Comments on the Executive Summary

The draft SOCCR2 Preface states that the report Summary is “designed for a broader, more general audience”. The Committee appreciates the challenges involved in distilling such a long, technically-oriented assessment down to a short, readable overview. Nonetheless the Summary is quite lengthy (27 pages), and in many places contains language and statistics that will be difficult for a lay reader to grasp. The text should be carefully edited by someone with a “popular science” writing background, to help make it more concise and more readable for the intended audience.

In many places, the Summary reads like an accounting exercise, which may be reasonable given its focus on characterizing carbon stocks and flows. But it would be helpful to readers to offer more context and guidance on why it is important to know more about carbon cycle dynamics. There is some good text to this effect in report Chapter 1 that could be adapted for use in the Summary.

Also, the Executive Summary misses some opportunities to integrate the physical and biological aspects of carbon cycle science and to integrate the natural science with social science, decision making, and actionable items. It would also be helpful to distinguish more clearly between carbon sources and sinks that one can control (e.g., urban emissions) and those that one cannot control directly (e.g., climate feedbacks).

The Summary should also be carefully reviewed to ensure consistency within the body of the SOCCR2 report. There are a few places where numerical values presented in the Summary do not match the related numbers presented in later report chapters (examples are provided below). There are a number of points raised in the Summary that seem to bear no clear connection to material in the later report chapters. This should be carefully checked. Also, a 2017 carbon budget has been released by the Global Carbon Project, and a summary on U.S. greenhouse gases up to 2015 has been released by the US EPA (EPA, 2017), and so the SOCCR2 numbers should be consistent with these reports where appropriate.

General Suggestions for the Summary Key Findings

Overall the Summary key findings come across as rather bland, in part because many of them focus primarily on the message that “we have learned a lot” about some aspect of carbon cycle science. It would be more informative to focus on articulating what has actually been learned (i.e., focus more on the actual outcomes of the research than on the research process itself). Further, statements such as “we have improved understanding of ….” or “knowledge gaps remain” are vague. Does improved understanding mean improved accuracy, or new mechanisms, or something else? The Committee suggests providing quantitative information, including uncertainties, where possible.

Where possible, social science and decision-making concepts that are discussed in the body of the report could be woven into the key findings.

It is suggested that confidence levels should reflect the insights and not the methodology. For example, it is not helpful to show high confidence that data and models do not converge on some estimate.

The time period covered by the assessment should be stated clearly at the outset of the Summary.

It would be helpful to start with a finding that provides context about global CO₂ and CH₄ trends (i.e., CO₂ now exceeds 400ppm; atmospheric CH₄ has more than doubled), including some explanation of

how we know that these trends are driven by anthropogenic emissions. These can link back to the National Climate Assessment report.

This finding can be followed by a clear statement about North American carbon budgets, that identifies the most important processes contributing to sources and sinks (e.g., for CO₂, biogenic versus energy-related sources; for CH₄, the importance of wetlands and estuaries), and that conveys the totality of changes in North American carbon flux (i.e., what is the net balance in terms of sources/sinks?). Figures ES1 and ES5 in principle provide this information, but they are not consistent and not described clearly in the text. There should be a separate key finding about methane.

Many topics discussed in the body of the report are not reflected anywhere in these key findings. While of course hard choices must be made regarding what is and is not critical to include in key findings, the following topics are worth reconsidering to add somewhere in this list:

• the point that terrestrial land sinks play a critical role in helping to offset anthropogenic emissions;
• the issue of decreasing capacity for land and ocean carbon sinks (i.e., describe the current sink capacity, the possible degradation of these sinks by land use changes and disturbances, and the levers available to protect these sinks);
• a statement reflecting the report’s discussion of tribal lands;
• a statement about the importance of greenhouse gas emissions from agriculture;
• a clear explanation of CO₂ and CH₄ contributions from the energy sector;
• a statement to convey some sense of what carbon sources/sinks we can and cannot control (i.e., what are the levers in the carbon cycle that we have opportunities to better manage?).

Comments on Specific Summary Key Findings

FINDING 1. Emissions from fossil fuels have declined slightly over the last decade, largely a result of decreasing reliance on coal, increasing reliance on natural gas, the global recession, and increased vehicle fuel efficiency standards. Economic productivity has continued to increase, demonstrating that CO₂ emissions can be decoupled at least partly from economic activity.

