439	P386/L4	S	Consider replacing thermohaline circulation with MOC or AMOC?
440	P386/L11	S	Mention uncertainty of impact on fresh water forcing on ocean circulation. See: Liu, W., S.-P. Xie, Z. Liu, and J. Zhu. 2017. Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate. Science Advances 3(1). DOI: 10.1126/sciadv.1601666.
441	P386/L17-35	R	Line 17 states high confidence, lines 31 and 35 seem to contradict this by stating very high confidence. Please reconcile
442	P386/L28	S	Recommend “fine spatial scale” rather and “fine regional scale.”
443	P389/Fig. 11.1	R	What are the thin green bars in the lower right inset? More significantly, comparing two individual years carries significant risks of cherry-picking. Recommend showing the classic September time series since 1979. This figure and Figure 11.2 are perhaps not the best choices for illustrating the key findings and main points of this section.
444	P389	V	As noted in Section II.3 for Figure ES.9/Figure 11.1, using a single year to compare with 2016 could be perceived as “cherry

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			picking” to maximize the difference. Perhaps better to use a 1980’s average.
445	P390/Fig. 11.2	R	Color scheme is strange, with no apparent logical progression. The positive scale, e.g., starts with increasingly dark shades of blue and then abruptly changes to greens. There does not seem to be any green on the map so perhaps this could be revised by simply eliminating the greens from the color bar.
446	P391/Fig. 11.3	S	This figure clearly shows that the coldest coastal soils are warming fastest, but it seems that what really matters is the increased area of permafrost at (or close to) 0ºC. If retained, recommend putting Centigrade scale on the right vertical axis.
447	P392/Fig. 11.4	S	The right y-axis should be explained (presumably it is for GRACE).
448	P392	S	Perhaps add an additional figure showing increased area of Greenland with net negative net mass?

12: SEA LEVEL RISE

#	page/line	V/R/S
449	P411/L26	R	Key Finding 4: “elevation thresholds” is ambiguous and should be defined.
450	P412/L7	S	Assessment of “change” is ambiguous. Consider replacing with “Assessment of vulnerability to rising sea levels….” Then, the last sentence could simply begin: “A risk-based perspective on sea-level rise points to the need for an emphasis on how changing….”
451	P412/L12	S	Consider rewording to read: “1) increased volume of seawater from thermal expansion of the ocean as it warms, and 2), increased mass of water in the ocean from melting ice in mountain glaciers and ice sheets…”
452	P412/L15	R	This is mildly esoteric and could be made more explicit by defining GRACE, and/or write “(altimeter and gravity measurements) and in situ water column measurements (Argo floats)…”
453	P412/L11-22	R	An important point to consider emphasizing is that in the last century, the largest contributor to SLR was thermal expansion, but now, “since 2005” loss of land ice has begun to take over.
454	P412/L27	S	When did this “weakening of the Gulf Stream” occur? Is this referring to the 2010 spike in sea level along the US. Northeast (NYC, Boston, etc.)? This could be articulated more clearly.
455	P412/L34	S	Perhaps this should read “...and the reduced gravitational attraction of the ocean toward the ice sheet”

