Chapter 2: The North American Carbon Budget: Past, Present, and Future

Overview/Main Issues

This chapter nicely summarizes and synthesizes the latest scientific information on the North American carbon budget by incorporating terrestrial, anthropogenic, aquatic, and coastal margin CO₂ and CH₄ dynamics. Starting with a historical context, the chapter summarizes current understanding of the magnitudes and trends of carbon stocks and fluxes at the continental scale. It also provides a regional context by stratifying the continent to countries and climate assessment regions and discusses the societal drivers, impacts, and carbon management decisions. Knowledge gaps and research needs are also identified. This chapter is well-written and clearly organized, and provides a broad context beyond individual chapters. Some of the main ways the chapter can be improved include the following:

• some work is needed on the Key Findings (discussed below);
• the goals and objectives should be explicitly described;
• critical content areas missing from the chapter are interannual variability of carbon fluxes and impacts of severe and extended droughts;
• indicators and feedbacks are missing from Section 2.4;
• consistent use of units is recommended;
• numbers with 3-4 significant digits over-state the confidence the reader should have, and all numbers should include uncertainties.

And one broader concern to note: This chapter follows the global overview in Chapter 1, where “sinks” are sinks in the cycle perturbed by anthropogenic CO₂ and CH₄, and the assumption is that globally, the net unperturbed background sinks are zero summed across all reservoirs. Yet in this chapter, “sinks” are net fluxes out of the atmosphere, background + perturbation. For the coastal ocean, inland waters, etc.—where lateral transport is significant—these sources and sinks include background/pre-industrial fluxes that are balanced by fluxes elsewhere. These distinctions must be made clear so that the reader is not given an impression of a greater or lesser sink for anthropogenic CO₂ than is there (e.g., P74, lines 4-6).

Statement of Task Questions

• Are the goals, objectives and intended audience of the product clearly described in the document? Does the report meet its stated goals?

The goals and objectives of this chapter are described in an implicit way at the end of the Introduction section. It would be better to rephrase that paragraph to clearly outline the goals and specific objectives. The intended audience is not clearly described. But this is perhaps something that only needs to be described in the Executive Summary. The report meets its stated goals to a large extent, with one exception—indicators and feedbacks are not clearly described.

• Does the report accurately reflect the scientific literature? Are there any critical content areas missing from the report?

The report accurately reflects the scientific literature to a large extent. For part of the Introduction and the Historical Context section, it is appropriate to cite some older but classical studies (e.g., Caspersen et al., 2000; Goodale et al., 2002, etc) for the historical context of the North American carbon cycle studies. One more recent study could be useful here is Zhang et al. (2012), which shows that on average the carbon sink in the conterminous U.S. forests from 1950 to 2010 was Tg C/yr with 87% of the sink in living biomass.

Two critical content areas missing from Chapter 2 are the interannual variability of the terrestrial carbon sink and the impacts of drought. It would be better to have a section to address the interannual variability of carbon fluxes. Besides disturbances, drought has significant impacts on ecosystem carbon uptake and has been studied extensively using modeling and upscaling methods in North America.

Another critical content area missing is indicators and feedbacks. The title of Section 2.4 is “Indicators, Trends, and Feedbacks”, but the section does not really touch on indicators or feedbacks. These two components should be added or strengthened; alternatively, the title should be changed to “Trends”.

Discussion of the methane budget should be expanded and updated in a few ways:

• Throughout the chapter, “carbon” refers to CO₂ and not methane. The chapter should mention the difficulty of unravelling methane sources and sinks, given that there are so many sources.
• The chapter should include references for fossil fuel methane emissions: e.g., Hendrick et al. (2016), Jacob et al. (2016), Kort et al. (2014), Turner et al. (2016). The chapter should also mention that there is not agreement about whether U.S. methane emissions or methane sinks are under-estimated or not; e.g., see Bruhwiler et al. (2017), Miller et al. (2013), Turner et al. (2015), and Wecht et al. (2014).
• Table 2.1 of the 2017 EPA report (EPA 430-P-17-001) shows 2015 has lower total emissions than 2005. The upticks in “natural gas systems” and “manure management” are nearly cancelled by downturns in emission from landfills. This has important information for managing the methane budget. Where possible, the numbers and conclusions should be consistent with this EPA report; departures from and updates of that report should be highlighted.