• Insert “North America” to avoid confusion with the global emissions, which have grown.
• The reference to economic “productivity” would be better worded as “activity”, assuming the authors are really talking about economic output measures such as GDP.
• Suggest dropping the reference to the recession here as that was too short-term of a change to have a major effect on global atmospheric concentrations.
• Add to this finding or add a new finding: North American fraction of global fossil emissions, and the trend since SOCCR1. CO₂ and CH₄ contribution from the energy and agricultural sectors.

FINDING 2. The results from top down and bottom up approaches to estimating the magnitude of the land carbon sink are converging because of improvements in data and methodology, though significant uncertainties remain in both approaches. The land sink appears to be persistent, but future impacts from land use change and disturbances, both natural and human induced, may diminish this sink.

• Starting with the “top down versus bottom up” message overemphasizes scientific process, as opposed to the actual information that people need to know. Suggest deleting this first sentence.
• This Finding should give quantitative estimates (including uncertainties) of the North American carbon budget and updates since SOCCR1.

• . Acknowledge here the fact that the estimates for Mexican forest fluxes have changed sign since SOCCR1, including discussion about the uncertainties in this finding.
• ‘Future impacts’ should include climate change.

FINDING 3. There have been marked improvements in the understanding of North America’s carbon sources and sinks and the partitioning of carbon forms in water environments, as well as the importance of carbon transfers in inland water environments and across land water interfaces. Significant emissions from inland waters and a large carbon sink in the coastal ocean have been quantified.

• This is another example of focusing on the process (“we’ve had improvements in understanding”) rather than focusing on what has actually been learned. Give quantitative estimates of fluxes and inventories in water environments, including uncertainties.
• Need to mention that fluxes associated with inland waters and coastal ocean include pre-industrial or background fluxes, and that there is significant lateral transport.

FINDING 4. Understanding of the CH₄ budget has much improved, although there are important knowledge gaps. Overall, observations indicate that global atmospheric concentrations of CH₄ are increasing, while North American CH₄ emissions are relatively stable.

• It is vague to say “understanding of the budget has improved” and “there are knowledge gaps”. Offer some concrete sense of what things we have learned, and what still needs to be learned. Be quantitative where possible.
• It is confusing to conflate the global and national emission trends. Better to focus on global emission numbers at the start of the key findings list, and then focus just on North American emission trends.
• It is not really clear what the point of this finding is. What do these stated trends mean? Can we link the North American CH₄ emission trend to mechanisms/drivers?
• Need to discuss the findings of significant methane emissions from oil/gas producing regions, and the existing discrepancies between emission observations with the apparently decreasing overall atmospheric methane trends.

FINDING 5. Analyses of social systems and how carbon is embedded in them demonstrate the relevance of carbon cycle changes to people’s everyday lives and reveal feasible pathways to reduce GHGs.

• The Committee questions whether this sort of statement really needs to be highlighted as a “key finding”, as it seems rather hollow.
• While explicit policy recommendations are not part of the mandate for the SOCCR2 assessment, it is reasonable to discuss the implications of policy decisions on the carbon cycle. Perhaps this is something that can be better addressed in the next SOCCR assessment.

FINDING 6. Urban areas in North America represent the primary source of anthropogenic carbon emissions, as well as an indirect source of carbon from the emissions associated with goods and services produced outside city boundaries for consumption by urban dwellers. Therefore, carbon monitoring and budgeting in urban environments is increasingly important, including the avoidance of double counting with sectoral data on CO₂ fluxes.

• This finding illustrates the problem of singling out urban regions as an emissions category. Given that the finding does not highlight other major sectors such as forests and agriculture, it is not obvious why “urban” is highlighted. The urban focus would make more sense if the

• finding identifies how particular components of carbon emissions are best controlled at the urban level (e.g., through steps such as restructuring urban development patterns to reduce driving), and if it is better integrated with the “decision making” discussion (Chapter 18) by identifying opportunities to effectively manage carbon emissions at the different governance levels (local/state/federal) or different system levels (forest, freshwater, etc).