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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456	P413/L1	S	“cores” may not be the best word choice here. Consider rewording to say “In areas once covered by the thickest parts of the great ice sheets of the Last Glacial Maximum…” and then, on line 3, replace “Slightly further away from the cores with “Along the flanks of the ice sheets, such as….”
457	P415/L5	R	As in the introduction, it may be simpler/clearer to keep everything focused on the 20^th century (1900) rather than post 1880. The implication that the rate of SLR was ~1.2 to 1.5 mm/yr during most of the last century, but is now twice that (~3mm/yr) is a critical point. The rate of SLR is accelerating and this should be emphasized strongly.
458	P415/L33	R	“heat storage” implies a total quantity of energy (Joules), not an energy flux (Wm^-2). Is this intended here (as in the rest of this section) to refer to, “rate of heat uptake by the ocean,” rather than “heat storage”? Clarify.
459	P416/L11-14	S	As worded, the sentence beginning, “On interannual scales, ENSO...” may appear to contradict itself to some readers. Consider rewording.
460	P416/L16	S	Consider avoiding the use of “stronger evidence.” “mounting evidence,” or “accumulating evidence” may be better choices.
461	P416/L19	S	Consider replacing “Input-output calculations” with “mass balance calculations.”
462	P416/L27	S	Add Wouters et al. (2015) to the list of references for ice mass loss in the Bellingshausen Sea region. Wouters, B., A. Martin-Español, V. Helm, T. Flament, J. M. Van Wessem, S. R. M. Ligtenberg, M. R. Van Den Broeke, and J. L. Bamber. 2015. Dynamic thinning of glaciers on the Southern Antarctic Peninsula. Science 348(6237):899-903. DOI: 10.1126/science.aaa5727.
463	P416/L23	S	Helm et al. (2014) adds support to mass gain in Dronning Maud Land. Helm, V., A. Humbert, and H. Miller. 2014. Elevation and elevation change of Greenland and Antarctica derived from CryoSat-2. Cryosphere 8(4):1539-1559. DOI: 10.5194/tc-8-1539-2014.
464	P417/L2	S	“Accelerating mass loss over the record….” What record? Consider clarifying that this is referring to Tedesco et al., 2013.
465	P417/15-16		Suggest that the authors clarify and expand on this important statement, because at least one reference cited here suggests an estimate lower than 250 cm.
466	P420/L24	S	Another paper worth citing to support the concept of a long-term sea-level “commitment” would be Golledge et al. (2015).

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			Golledge, N. R., D. E. Kowalewski, T. R. Naish, R. H. Levy, C. J. Fogwill, and E. G. W. Gasson. 2015. The multi-millennial Antarctic commitment to future sea-level rise. Nature 526(7573):421-425. DOI: 10.1038/nature15706.
467	P420/L24	R/S	Importantly, some of the “emerging science” discussed here (e.g., DeConto and Pollard, 2016), shows that the loss of marine-based ice is permanent on the timescales being considered here, because of the slow thermal recovery of the ocean. In other words, if lost, marine-based ice will not regrow until the oceans cool enough to allow the regrowth of buttressing ice shelves… which will take centuries to millennia. Consider including some discussion on this point.
468	P421-422	R/S	Regional Projections. The list (#1-6) is accurate, clear, and concise. However, it mostly emphasizes the gravitational fingerprint of ice sheet and glacier loss. It might be worth considering the addition of a bullet, regarding the expected distribution of near-term, stearic-driven sea level rise (which could especially impact U.S. interests in the western Pacific). See Figure 12.2.c. The impact of ocean dynamic effects on the U.S. Northeast Coast might be worth a bullet too, as it could be in the ~1 to >10 cm range by 2100. For example, see: Yin, J. 2012. Century to multi-century sea level rise projections from CMIP5 models. Geophysical Research Letters 39(17). DOI: 10.1029/2012GL052947. Yin, J., and P. B. Goddard. 2013. Oceanic control of sea level rise patterns along the East Coast of the United States. Geophysical Research Letters 40(20):5514-5520. DOI: 10.1002/2013GL057992.
469	P427/L17	V	Key Finding 2: The list of RCP’s appears to be backwards.
470	P430/L9	V/R	This should read: “… regarding the stability of marine-based ice in Antarctica.” Ice in both West and East Antarctic outlets and deep basins are vulnerable.
471	P430/L11-19	R	This would be a good place to reiterate the important point that most of North America will experience substantially more relative SLR from an equivalent loss of ice on Antarctica than from Greenland.
472	P430/L35	S	This statement could also be listed under Key Finding 2.
473	P432/L23	S	Check grammar: “to do so..”
474	P433/L19	S	Check grammar: “at specific locations”
475	P434/Fig. 12.1	S	Consider citing the original source of the GIA solution in panel e? This may be from Hay et al., 2015? There are two Kopp et al., 2015 references. Label 2015a and 2015b?

