This chapter summarizes the current knowledge in high-latitude (mostly permafrost) carbon storage and dynamics. Arctic and boreal regions contain large carbon stock, especially in permafrost soils. The factors that control carbon storage have been changing rapidly over the last several decades. As a result, this large carbon pool is highly vulnerable for carbon loss in a future warming climate. There are major needs to reconcile model and observations in assessing permafrost carbon balance and in understanding the importance of abrupt thaw of permafrost.
The authors have done a commendable job in providing an updated synthesis of data and knowledge in high-latitude/permafrost carbon dynamics. It is very well written in general. The chapter provides a clear circumpolar/global perspective to provide context for the discussion of North American carbon cycle. The Committee makes the following suggestions to help improve the chapter.
- There is some discussion of long-term carbon accumulation processes (in 11.3.3) and discussion of projected future change in the year 2300 (in 11.4.2), but further discussions of longer-term past and future perspectives would be useful. For example, the permafrost carbon pool has been accumulating and has been relatively stable over the last several thousand years at least, but recent abrupt changes in controlling factors (warming, disturbances) may cause instability and degradation that the system hasn’t experienced in thousands of years. See wording suggestions on subsection 11.3.3 and key findings below.
- The key uncertainties are clearly documented, but progress could be enhanced by ranking which research questions are most amenable to solution in the near term. It would also be useful to discuss ongoing research campaigns (i.e., ABoVE7) that may contribute to progress, and to provide clearer guidance to federal agencies about where they can most effectively use their resources.
- The writing and organization are clear in general, especially in the early part of the chapter. However, it is uneven in the late half of the chapter when discussing carbon fluxes, suggesting a lack of adequate editing on subsections written by different lead authors. Note that subsection 11.5 is missing. Compared to subsections 11.3.1 and 11.3.2 on soil and vegetation carbon pools, subsection 11.3.3 on carbon pool change lacks detail. To maintain balance with the other two subsections, the authors could expand this subsection into multiple paragraphs, rather than just one densely-packed paragraph. It may be useful to state this is an overview of natural drivers affecting permafrost carbon pool, to distinguish from discussions on carbon fluxes in subsection 11.4.
- The chapter only cites Harden et al. (1992) on the subject of long-term accumulation histories, but there are more recent publications on this subject that could be referred to, at least for permafrost peatlands (e.g., Loisel et al., 2014).
7 The Arctic-Boreal Vulnerability Experiment, a NASA–led field campaign.
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 the chapter are clearly stated in the first paragraph, and the chapter meets the stated goals.
- Does the report accurately reflect the scientific literature? Are there any critical content areas missing from the report?
Yes, the chapter’s content and key findings accurately reflect the scientific literature. Some suggested references are included in the line comments.
- Are the findings documented in a consistent, transparent and credible way?
Yes, the key findings are presented in a consistent, transparent and credible way.
- 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?
In general, the chapter key messages and graphics are clear, but see comments below on figures. The key messages reflect supporting evidence, with appropriate assessment of likelihood. The figures/tables are effective in communicating the messages. However, the graphics and resolution of Figure 11.1 needs improvement. Other Figure suggestions are noted below.
- Are the research needs identified in the report appropriate?
The research needs are not stated explicitly as part of Key Findings, but are discussed at the end of the chapter (11.7). In particular, the chapter identifies the importance of: (1) reconciling model and observation difference in Artic vegetation greening and soil carbon stock change, and (2) emerging research on disturbance of permafrost soils by abrupt thaw. Timescales appear to be a major factor in discussing vegetation and soils carbon pool change and carbon sequestration. The point that vegetation greening and shrub expansion may have limited or no impact on long-term soil carbon sequestration could be made clearer. Identifying and modeling processes that control long-term carbon balance seem to be important research needs. “Abrupt thaw” is an important disturbance event that causes instability for a system that has been stable over thousands of years.
- Are the data and analyses handled in a competent manner? Are statistical methods applied appropriately?
Yes. The results and findings are all from the peer-reviewed literature.
- Are the document’s presentation, level of technicality, and organization effective? Are the questions outlined in the prospectus addressed and communicated in a manner that is appropriate and accessible for the intended audience?
In general this is a well-presented chapter.
- Are the key findings in your chapter well stated and supported by the detail provided in the chapter?