• The finding could be improved by incorporating some of the information articulated in Box ES5—for instance, the point that monitoring could help inform the emission reduction pledges made by cities.

FINDING 7. Overall, research has led to an improved ability to attribute observed changes in the carbon budget to specific causes, including social and economic factors, technological change, climate variability, and management practices. Understanding these processes and their interactions aids in projecting future changes in the carbon cycle and developing adaptive capabilities. One projection is of significant concern the 5% to 15% of the carbon stored in soil pools in the circumpolar permafrost zone is considered to be vulnerable to release to the atmosphere by the year 2100, considering the current trajectory of global and Arctic warming.

• The last sentence of this finding is the critical point to emphasize. The first two sentences are perhaps not even needed.
• It would help to delineate here the potential impacts of permafrost thaw on CO₂ and on CH₄ separately (rather than on carbon collectively), and to translate the percentages into absolute amounts (Pg, Tg of carbon) and/or into ppm in the atmosphere.
• Add a finding about projections – decreasing capacity of land and oceans to absorb CO₂.

FINDING 8. There are still regions and ecosystems that are less well understood that would benefit from additional research and monitoring (e.g., the Arctic and boreal regions, grasslands, wetlands, inland waters, and tropical ecosystems among others described in SOCCR 2). Uncertainties for particular sources, sinks, and fluxes must be reduced to provide consistent and accurate inventory (bottom up) and verification (top down) estimates. Filling these gaps will be important milestones for the third SOCCR a decade from now.

• This finding offers little substance. A finding focused on research needs could be useful if it avoids being a vague list that could potentially encompass anything. Instead it could identify specific knowledge gaps that could feasibly be addressed with a focused research agenda.
• This Finding should include consideration of what research and support is needed to advance our understanding of carbon cycling and resource management on Tribal Lands, and to advance the integration of social science with the natural sciences of the carbon cycle.

Comments on Summary Figures / Tables

Figure ES1: This figure shows that the domain of this report includes Puerto Rico, Hawaii, and U.S. Pacific islands, yet the report provides little information about these locations. The authors should redraw ES1 to exclude these places, or mention in the text their carbon significance.

Figure ES2: This is potentially a helpful figure, but the following improvements are suggested:

• Augment to also show lateral fluxes of carbon – especially given how the report emphasizes these lateral fluxes as one of the important scientific advances of recent years.

• The box labeled “Atmosphere” should be “Global Atmosphere”. As is, one could mis-interpret the +1032 to refer to atmosphere over N America. The authors may wish to consider putting parenthesis around (+1032) to indicate that it is an estimated quantity.
• The figures are reported with too many significant digits. The numbers should include uncertainties.
• The caption is unclear, and needs to distinguish between inventory and fluxes (e.g., arrows are fluxes).

Figure ES3: The graphics should be improved in several respects:

• The figure is missing information about the CO₂ source. It should show CO₂ and CH₄ separately. Most importantly, the CO₂ figure should show fossil fuel carbon and land use emissions over time.
• It shows emissions as “negative sinks”, which will be very confusing to most readers.
• Many readers may be unfamiliar with the “micromole per mole” units used. The report should include a note about units for reporting gases and consistently use one type of unit that is most familiar to lay readers (perhaps ppm and ppb).

Figure ES4:

• Change left Y-axis label to “Annual emissions (PgC)”, as now it only indicates unit, but not parameter/variable.
• Figure legend: Change “North America” to “North America Total”, to make this more clear.
• The figure graphic quality can be improved, for example, by adding x-axis major tick marks for every 5 years, and leaving space between symbols and letters in the legend.

Figure ES5 should be re-considered. This figure pertains to the total fluxes into or out of the atmosphere, but one could easily mis-interpret this figure to say that the North American net CO₂ flux is approximately equal to the emissions from fossil sources—as the forest sink and coastal ocean sink are countered by outgassing from inland waters. This figure would appear to contradict the terrestrial sink (land and water components). Other suggestions:

• y-axis label should be “Carbon dioxide fluxes (Tg C per year)”
• change x-axis category labels
• change from “Fossil fuels” to “Fossil fuel emissions”
• change from “Forest sector” to “Forests”
• change “Inland water outgas” to “Inland water”
• change “Arctic/boreal” to “Arctic/boreal permafrost”

In Figure ES2, ES4, and ES5, check the numbers/units for consistency. Even within the Executive Summary figures, there is inconsistency in the units used (a mix of Pg, Tg, other units). The Committee recommends adding a figure showing the changes in the mix of energy sources and associated CO₂ emissions over time (at least in Chapter 3 [Energy Systems], if not in the Executive Summary).