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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13: OCEAN CHANGES: WARMING, STRATIFICATION, CIRCULATION, ACIDIFICATION, AND DEOXYGENATION

#	page/line	V/R/S
476	General	R	A few examples of words that should be defined are: autotrophic, saturation with respect to aragonite, bathyal
477	P452/L30	V	. . . global average surface ocean acidity . . .
478	P452/L30	V	A definition of “global ocean acidity” is needed.
479	P453/L18-21	R	Recommend using “increases” instead of “changes”? All of the effects outlined in this sentence are in a single direction, implying that only one direction of changes in stratification can be responsible. Otherwise an equally valid reading of the sentence is that decreases in stratification would also have the same effects.
480	P454/L14	V	The full citation to “Rykaczewski et al., 2015” is not included in the References section
481	P454/L16-18	S	Converting these changes to degrees/decade would allow more direct comparisons over the different time periods, although trends over such a short interval as 1982-2006 have generally poor signal-to-noise ratio. “deeper waters” suggests that these are measurements over some depth, so either this should be specified, or if SST is meant then it should be stated.
482	P454/L22	R	Have glaciers also thinned, in addition to melting at “their fringes”?
483	P455/L1-14	S	These paragraphs stray from the topic of this section, viz., warming, stratification, and circulation changes.
484	P455/L12-14	V	This assertion needs a citation.
485	P456/L6	V	Delete “more”
486	P456/L6-13	R	This paragraph contains long, complex sentences and should be revised to improve flow.
487	P456/L8-13	R	Not clear what is meant by “rate of acidification”; what is typically observed is that the changes in partial pressure of CO₂ in the surface ocean tracks those of the atmosphere on a seasonally-average basis, but with a geographically varying “disequilibrium.”
488	P456/L36-P457/L11	R	This paragraph is confusing to read, and would benefit from careful editing/rewriting.
489	P456/L36	R	Not clear what is meant by “less buffered against pH change.”
490	P457/L7-8	R	What is the difference between “sensitivity to ocean acidification” and “lower buffering capacity”? The use of the word “sensitivity” seems more appropriate to organisms or ecosystems than to seawater.

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
491	P457/L15	V	Specify whether the CO₂ increase referred to here is in the ocean or in the atmosphere.
492	P457/23-25	S	This final sentence doesn’t seem to fit here.
493	P457/L36	R	The word “tremendous” should probably be omitted, it is not clear at what point “pressure” would rise to “tremendous pressure,” and unless the cited reference addresses this, it seems overstated.
494	P457/L15	V	Clarify whether this is oceanic or atmospheric p(CO₂).
495	P457/L20	R	Recommend using Gt instead of Pg for greater familiarity with the wider scientific and policy community.
496	P458/L20-21	R	This sentence is ambiguous—is the driver “CO₂ emissions” intended to refer to “climate-induced” (as above)? Increased discussion of “anthropogenic nutrient input” as a driver for ocean deoxygenation would be beneficial.
497	P458/L23	S	Anaerobic respiration is of course possible too. Clarify.
498	P459/L6-9	V	Has this been shown? If noting this, also need to state that plant WUE also increases with climate (CO₂) change (discussed on page 341).
499	P459/L6-7	R	Warming on land increases plant WUE (see Chapter 10)—but this is far from the only hydrologic effect of warming. Changing seasonality and increased ET are bigger effects. Clarify (quantify?) how these processes play a role in increasing nutrient transport to the coastal ocean.
500	P459/L22-23	S	It seems likely that the rates of net loss of wetlands are too small to be a factor?
501	P459/L35	V	Should this be “nitrite” here, or “nitrate”?
502	P459/L38	R	Recommend providing the comparison of the rates of N₂O production through this mechanism and terrestrial anthropogenic production
503	P462/L16	V	The full citation to “Rahmstorf et al., 2015” is not included in the References section.
504	P463/L20	V	This web-site for CDIAC ocean data is in the process of being subsumed into NOAA, and may be unavailable soon. Recommend additional citations, if possible.
505	P463/L26	V	Do these citations really claim increases in upwelling? The cited Feely et al. (2008) is based on a single cruise and Harris et al. (2013) on a 5-y time series. Suggest inclusion of additional citations if available.
506	P463/L32	V	Should be “were” not “where”
507	P463/L30-32	V	Minor revisions: “remain”…”yr^-1”…”were”.
508	P465/L8	R	It is not clear what “naturally corrosive materials” might be

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			present as “riverine loads.” Is this intended to describe the CO₂ composition of the rivers (and how they differ from the ocean)? Please clarify.
509	P466/L5	V	Might be clearer if the amount 6 Sverdrups were parenthetically equivalenced to 6 x 10^⁶ m^³/s, rather than simply an equivalence for a single Sverdrup.
510	P466/L6	V	Is the change of 100% to 150% from present day? Clarify.

