Summary

The second “State of the Climate Cycle Report” (SOCCR2) aims to elucidate the fundamental physical, chemical, and biological aspects of the carbon cycle and to discuss the challenges of accounting for all major carbon stocks and flows for the North American continent. This assessment report has broad value, as understanding the carbon cycle is not just an academic exercise. Rather, this understanding can provide an important foundation for making a wide variety of societal decisions about land use and natural resource management, climate change mitigation strategies, urban planning, and energy production and consumption.

SOCCR2 is part of a broader suite of assessment activities carried out within the U.S. Global Change Research Program’s (USGCRP) National Climate Assessment activities. To ensure that SOCCR2 is scientifically credible and effectively communicates, the USGCRP asked the National Academies of Sciences, Engineering, and Medicine to review the draft document (during the same time period in which the draft SOCCR2 report was available for public comment). The Academies appointed an ad hoc expert Committee to conduct this review, and offered here are the Committee’s findings and recommendations.

The Committee finds that the draft SOCCR2 report is an admirable effort to distill a huge volume of research into a helpful overview of the available data and the current state of knowledge. Many of the individual chapters are well written and organized. In most cases, the key findings are clearly stated and are amply supported with evidence. The report provides a good sense of how the relevant science has advanced since the first SOCCR report was released a decade ago. At the same time, the Committee finds many ways the draft report could be improved (as is often the case for scientific reviews of draft assessment reports).

For all of the draft report chapters, a variety of suggestions are made herein for editing, clarifying or expanding key points. A few chapters raised particular concerns among the Committee, however, which we highlight here:

• Executive Summary. The draft report Summary should be more concise and more accessible to a general audience. Many instances of technical jargon and confusing wording could be improved with the services of a good science writer.
• Chapter 3 (Energy). This chapter should be organized more tightly on the issue of energy as a source of carbon emissions and on the potential for mitigation of energy sector emissions. The Committee has suggested some figures that would help in this regard.
• Chapter 6 (Social Science). There should be a clear acknowledgement that this chapter does not address economics research and focuses only on a few components of the carbon cycle. The chapter would be improved by less discussion about social science “process” issues and more examples of the actual insights being gained from this research. The chapter may fit better later in the report (after Chapter18).
• Chapter 7 (Tribal Lands). This chapter lacks depth of treatment of many key issues. Given the challenges of integrating Traditional Knowledge with assessment approaches that rely heavily on peer reviewed literature, and the wide diversity of tribal communities across the U.S., Canada, and Mexico that must be considered, it may be better to re-focus this chapter more on exploring how to support and empower indigenous communities to advance sustainable carbon management policies and programs.

• Chapter 13 (Terrestrial Wetlands). This is the only chapter in which the SOCCR2 authors did their own original numerical analyses of primary data and used these analyses as basis for their assessment. The Committee believes it would be better to instead focus on presenting results from the available published literature. There is also a need to address several statements in the Chapter that appear inaccurate or poorly worded.

The most significant cross-cutting suggestions for improving the report are described here in general terms. Details and specific examples are provided within the body of our review.

• There are many places in the draft report where flux estimates and other numbers are inconsistent within chapters (e.g., between the text and the figures), among different chapters, and between the Executive Summary and some chapters. There are also numerous instances of inconsistencies in the units of measurement used across different parts of the draft report—for instance when discussing carbon fluxes and energy issues. It is critical that the draft report be carefully checked to identify and address such inconsistencies. It may be helpful for the SOCCR2 organizers to convene representatives from across the different chapter teams, to construct a diagram that puts all the different types of flux estimates into one framework. Figure ES2 could be a starting point for such a diagram, if it were more directly connected to, and inclusive of, the different types of flux values presented across the report chapters.
• The use of terms “C uptake”, “C sequestration”, “C emission of –xx Tg”, “C sink of –xx Tg”, “C sink of xx Tg” are used variably through the draft report, particularly with respect to discussion of forests, soils, and agriculture. The SOCCR2 authors must find a way to standardize the definitions and usage of these terms across all chapters.
• The Committee has concerns with how carbon fluxes related to inland and coastal waters in particular are presented. Figure ES5 presumably shows total (background + perturbation) net (difference between incoming and outgoing) fluxes, which could lead to a mis-impression that the forest sink is countered mainly by outgassing from inland waters, and that the net CO₂ flux from North America approximately equals the fossil fuel flux. This would appear to contradict, for example Figure ES2, and Chapter 2 that one-quarter to one-half of North American fossil fuel source is removed by natural sinks. Furthermore, the contributions by forests, agriculture, wetlands, and arctic boreal systems do not add up to -937 shown in Figure ES2. The numbers need to be consistent throughout the draft report.
• The geographic scope in the draft report is ambiguous. The assessment aims to address North America as a whole but the inclusion of Canada and Mexico is very uneven. The Executive Summary in the draft SOCCR2 report states that the geographic scope includes Hawaii and U.S. territories, yet these areas are not mentioned anywhere else in the document. The draft report should be revised to provide more clarity about the intended geographic scope, and where possible, to provide a more even treatment of the regions included in the chapter discussions.
• The Committee disagrees with the practice of assigning confidence levels to direct factual information, such as the observations that atmospheric concentrations of CO₂ and CH₄ are increasing. This actually undermines these incontrovertible observations. In some cases, this could be clarified by ascribing confidence levels to specific parts of a finding, rather than to the finding overall.
• The research needs identified in the draft report cover a very broad array of possible research topics. The recommendations should be better focused on prioritizing specific advances that could be feasibly made over the coming few years with sufficient research investments.

