Chapter 17: Consequences of Rising Atmospheric CO₂

Overview/Main Issues

This chapter examines some of the impacts of rising atmospheric CO₂ levels, in particular on ocean acidification and on dynamics of terrestrial vegetation. The limitations in the focus of this chapter are striking, as it is hard to discuss these particular consequences without considering the concomitant impacts of rising CO₂ levels on factors such as temperature and precipitation changes, hypoxia, sea level, and changes in terrestrial biosphere. The Committee understands the limitations that the SOCCR2 authors faced when writing this chapter (i.e., they did not want to impinge upon topics that fall squarely within the domain of other National Climate Assessment reports). Yet for the chapter to be useful, it needs to be more well-rounded in terms of considering the full impacts of rising atmospheric CO₂ emissions. One possible solution would be to summarize key insights from the Climate Science Special Report more significantly at the beginning of this chapter, and to build the discussions from there. The authors may also consider changing the title of this chapter to “Direct Consequences of Rising Atmospheric CO₂ on the Biosphere”, to contain the expectations of the reader.

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?

This chapter is in alignment with the overall goals and objectives of the report as stated in the Executive Summary and Report Preface. This chapter is generally well written and is scientifically sound. However, as stated above, the Committee finds a chapter solely on CO₂ consequences for ocean chemistry and terrestrial vegetation—without considering the inevitable broader consequences of climate change, sea level rise, changes in ecosystem structure etc.—to be not very useful for policy makers.

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

In general, the report is accurate and is well supported by scientific literature. The Committee did find a few content areas that could be expanded or included further. For example:

• The section labelled “Limits in Ocean CO₂ uptake” does not actually address that topic (this is further addressed in another chapter, but should be summarized here).
• The chapter needs an expanded view of biological impacts of ocean acidification and CO₂ increase (not limited to calcification).
• Section discussing Geologic History is oversimplified to the point of potentially being confusing. Also, the paragraph at the bottom of this page is misleading—the multi-stressor context of the geologic record is a strength, not a weakness. Report should reflect this important role that the geologic record may play in understanding impacts of future change. Also, this section does not result in a Finding, which is inconsistent with the rest of the chapter.

• Sea level is discussed in terms of “coastal protection” but not impacts on carbon sinks—please expand this section to discuss impacts on future carbon sinks/sources.
• The Chapter should address acidification in freshwater (not only in oceans). Though the data are insufficient to offer conclusive proof of acidification trends, the basic chemistry clearly indicates the potential for acidification is the same as in the oceans (Phillips et al., 2015). There is growing evidence of likely ecosystem impacts (Hasler et al., 2015; Weiss et al., 2018). Data are very sparse, and long timeseries are needed to understand these trends. Thus it is important to discuss this emerging evidence in SOCCR2.

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

The Findings are generally appropriate and are in alignment with the supporting text. However, there are some ways the Findings could be improved:

• The discussion of fisheries/aquaculture impacts appears as a Finding but not extensively discussed in text.
• Finding 4 needs rewording for accuracy.
• Finding 2 is so generic as to be meaningless.
• Findings, generally, are not very quantitative here compared to other chapters (could use some quantification and statement of uncertainties).
• The Key Findings could be re-ordered: Finding 1 and 3 on the oceans, Finding 2 on land, and Finding 4 on carbon-climate feedbacks.

• 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?

Figures are appropriate.

• Are the research needs identified in the report appropriate?

The research needs and gaps were appropriate.

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

The authors appropriately summarize and cite published analyses.

• 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 level of technicality is appropriate for an interested scientific audience (undergraduates, graduate students, or scientists from other fields). The main problem is that the chapter does not directly refer to or summarize other impacts of rising CO₂, so as written the information is hard to interpret.

Line-Specific Comments

P695, Line 28-32

We suggest using “would increase”, “would likely change” … Without climate change and anthropogenic disturbances, “will” conveys too much confidence.

P696, Line 13-20

Need to bring in downstream effects— increased litter and greater decomposition/respiration.

P696, Line 22-23

Need to add brief statement/paragraph on the impact of CO₂ on ocean biota.

P697, Line 2

Have *at times* been well in excess of …

P697, Line 3

Add something like “Human civilization (which developed approximately X thousand years ago) during a time...”

P697, Line 34

Replace “rapid rise …” with “Solution of atmospheric CO₂ in sea water forms carbon acid...”

P697, Line 1-5

The decrease in atmospheric oxygen confirms the combustion.

P700, Line 28

Define residual land sink

P701, Line 15

Add reference to Swann et al. (2016).

P701, Line 19-28

Need to mention water here.

P702, Line 1

Suggest title change to “Indirect thermal effects of rising CO₂ on ecosystems.”

P702, Line 23

Taken up or released by ecosystems *and the oceans*.

P705, Line 14-27

Burke et al. (2015b) shows nonlinear dependence of agriculture on temperature.

P706, Line 6-8

The carbon sink varies with climate change as well.

P706, Line 18

Coastal wetlands as well?

P707, Line 1-3

Add references to Burke et al. (2015a); Hsiang et al. (2011, 2013).