14: PERSPECTIVES ON CLIMATE CHANGE MITIGATION

#	page/line	V/R/S
511	P481/L10-18	V	This finding misses the key point that, independent of the warming target, stabilizing warming requires that CO₂ emissions go to zero.
512	P482/L7-8	R	Statement would benefit from clarification that the Paris goal is not exactly the same as 2°C or 1.5°C, it is “Holding the increase in the global average temperature to well below 2°C above preindustrial levels and to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels.”
513	P482/L28-37	V	Distinguishing between committed warming and committed emissions is important here. The different scenarios diverge slowly first because the problem is intrinsically one of cumulative emissions and second because, in the near term, annual emissions on trajectories of ambitious mitigation and continued high emissions are similar and diverge only through time.
514	P483/L1-15	V	The story in this paragraph is somewhat oversimplified. The key points that should be addressed are that (1) some SLCPs are coupled to CO₂, (2) some SLCPs are coupled to economic development, (3) some SLCPs can be tackled independent of CO₂, and (4) because SLCPs are short-lived, they can intrinsically be tackled any time (long-term climate changes are largely indifferent to cumulative SLCP emissions).
515	P483/L17-22	R	The framing of this paragraph is more appropriate for a key finding than the framing of the current Key Finding 2.
516	P483/L23-34	V	It is important to state the underlying probability when discussing allowable emissions for a target. The current wording could imply 100% confidence in staying below the target, when the numbers seem to be based on the “likely” range.
517	P483/L23-34	V	It is misleading to start by providing a CO₂ budget with no mention of other GHGs. This section could be improved by first introducing a budget based on a reasonable (and explicit)

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			projection of non-CO₂ GHGs and then potentially mentioning that the budget would be bigger if emissions of non-CO₂ GHGs were smaller.
518	P483/L24	R	Clarification on whether quantities are presented in units of C or CO₂ is needed. While it is clear that units of C are better aligned with the physical and biological processes, the emphasis in the policy world on emissions in terms of CO₂ equivalents provides a strong motivation for converting everything in units of CO₂.
519	P484/L3-8	R	Any conclusion about untapped reserves of oil, gas, and coal depends strongly on weak assumptions about future relative preferences for the three fossil fuels. From a climate or a health perspective, it would make a lot more sense to think about utilizing more of the gas and less of the coal. Suggest including a caveat about uncertainties in future consumption patterns.
520	P484/L32	V	It is a little misleading to say that the concept of balance between sources and sinks in the Paris Agreement implies that CO₂ emissions need to drop to zero. The definition of a range of warming targets (any warming target) implies that CO₂ emissions need to fall to zero. The concept of balance in the Paris Agreement is an acknowledgement of this.
521	P484/L32-34	R	It should be noted that the ocean plays an important role in the C cycle and acts as a C sink. Marine ecosystems and species in the open ocean and deep sea, play a significant role in absorbing, moving, and storing carbon but are currently not considered or suited to be part of UNFCCC accounting mechanisms. Ignoring the ocean in mitigation strategies can create additional problems and/or acceleration of changes in internal dynamics of the coupled atmosphere-ocean system.
522	P485/L3-14	V	Discussion of allowable emissions budgets needs to be accompanied by a clear presentation of the associated probabilities of staying under the temperature targets.
523	P485/L15-24	V	It is important to make the point that none of the trajectories has a high probability of limiting warming to 2C or less.
524	P486/L1-7	S	Examples that demonstrate policy interactions that can enhance or degrade other efforts would be useful here.
525	P486/L9-15	V	Need probabilities and CO₂ units.
526	P486/L21-31	R	It would be very useful to add a comment about the magnitude of the projected removals in comparison to current emissions. Without such a comparison, it is hard to get a sense of the truly vast scale of the removals in the integrated assessment models.
527	P487/L3	V	One of the main conclusions form the IPCC AR5 (2013) is that adapting to a world with warming much greater than 2^oC is unlikely to be possible. It is important to avoid constructions that imply the opposite.
528	P487/L21-	R	This sentence requires clarification. Particularly effective in