Table ES1 on trends, indicators, drivers from the energy system seems oddly placed in the Summary, given that there is nothing comparable presented for trends, indicators, drivers, impacts of other major components of the carbon cycle (e.g, forestry, agriculture, land use changes).

Line-Specific Comments

P18, Line 16-17 [Preface]

CO₂e is defined relative to a time horizon, typically 100 years or 25 years. Most usage in the chapters assumes 100 years, though some mentions a shorter time horizon. This has to be consistent throughout the report. Here, it would be useful to give actual numbers for methane and N2O for the two time horizons.

P21, Line 12-13

Interestingly, this statement is an expression of world views characteristic of those held by indigenous communities—interconnectedness of humans and the environment. Additionally, it may be worth combining with text at p.25, l16.

P21, Line 15

Replace “improved understanding” with “advances in our understanding”.

P21, Line 28

It should be possible to quantify these. Can estimates be provided?

P21, Line 30

Discussion of ecosystem impacts is sparse. These impacts are complex and multi-faceted, involving spatial, temporal, and place-resource dependent considerations. Factors such as species displacement and migration, alteration of phenological behavior, impacts of water timing and availability, extreme events, and impacts such as introduction of genetic strains are not addressed.

P23, Line 20 – P25, Line 15

These main findings are disconnected from the final few pages of this chapter (beginning with p.38).

P27, Line 4

Forests typically are sinks, so reverse the analogy.

Box ES2, paragraph 2.

Some of the chapters use different units [g/m²]. More consistency is needed. Also methane units need to be included. Definition of CO₂e should include the time horizon (typically 100 years).

P28, Line 29-36

This is largely repetitive of concepts presented at p.23, lines 9-16.

P29, Line 8-18

While the numbers appear consistent with those in the literature, information on the significance of these increases would be valuable for a lay audience. For example, what is the importance of these increases for ecological processes, human health, food supplies, quality of life, and habitability? Might these statistics be more effectively presented at p.32?

P29, Line 5-28

These concepts are described in detail in other USGCRP documents, is it necessary to repeat in SOCCR2?

P29, Line 2

Suggest changing to “Evidence strongly suggests that changes….” (add “that”).

P29, Line 10

The unit for the atmospheric CO₂ concentration here (ppm) is different from the one in Figure ES3 (umol/mol).

P31, Line 5-7

The confidence statement used here should clarify that the magnitude of sources/sinks contains uncertainty, but not the process.

P31, Line 12

Discuss methane after this line.

P32, Line 22-30

Need to mention that CO₂ fertilization is transient storage, as it is followed by greater litter inputs and enhanced decomposition and CO₂ efflux from soils.

P33

This section should have a sub-section on CO₂ and one on methane. Here methane sources and sinks appear as a single bullet (p.35) interspersed between CO₂ fluxes and stocks. The methane subsection should include the recent studies of U.S. methane sources (e.g., Kort et al., 2014) and other references listed below for Chapter 2). Contrary to p.36, line 2, Turner et al. (2016) finds a trend in U.S. emissions.

P33, Line 2

Shouldn’t the focus be on carbon flux instead of atmospheric concentration when discussing sources and sinks?

P33, Line 22

The use of “now” refers to what period? 2004-2013?

P33, Line 22

An important factor is the declining trend in North American contributions as a percent of global emissions; this deserves some elaboration.

P35, Figure ES5

It is not clear how one gets 634 Tg from Figure ES2. Please check numbers and ensure their compatibility across the document.

P35, Line 11-14.

This contradicts Key Finding 3 of Chapter 2 (between ¼ and ½ of fossil fuel emissions were offset by natural sinks on North American land and adjacent coastal ocean. The authors should re-think how to present the information.