• Key Findings throughout the draft report place considerable emphasis on noting that research has progressed (“we’ve learned a lot about….”). Emphasis should instead be placed on explaining what new insights have actually been learned from this research, in quantitative terms where possible.
• The SOCCR2 authors should reconsider presenting numbers with 3 or 4 significant digits, as this overstates the confidence one should have in such numbers. The authors should also convey more consistently the confidence and uncertainties in estimates that are presented.
• There are several places where new figures could be added to the draft report to help illustrate key concepts, and a number of edits/improvements are also suggested herein for several of the existing figures.
• A few topics should be better addressed in this assessment, including: important recent research on U.S. methane sources and sinks; integrated assessment modeling research; impacts of climate change, especially of changing precipitation patterns, on carbon cycle dynamics; and Arctic coastal zones as a potentially important biogenic carbon source. Also the report Summary should more clearly frame ocean carbon dynamics as a critical part of the global carbon system, since the magnitude of the North American carbon sink is constrained by the magnitude and geographic distribution of the ocean sink

Below are some additional issues that the Committee believes should be considered, although we recognize that some might be challenging to fully address in the limited time available for the SOCCR2 authors to complete their report revisions. At a minimum, these issues could be acknowledged as important considerations within the SOCCR2 report—perhaps in a short section about “future challenges”. Some suggestions might be taken as suggestions for shaping the next round of assessment work (SOCCR3).

• The draft report discussion of management decisions that affect carbon dynamics is uneven. For instance, there is discussion of how urban-scale actions can affect carbon emissions but little comparable discussion of actions at state, federal, and international levels. There is extensive coverage of decision-making regarding Agriculture, Forestry, and other Land Uses (AFOLU), but little discussion of how this integrates with other components of the carbon cycle to support decisions about CO₂ mitigation. There is very limited explanation of the opportunities that exist for more effective management of carbon sources and sinks. The Committee strongly encourages the authors address such gaps, as this would greatly enhance the usefulness of the assessment for informing governance and management decisions that affect carbon sources and sinks.
• The Committee recommends re-examining the policy that all chapters must have some minimum number of key findings, as it results in many findings that are obvious statements that do not offer specific new insights or do not convey a clear message to the reader.
• Discussions of future emissions scenarios should consider a wider array of scenarios, including scenarios that examine the actions needed (by reducing certain carbon emissions, enhancing certain carbon sinks) to avoid a 2℃ global temperature increase.
• Some biological, physical, and societal processes discussed in the draft report are treated as isolated subjects that are not well-connected to each other or to the central issue of understanding the carbon cycle. For instance, it is important to discuss how warming temperatures and changing precipitation patterns affect carbon emissions from key terrestrial

• and aquatic ecosystems; how expanding biofuel production affects the management of grasslands, forestry, agriculture; and how energy use contributes to the carbon budget overall. Giving greater attention to these integration concerns will help assure the overall report is more than just the sum of its individual pieces, and that the report may be useful for informing mitigation and adaptation policies and management decisions.

• The Committee understands that time to make revisions is short, but if there were sufficient time—and certainly for a future report (SOCCR3)—it would be desirable to consider an alternative organization in which social science research issues are woven throughout the report, rather than presented as a stand-alone subject. There should also be consideration of relevant economics research that provides important insights about human influences on the carbon cycle—for instance, regarding costs as a key determinant of behavior related to energy use and resulting carbon emissions.

Finally, the Committee notes some issues related to the relationship between SOCCR2 and the fourth National Climate Assessment, Volumes 1 [Climate Science Special Report] and 2 [Climate Change Risks, Impacts, and Adaptation, NCA4]. The efforts to avoid overlap with NCA reports leads to some frustrating limitations in the SOCCR2 scope—for instance, regarding the discussion of carbon emission mitigation strategies, and of the consequences of rising CO₂ levels. For future USGCRP assessment efforts, consideration should be given to whether the carbon cycle should be more interwoven into other assessment products, or whether there are better ways to structure future SOCCR reports to be more distinct from other products.

The Committee commends the SOCCR2 authors on the tremendous amount of work that went into the production of this assessment, and we hope the suggestions offered herein will help assure the final report is as robust and as useful as possible to a wide variety of stakeholders.