Coastal oceans were reported as a carbon source in SOCCR1 but as a carbon sink in SOCCR2. This can perhaps be considered as a key finding. In addition, it would be useful to have a key finding on CH₄ budget.

• Are the findings documented in a consistent, transparent and credible way?

The way that Key Finding 5 is documented needs a bit of work. “Significantly” is typically associated with a statistical test. Is the average carbon sink estimated by top-down approaches not statistically higher than that by bottom-up estimates? If a test couldn’t be done, “significantly” should be replaced with something like “quite” or “substantially”.

• Are the report’s key messages and graphics clear and appropriate? Specifically, do they reflect supporting evidence, include an assessment of likelihood, and communicate effectively?

Key Finding 1 is undoubtedly true but this has been known for a long time, thus it may be unnecessary as a key finding. It includes a new estimate (1,032 Tg C per year), but this number is already reflected in Key Findings 2 and 3. Note also this should say “net source of CO₂” (not carbon).

Key Finding 4 should reflect the fact that there is significant interannual variability in the carbon sink. Also it is unclear what “natural terrestrial carbon sink” actually refers to. Does this include natural ecosystems such as forests and grasslands? Are agricultural soils included? Even forests/grasslands are managed to a large extent. Does this include wood products, land use changes, etc., which may be considered anthropogenic rather than natural? The authors should either specify what is included (in parenthesis following “natural terrestrial carbon sink”) or slightly rephrase the term as something like “land/ecosystem sink”. This also applies to Key Finding 3.

Key Finding 4 and 5 should include quantitative estimates and uncertainties. No need to emphasize the approach (top-down, bottom-up), just focus on the actual outcomes.

In the regional context, it would be good to add a figure to quantitatively illustrate how much the size of the carbon sink is in each national climate assessment region, how much fossil fuel emissions are released in each region, and what percentage of the emissions is offset in each region.

• Are the research needs identified in the report appropriate?

Research needs could include how to better integrate modeling approaches with observations and how to reduce the uncertainty in carbon sink/source estimates.

• Are the data and analyses handled in a competent manner? Are statistical methods applied appropriately?

As mentioned above, the authors should test whether the mean of the carbon sink estimates from top-down approaches is not significantly higher than that of the sink estimates from the bottom-up methods. Otherwise, the statement should be rephrased accordingly. In addition, both Tg C and Pg C are used for carbon fluxes (larger than 1,000 Tg C or 1 Pg C). It would be better to use the same units (Tg C) throughout the chapter. In addition, both Tg C per year (p. 80, line 11) and Tg CH₄/yr (Figure 2.4) were used. While this is commonly done in the scientific literature, it may be helpful to include in parenthesis CO₂-equivalent for both CO₂ and CH₄.

• Are the document’s presentation, level of technicality, and organization effective?

There are repetitions or mistakes in sentences. For example, the statement on p. 82, Lines 14-16 is essentially the same as the first two sentences of the following paragraph. These two paragraphs should be combined and modified. In addition, Key Finding 1 is exactly the same as Key Finding 5 and should be corrected.

This chapter can be difficult to read at times, with awkward use of language in many places (e.g. Key Finding 2: “a level of magnitude”). The chapter would benefit from editing.

• What other significant improvements, if any, might be made in the document?

• For Section 2.4, the first two paragraphs would fit better into Section 2.3.
• Section 2.4 should also discuss the interannual variability of carbon fluxes besides indicators and feedbacks.
• Section 2.5.2 should quantitatively describe the regional carbon sinks, fossil fuel emissions, and the percentage of fossil fuels emissions are offset by ecosystem carbon uptake.

• The discussion throughout the chapter on “top-down” and “bottom-up” is not informative to the general readership of the report, and could be shortened considerably.
• The authors are advised to carefully check the “Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2015”¹, and see whether the estimates of emissions and sinks should be compared against SOCCR2 estimates and whether there are nice figures that can be used in SOCCR2.

Line-Specific Comments

P71, Line 21-25

Why not use Pg C here, as is used elsewhere (e.g., p.73, Line 41). Having more consistency in units will make the report less confusing to the audience.

P71, Line 30-33

This key finding ignores the previous research findings showing interannual variability in ecosystem carbon fluxes caused by drought and disturbances.