Comment on Key Finding 2. Timescales are probably an important parameter to differentiate permafrost carbon pool from forest or other biome biomass carbon pools (or even mineral soils carbon pool). This chapter is a place to put the contemporary carbon cycle in the context of long-term geological carbon cycle dynamics. Indicating the long-term accumulation of permafrost carbon helps put carbon release through recent and future disturbance, especially related to abrupt thaw, into a broader context. The Committee suggests augmenting this finding with a statement such as: “This large soil carbon pool in the permafrost zone has accumulated over hundreds of thousands of years.”
P413, Line 24
Change “145” to “146” to be consistent to 10% of 1460 GtC.
P414, Line 19
Change “Arctic ecosystems” to “the Arctic”.
P415, Line 3
Citations can perhaps be changed to “Romanovsky et al., 2010, 2016”, without repeating the author names. (A similar formatting change could be made throughout the chapter).
P415, Line 7
It is a bit redundant to say both “the Arctic” and “high Arctic”. Rephrase.
P417, Line 29
Perhaps change the subheading to “Characteristics of Permafrost Carbon”?
P418, Line 24
Change “Ice covers” to “Ice sheets cover”.
P419, Line 4
Switch order of fluxes and stocks by changing to “Current Understanding of Carbon Stocks and Fluxes”, as this is the order of description below. Also, why use different terms “stocks” and “pools” here?
P419, Line 7
Change to “peatlands (>20% carbon)”… and “mineral soils (<20% carbon)”, as organic matter rarely contains much more than 50% carbon and it is redundant and inprecise to indicate <1% for mineral soils.
P419, Line 14
Change to “soils of many meters thick”.
P420, Line 16-17
Change “sea levels were” to “sea level was”.
P422, Line 5-6
The values seem to be inconsistent, as 15.07 PgC may only refer to boreal biome, and tundra vegetation contains another 1.36 PgC (as in Table 11.2) that is not included here.
P422, Line 15
Change heading to “Natural Drivers of Carbon Pool Change” to distinguish from subsection 11.4.
P422, Line 15-40
This paragraph is weak, especially compared to previous two subsections 11.3.1 and 11.3.2. This could be expanded by discussing long-term historical drivers. Focusing on just the past few centuries and millennia is likely too short, as there are many recent synthesis of well-dated peatland records showing that these ecosystems have accumulated carbon over >10,000 years (e.g., Loisel et al., 2014; Treat et al., 2016). See general comments above.
P423, Line 2
Change to “11.4.1 Carbon Fluxes in Recent Decades”?
P423, Line 15-28
May need more discussion of boreal forest in this section, to balance the focus on tundra. Also, perhaps cite some more recent references after the 2012 synthesis, such as Euskirchen et al., 2017.
P424, Line 5-10
Perhaps move this to section on future projections?
P424, Line 18
Delete “soil area”.
P427, Line 29
Need more discussion on peatland fires (e.g. drawing upon Turetsky et al. publications).
P427, Line 31-34
There are no discussions of insect outbreaks in the chapter elsewhere. Also, provide an overview of the three approaches used for future projections at the end of this paragraph.
P429, Line 41
This subsection is weak, except perhaps the first paragraph, it is not really focused on upscaling.
P430, Line 6-8
The sentence is unclear.
P430, Line 12-18
Is that more suitable for the overview?
P430, Line 31
Missing subsection 11.5, between 11.4 and 11.6. Reorganization is needed.
P430, Line 32
Should this be indicated as a Case Study in the subsection heading?
P431, Line 15
Change to “Observations and modeling results summarized in this chapter”.
P433, Line 27-35
Two different statements “high confidence” and “very high confidence”. Need more consistency.
P434, Line 10
Gorham (1991) may not be a correct reference for this statement. Please check.
P435, Line 9
Change “Observational data” to “Experimental data”.
P449, Line 4
Change to “Tundra area data”.
P452, Figure 11.3
The Y-axis labels can benefit from adding “/year” on both sides.
P453, Line 5-6
The sentence in the figure caption is unclear. Also, change “carbon with the text” to “carbon in the text”.
P454, Line 9
Change to “(see Table 11.1 for references and data source)”.
P454, Figure 11.5.
The authors may want to use white-color outer band for the category ‘Various (Mineral)’, while leaving Histosol (organic) gray. This way, three first-order subdivisions (Gelisol, Histosol and Various) are represented by three different colored outer bands.
P457, Figure 11.8
Change the Y-axis labels to “Area (km2)” and “Area (acres)”, by showing the variables, rather than just indicating measurement units.
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