P36, Line 13

As stated this sentence indicates that “land” sinks include inland waters and the coastal ocean. The word “land” here should be eliminated, or perhaps replaced with “continental”.

P36, Line 36-37

Why is the term “reservoirs” used? Is there an intended difference from sinks?

P37, Line 1-4

Carbon storage and risks of greenhouse gas and soot emissions from forests is heavily dependent on vegetative management practices, such as prescribed burning, mechanic removals, and species manipulation.

P37, Line 13

Why aren’t tillage practices mentioned?

P37, Line 35

Why isn’t methane emission from reservoirs mentioned ?

P37, Line 21-22

The results and findings presented here are inconsistent with ones in Chapter 13: the stated net carbon sink from terrestrial wetlands of 64 TgC/yr (36 TgC/yr by nonforested wetlands, plus 28 TgC/yr by forested wetlands) is different from the value of 53 Tg/yr presented as Key Finding 2 in Chapter 13—despite the fact that the Executive Summary indicates Chapter 13 as source for that information. The authors need to check for consistency with the latest version of relevant chapters of the report.

P37, Line 25

The methane source from terrestrial wetlands [21 Tg CH₄/yr] is different than in Key Finding 2 of Chapter 13 [18 Tg CH₄/per year].

P38 [section: “A systems approach to linkages between the carbon cycle and society”]

• Despite the definition of “systems” in the footnote, there is no discussion of economics in this section or throughout Chapters 6 and 18.
• As this section provides background information and little new insight, it could be shortened significantly.
• The discussion of CO₂ from urban areas confuses the terms “drivers” and “sources”. Urban populations drive CO₂ emissions elsewhere. (Page 40 – The CO₂ that is emitted locally from urban areas are from transportation and natural gas consumption at residential and commercial establishments. CO₂ from fossil fuel combustion is released at power plants far away).

P38, Line 1-6

Consider adding discussion of impacts of carbon on plankton production and food chains, ocean acidification, hypoxia (related to land use and chemical fertilizer application), and harmful algal blooms.

P38, Line 8-24

Much of this appears to be repetitive.

P39, Figure ES6

In the Box “Climate Drivers”, delete “annual” from weather.

P40, Line 41 – P42, Line 18

These projections seem to be based on the scenarios that have socio-economic responses embedded into physical climate models. Should references be included?

P40 [Section: “Projections of the Future …”]

• Is important to mention that the capacity of the land and oceans to act carbon sinks decreases with projected climate change.
• Need to include projection of methane.

P41, Line 16-21

Is important to mention that CO₂ fertilization effects are likely overwhelmed by climate change effects.

P42, Line 10-18

Perhaps note the challenges in carbon accounting involving the world commons. Under IPCC rules, coastal states do not get credit for ocean sinks.

P42, Line 19-40

This discussion seems disconnected from SOCCR2. Issues of carbon management, systems of governance, etc. could be more fully integrated with discussions in other chapters. While the authors understandably shy away from prescriptive statements, SOCCR2 could usefully inform decision makers about actions that make significant contributions to reducing GHG emissions.

P43, Box ES.3

This box is a bit perplexing. Why is the discussion limited to cities? The presence of states, businesses, and tribal governments at Bonn and participating in various climate-related initiatives is likely more substantial and significant. As indicated in Chapter 4, a main obstacle to the ability of cities to influence carbon-flux is jurisdictional fragmentation and the lack of a multi-level system of carbon governance.

P43, Line 8

The 3rd option involves storage in geologic reservoirs as well.

P43, Line 8

The reference to geoengineering needs to be more carefully worded to avoid giving a mis-impression that solar radiation management techniques could directly affect the level of carbon gases in the atmosphere (they cannot).

P45, Table ES.1

Why is this included while corresponding treatment for other chapters is absent?

P46, Line 37

This statement suffers from the paucity of data on effects of tribal management practices on carbon fluxes and the lack of a means to “upscale” actions to determine their significance.

P47, Line 2-37

These co-benefits and trade-offs suggest the existence of an integrated cross-boundary economic and jurisdictional system, which does not exist. Should the focus instead be on multi-level integration

involving local communities, urban areas, regional, national and international carbon accounting and decision-support systems?