P72, Line 5

Change “3 centuries” to “three centuries”.

P72, Line 9-10

This statement needs to be rephrased. Continental carbon sources are only partly offset by sinks from natural and managed ecosystems.

P72, Line 21

The focus here should not be North American carbon balance, but North American carbon sink, or the carbon sequestration capacity of North American ecosystems.

P72, Line 23-25

Since this statement is put in the historical context, it is better to cite some older and classical studies (e.g., Caspersen et al., 2000; Goodale et al., 2002).

P72, Line 38

The authors should mention that atmosphere-based estimates provide limited information on the exact location of carbon sinks/sources.

P74, Line 6

Change “50%” to a specific number (in units of teragrams).

P74, Line 9

The phrase “be of sufficient magnitude” is redundant and can be removed.

P75, Line 29-42

This paragraph is somewhat disconnected. The first two sentences are on the missing components of synthesis efforts, while the following sentences are on inventories or methane. Is methane one of missing components? This paragraph should be modified to improve logical flow.

___________________

¹ EPA 430-P-17-00: https://www.epa.gov/sites/production/files/2017-02/documents/2017_complete_report.pdf

P76, Line 18-21

It would be better to briefly summarize how the estimates across various scaling approaches were reconciled.

P76, Line 22

It is better to switch “fluxes” and “stocks” in the section title because the first paragraph is on stocks, not on fluxes.

P76, Line 41

“There is very high confidence that” can be removed.

P77, Line 32-33

What is the difference between “uncertainty and “error”? It would be good to make this clear.

P78, Line 20

Starting the paragraph with “Figure 2.3” makes the paragraph look like a figure caption. (May be confusing to readers).

P78, Line 27

“-274 Tg C per year” should be changed to “274 Tg C per year”.

P80, Line 25 – P81, Line 24

These two paragraphs are not on indicators, trends, or feedbacks (except the last couple of sentences of the latter paragraph). Perhaps integrate these paragraphs into Section 2.3?

P80, Line 33-35

This is actually something new and could be considered a key finding of the chapter.

P81, Line 17-19

The so-called “browning” and “greening” trends should not be interpreted as evidence for vegetation carbon gains or losses. These trends are based on the normalized difference vegetation index (NDVI). They can indicate trends in photosynthetic activity or gross primary productivity, but not net carbon uptake.

P84, Line 2-6

Surprisingly, drought was not mentioned here or anywhere else in the chapter.

P86, Line 20

The authors should provide a few examples of observational networks, particularly those that have emerged since SOCCR1.

P86, Line 21-29

Perhaps provide an example about how interoperability can benefit carbon management decisions?

P86, Line 34

Why is “very likely” used here? This should be definitely true.

P87, Line 20-21

This statement should be reflected in Key Finding 4.

P88, Line 10

“land and water fluxes” should be changed to something like “carbon fluxes from land and water” to avoid confusion. Water fluxes are generally used to indicate evapotranspiration, river discharge, etc.

P88, Line 15-19

These sentences clearly indicate that there is large interannual variability in ecosystem carbon fluxes, which contradicts Key Finding 4 (that the magnitude of the terrestrial carbon sink has persisted at a similar magnitude over time). The interannual variability in ecosystem carbon fluxes is overlooked in this chapter and should be highlighted. Upscaling of flux observations and biosphere modeling studies also show significant interannual variability.

P90, Line3-5

Key Finding 1 here is exactly the same as Key Finding 5.

P103, Figure 2.1.

Figure Legend: Spell out NCA; change “Forest” to “Managed Forest”?; change “Other Land” to “Other Land Ecosystems”?

P104, Figure 2.2.

The graphics can be improved. Change left vertical axis label to “Fossil fuel emissions (PgC)”.

P105, Figure 2.3.

Figure caption: The numbers for net carbon uptake (top to bottom) are written in the opposite direction to those for carbon release (bottom to top), which makes the figure a hard to read. Why not show all these numbers in the same direction (from bottom to top)?

P106, Figure 2.4.

Figure caption: change “million tons” to “Tg”

Wetland CH₄ emission of 47 Tg is much higher than the value in Chapter 13 (18 Tg CH₄/yr) and the Executive Summary (21 Tg CH₄/yr).

P109, Table 2.2.

In the heading, change “central estimates” to “Median Estimates” (or some other appropriate description).