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
	12		comparison to what? If the idea is that smokestack capture looks more feasible than direct air capture, it would be good to say this.
529	P487/L32	S	“Leading” is too normative in this context.
530	P488/L8-17	S	It could be misleading to start discussing technical feasibility of solar radiation management before introducing the challenges of governance. If the point is that the technical issues are unlikely to be the main constraint, this should be stated more clearly.
531	P488/L32-P489/L3	R	Here as elsewhere in the report, this chapter would benefit from greater discussion of coupled system responses. An atmosphere and surface focus can have serious implications for atmosphere-ocean coupling, troposphere-stratosphere exchange and the changes that would incur in the earth system response.
532	P489/L4-15	R	It is worth mentioning that the quantity of available literature and analysis of all of the climate intervention options is a tiny fraction of that on climate change. Just as progress on climate change requires extensive science, so will balanced consideration of climate intervention.
533	P489/L16-25	R	Recommend using this paragraph as the introduction to climate intervention, not the concluding one.
534	P490/L9-13	R	This description of the available evidence misses the importance of cumulative emissions.
535	P492/L5-11	R	The comment about required emissions reductions even for stabilizing at less than 4^oC is important, but it is incorrect as stated. The essence of a cumulative emissions budget is that CO₂ emissions need to go to zero. The conclusion that the required reductions are smaller for a higher target is only temporarily correct. In general, it is important to make sure that readers are aware of the distortions that arise from acting as if we care about this issue only through December 31, 2099.

15: POTENTIAL SURPRISES: COMPOUND EXTREMES AND TIPPING ELEMENTS

#	page/line	V/R/S
536	P500/L15	R	Typo: “… than can BE well quantified”
537	P500/L16-17	R	The terminology “correlation of extremes” then “changing correlations” (used later in chapter including page 501, lines 5-6), then “compounded extremes” (section 15.3) is confusing. “Compound extremes” makes much more sense given the examples that are showcased.
538	P500/L19	R	The notion that models tend to error on the “underestimate” side is also well documented/supported in the paleoclimate literature. This is mentioned near the end of the chapter but could be

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			mentioned earlier.
539	P500/L28-33	R	Recommend revising the framing of these sentences in terms of earth system models. While earth system models are increasingly becoming more complete, they do not include or fully represent all known processes of a fully coupled planetary system. Noting that even if these models were complete, this is a complexity problem and all complex systems inherently have the element of surprise would benefit the message of this section.
540	P501/L14	V	Add the meridional overturning circulation to the list… perhaps replacing the ENSO example.
541	P501/19-23	R	These sentences are inconsistent. In one sentence the discussion is limited to the instruments observation record—why? In the next paragraph the reference is to observational record not just instrumental record.
542	P501/L26-31	R	Is this basically curve-fitting and extrapolation? An even greater weakness is that they also assume stationarity.
543	P502/L1-15	R	If there are land processes incorporated including vegetation dynamics, why aren’t these models earth system models with bio-physical processes? Another feedback that the models do not include is the ocean-ice dynamics coupled system in the Arctic Greenland ice sheet. Recommend mentioning these limitations of the models.
544	P503/L17-18	R	Clarify whether this analysis looked at univariate or coincident occurrence?
545	P503/L24	R	The reference to Chapter 11 implies that the Fort McMurray fire was covered there but it was not explicitly mentioned nor quantified.
546	P503/L33-38	R	The example of compounded droughts is a good opportunity to mention the issue of non-resilient human communities.
547	P503/L35-38	R	The reference to limited resolution and increase in frequency of events ignores the possibility of inadequate incorporation of processes in the models that would produce the compounded events.
548	P504/L37	R	A better/additional reference to the warming hole would be: Drijfhout, S., G. J. van Oldenborgh, and A. Cimatoribus. 2012. Is a Decline of AMOC Causing the Warming Hole above the North Atlantic in Observed and Modeled Warming Patterns? Journal of Climate 25(24):8373-8379. DOI: 10.1175/jcli-d-12-00490.1.
549	P505/L3-5	R	This should also be stated in Chapter 12 on SLR and currently is not.
550	P505/L16-27	V	An example of something like a tipping point was the accelerated loss of Arctic sea ice about 10 years ago and the

