Chapter 19: Future of the North American Carbon Cycle
Overview/Main Issues
This chapter nicely summarizes current understanding of future changes in carbon fluxes and stocks in North America and over the globe. The chapter also examines the various factors that will control future carbon fluxes/stocks such as climate, atmospheric composition, land use change, nutrient availability, and resource management. Critical carbon cycle vulnerabilities and key research needs are also identified. Finally, this chapter briefly describes the future methane cycle and the improvement of model projections. The Committee has some suggestions on how the chapter can be improved; in particular: the methane cycle deserves more attention; some tables and figures that do not seem fully relevant could be removed; the topic of research to improve terrestrial biosphere/earth system models should be included; and it would be helpful to add a summary/outlook section at the end of the chapter.
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 are implicitly referred to in the second paragraph of the Introduction, but it would be better to describe these more explicitly. The report meets the goals that were implicitly mentioned.
- Does the report accurately reflect the scientific literature? Are there any critical content areas missing from the report?
This chapter accurately reflects the scientific literature to a large extent. However, the methane cycle (sinks as well as sources) should receive more attention.
- 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?
Table 19.2 is good. Tables 19.1 and 19.3 do not seem appropriate for this chapter; they might be a reasonable fit for Chapter 2.
Table 19.3 is incomplete particularly for the major drivers of change. The Table should include certainty bounds, and the timeframe for “future’ should be specified.
Figures 19.1-19.4 are not necessary or relevant to this particular chapter. These figures may be appropriate for Chapter 1 or Chapter 2.
At first glance, Figure 19.5 suggests negligible trend in the different land cover categories. Could the figure be replotted to highlight the growth of urban areas?
Figures 19.8 and 19.9 are good. Perhaps even more useful to the reader however, would be additional analyses/figures related to projections of carbon stocks and trajectories of the percentage of fossil fuel
emissions offset by carbon sinks (focusing on North America and stratifying by country and biome type).
- Are the research needs identified in the report appropriate?
The key research needs identified in Section 19.8 are appropriate. We suggest, however, that this section also address the topic of research to improve terrestrial biosphere/earth system models and coastal ocean biogeochemistry models. This is important given the large uncertainty in model simulations and large discrepancies among models.
- 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?
The Introduction section provides insufficient information on why we need to project the future of the North American carbon cycle. It would be good to have one paragraph focusing on the impacts of the carbon cycle on regional/global climate and one paragraph focusing on the effects of climate change (and other global change agents) on the carbon cycle.
The information presented on p.776 largely looks at the global scale, but given the focus of the SOCCR2 assessment, would it not help to also offer some discussion that focuses specifically on North America, including Canada and Mexico?
The chapter ends rather abruptly. Some sort of summary/outlook section could perhaps be added at the end of the chapter.
- What other significant improvements, if any, might be made in the document?
Section 19.2 (Historical Carbon Cycle Changes) is not needed, as this has been covered by Chapter 1 and/or Chapter 2. Removing this section will help shorten this chapter.
The methane cycle is only briefly described as a sidebar at the end of the chapter. More attention should be paid to the methane cycle, given that this chapter is about the future of the North American carbon cycle. It would also be nice to include one figure on the future projections of methane fluxes based on published data.
- Are the key findings in your chapter well stated and supported by the detail provided in the chapter?
The Key Findings are well stated and supported by the detail provided in the chapter. But it may be best to switch Key Findings 1 and 2, so that fossil fuel comes first, followed by sinks. And Key Finding 4 – Line 6 - need to mention hydrologic changes
Line-Specific Comments
P771, Line 22-25
The “;” should be replaced with “,”.
P771, Line 27
The word “that” is needed after “meaning”.
P773, Line 20-24
The discussion of CO2 fertilization should be accompanied by mention of enhanced respiration.
P775, Line 4-15
These are the recent and current impacts of land use/land cover change, rather than changes for the future. Thus they are more appropriate for Chapter 1 and/or 2.
P776, Line 5-10
These results are for the global scale, not for North America. It would be helpful for the authors to also focus specifically on North America (by looking at Canada and/or Mexico).
P776, Line 20-27
Again, why not offer some focus on North America specifically (by touching Canada and/or Mexico), in addition to the global focus?
P777, Line 26
“In summary,” should be removed, as the sentence is not a summary of the paragraphs above.
P778, Line 3-18
As above, would be helpful to put this in the context of North America specifically, in addition to the more general global-scale context.
P780, Line 1-38
Here too, this section (19.5.1) could focus on North America, in addition to the focus on the globe in general. It would also be better to use model ensembles to specifically examine the future responses of land carbon cycle to rising atmospheric CO2 over North America.
P789, Line 15-37
The growing evidence that acidification may put ecological pressure on freshwaters could be cited here (Hasler et al., 2015; Phillips et al., 2015; Weiss et al., 2018).
P791, Line 23 – P792, Line 21
This section lists three sources of uncertainties in models: model structure, model parameterization, model evaluation. While model evaluation is important to discuss, it is confusing to characterize model evaluation itself as a “source of uncertainty”. We suggest this list instead discuss ‘model inputs’ as a source of uncertainty. The SOCCR authors may wish to also consider recently published work that formally separates uncertainty into Model Structure, Internal Variability and Scenarios (Bonan and Doney, 2018; Hawkins and Sutton, 2009; Lovenduski and Bonan, 2017; Lovenduski et al., 2016). Scenarios could potentially be discussed as part of the “model input” category recommended above, and the authors may wish to add a paragraph on internal variability.
P791-792 , Section 19.2
This section should be revised to be more balanced between land and ocean perspectives. In the ocean, uncertainty in the projections of ocean circulation change, in addition to biogeochemical changes, should be mentioned.
P798, Lines 6-12
This section does not adequately explain the expected future slowing of the ocean sink. The current discussion of changing biology in the open ocean has minimal impact on carbon uptake. References such as Randerson et al. (2015) can provide an adequate global context.