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			models did not predict it. Consider using here an example?
551	P505/L16-30	V	Loss of Arctic sea ice may also accelerate the loss of Greenland land ice. For example: Koenig, S. J., R. M. DeConto, and D. Pollard. 2014. Impact of reduced Arctic sea ice on Greenland ice sheet variability in a warmer than present climate. Geophysical Research Letters 41(11):3933-3942. DOI: 10.1002/2014gl059770.
552	P505/L32-33	V	This is an inaccurate representation of the findings of the cited Schuur et al. (2016) paper. While the quantity of C stored in permafrost soils is estimated at 1300-1600 Gt C, the paper indicates that only 5-15% is vulnerable to being released this century (although there is uncertainty). Therefore, it is very unlikely all this C would be released, as is suggested by the language in this sentence.
553	P506/L3-5	R	Refer back to the passage in Chapters 11 and 13 on hydrates (11.3.3 and 13.3.2).
554	P506/L21-24	R	Would add that it also depends on ice-ocean dynamics.
555	P506/L21	V	This sentence is misleading. Greenland responds “relatively slowly,” but Antarctica is different, because so much ice rests on bedrock far below sea level.
556	P506/L24	V	Robinson et al., 2012 report that even with an imposed 8ºC of warming, it takes ~1500 years for Greenland to loose ~85% of its ice. Recommend using the word “millennia” and not “centuries” for Greenland.
557	P506/L28-29	V	It is extremely important to bound this statement with rough timescales (centuries? millennia?)
558	P506/L29	V	This should read “… involving ocean-ice sheet-bedrock interactions.” Marine ice sheet instability works in places where the ice-sheet bed slopes downward toward the continent.
559	P507/L7-11	V	This section is terrifying, though understated. At some point between present conditions and dramatically more CO₂, the planet does something completely different—and our climate models are missing whatever processes lead to that different state. This means that we cannot estimate at what point in the future we might activate those unknown processes. In contrast, the language in the executive summary is soothing. Suggest being consistent in how this issue is discussed
560	P507/L25	R	Some estimates of Pliocene sea level are 10-30m higher than today, requiring a substantial contribution to sea level from Antarctica. This also implies substantial polar amplification in both hemispheres (not just the Arctic) and extreme ice sheet sensitivity to modest warming.
561	P507/L34	R	Note that the referenced Huber and Caballero, 2011 paper

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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#	page/line	V/R/S
			reported 16xCO₂ to reproduce polar warmth in line with climate proxies.
562	P511/L6	R	Typo: “… than can BE well quantified”
563	P511/L19-21	V	Why is there no discussion of the known unknowns in science? Isn’t the lack of this knowledge also a threat to our understanding of tipping points? Models do not yet incorporate all processes and coupling and there are known earth system science gaps that require attention.

SUGGESTED GLOSSARY TERMS

Aerosol-cloud interactions	Aerosol-radiation interactions	Agricultural drought
Albedo	Anticyclonic circulations	Atlantic meridional overturning circulation
Atmospheric blocking	Atmospheric river	Baroclinicity
Bias correction	Carbon dioxide removal	Climate intervention
climate sensitivity	CMIPs (general description)	CO₂ equivalent
CO₂ fertilization	Cryosphere	Denitrification
Deoxygenation	Dynamical downscaling	Earth system models
Effective radiative forcing	Empirical statistical downscaling models	Eutrophication
Extratropical cyclone	Geoengineering	Global temperature potential
Global warming potential	Hydrological drought	Hypercapnia
Hypoxia	Ice wedge	Instantaneous radiative forcing
Intended nationally determined contributions (INDCs)	IPCC	Long wave cloud radiative effect
Meridional temperature	Meteorological drought	Mode water
Model ability/model skill	Model bias	Model ensemble
Model independence	Model uncertainty	Negative feedback
Nitrogen mineralization	Ocean acidification	Ocean stratification
Oxygen minimum zones	Parameterization	Parametric uncertainty

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Suggested Citation:"Appendix A: Line Comments." National Academies of Sciences, Engineering, and Medicine. 2017. Review of the Draft Climate Science Special Report. Washington, DC: The National Academies Press. doi: 10.17226/24712.

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Paris Agreement	Pattern scaling	Perfect storms
Permafrost	Permafrost active layer	Petagram
Positive feedback	Proxies	Radiative forcing
Relative sea level	Representative concentration pathways	Rossby waves
Saffir-Simpson storms	Scenarios	Sea level pressure
Shared socioeconomic pathways	Shortwave cloud radiative effect	Snow water equivalent
Solar radiation management	Special Report on Emissions Scenarios	Structural uncertainty
Teleconnections	Thermohaline circulation	Thermokarst
Tipping elements	Tipping points	Transient climate response
Tropopause	Undersaturation (vs. saturation)	Urban heat island
Zonal mean