Chapter 1: Overview of the Global Carbon Cycle

Overview/Main Issues

This chapter contains high-level background on the importance of the global carbon cycle, the major pools and fluxes of carbon, what’s changed over time, and perturbations of the carbon cycle. It also has a section that pertains specifically to the North American carbon cycle. The chapter represents a considerable effort on the part of the authors to distill the literature and key points. While it does a good job giving a high-level overview, it could be improved by the harmonization of the data in the text with the figures and other chapters in the draft report as well as by adding new conceptual figure(s) and a table. Some other general suggestions include:

The chapter is not balanced between CO₂ and CH₄. There needs to be a key finding about CH₄. The term “carbon” often refers only to CO₂, and not methane (e.g. Section 1.3.2).

For methane, it is important to mention recent studies that have used “top down” observations to derive emission estimates that are higher than the “bottom up” EPA inventory estimates (e.g., Kort et al., 2014; Miller et al., 2013; Turner et al., 2016). Studies that have fused large collections of airborne, ground-based, and satellite data should be given particular attention.

Further editing to clarify and shorten the key findings as well as some of the text would also improve the chapter.

There was not always a consistent mention of cement production and inland waters as important sources of carbon emissions to the atmosphere. And there is some problem with consistency for fluxes within this chapter and Figure 1.1, as well as across other chapters.

Some sections are not well organized or clearly written. For example, Section 1.1 should have, at a minimum, a much stronger introductory sentence that illustrates the fundamental importance of the carbon cycle. See, for example Cole, 2013.

Some of the headers are ambiguous and, in some cases, do not describe well the content of the paragraphs that follow. For example: Section 1.1 (The Role of Carbon)—where, and for what? Section 1.2 (The Natural Carbon Cycle)—the 2nd paragraph talks about human influence on the carbon cycle.

The Chapter is missing a conceptual figure to illustrate feedbacks. The nature and magnitude of feedbacks are likely to be crucially important, which is identified in the text

Statement of Task Questions

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

One of the major new pieces of information for SOCCR2 is the emissions contribution from inland waters. This was not highlighted in the key findings, but should be. The authors should be careful to explain that this flux includes the background flux, not just fluxes in response to anthropogenic emissions. Also the discussion of methane is very slim and should be enhanced.

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

The suggestion that “global emissions became slightly more uncoupled from economic growth”…requires a synthesis figure and/or references to support this finding.

Often time frames are missing (see above and line comments)

• 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?

It would be helpful to add:

• a figure (conceptual or more specific) that shows carbon cycle feedbacks;
• a figure or table illustrating Key Finding 3, “uncoupled from economic growth”
• a table or figure that show the relative radiative forcings of greenhouse gases (including non-carbon) as per Key Finding 2—although the authors may wish to consider whether discussion of radiative forcing is even appropriate for this report, or whether that topic should instead be restricted to the NCA report.

• Are the research needs identified in the report appropriate?

They were not explicitly part of this chapter.

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

Yes, with the exception of Key Finding 3, about which it is not possible to tell.

• 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?

Some improvements are necessary, see above.

• What other significant improvements, if any, might be made in the document?

In addition to figure and table suggestions noted above, it would be good to add an explanatory sentence or two about radiative anthropogenic forcing on page 5, should the authors decide to keep radiative forcing in this report.

• Are the key findings in your chapter well stated and supported by the detail provided in the chapter?

In general, we find the key findings are unnecessarily long, and a few are not as clear as they could be. Specific suggestions include the following:

Key Finding 1.

• Make clear time frames; e.g., give year or range of years.
• This should echo key finding in the NCA report: CO₂ over 400 ppm, highest during human history, fastest growth rate, etc. Same for methane.

Key Finding 2.

• The authors should discuss whether radiative forcing belongs in SOCCR2, or whether that topic should be restricted to the NCA report. If this topic is not removed altogether, then the Key Finding should mention other (non-carbon) GHGs here and their contribution to total radiative forcing. For this message, the unmatched time frames are confusing (e.g., relative to year 1750, and then 1880-2012).
• There was no citation in the supporting evidence for the “high confidence” relationship between radiative forcing and increase in global average temperature.

Key Finding 3.

• If this “uncoupling” between economic growth and fossil fuel emissions is taken from published studies and syntheses, a figure or table should be included in support of the statement.
• There was no mention of CO₂ emissions associated with cement production and changes over time, although they are part of Figure 1.1. Are they assumed to be part of the global fossil fuel emissions?
• “Growth in emissions (of) -0.1%” could easily be misconstrued by some readers. How about “For 2015 and 2016 emissions remained constant at ...”. It is best to avoid negative signs that given an impression of a net sink.
• The authors should provide quantitative information. For instance, noting the current fossil fuel carbon emissions and cumulative emissions since pre-industrial would help tie this to Key Finding 5.

Key Finding 4.

• The numbers in this key message do not appear to match the numbers in Figure 1.1.
• Missing in the text is mention of the flux to the atmosphere from inland waters, which is identified as an important new piece of information and also appears in Figure 1.1. Note that the flux estimate is different from what is identified in the inland water section. This also needs to be consistent with Key Finding 1.
• The statement that the magnitude of the future ocean sink is uncertain “because the responses of the carbon cycle to future changes in climate are uncertain” could use more nuance. The magnitude of the future ocean sink also depends on other factors such as what GHG emissions pathway is followed, possible ocean circulation changes, and natural internal system variability.

Key Finding 5.

• The message could be edited for clarity. There is an important reference to feedbacks here, ergo the suggestion to include a conceptual feedback figure which details some of the feedbacks of concern.
• The first sentence (regarding T response per 1000 PgC) could be deleted, as this is a better fit for the NCA report.
• This is an important Key Finding but the message is blurred and most will not grasp the meaning of “this limit could be reached in as little as 20 years.” Is that to mean all fossil fuel CO₂ emissions world-wide must go to zero by 2037? Or that emissions then can only be those that are matched by say equal ocean uptake, etc?

Line-Specific Comments

P49, Line 14-20

Please reference years or range of years in Key Finding 1 as is done for Key Finding 2.

P49, Line 35

Change “slowed” to “decreased”, since there is a negative value for percentage change.

P49, Line 36

Flat growth is an oxymoron, it seems.

P50, Line 8-9

Please clarify, e.g., mitigation activities, such as…

P50, Line 13-15

This wording is awkward, rewording is suggested.

P49, Line 13 – P50, Line23

Missing from the findings are a direct reference to (1) new global emissions estimates from inland waters and (2) the proportion of cement production emissions contributing to global CO₂. The data for (1) are in Fig 1.1

P50, Line 24

This chapter should start with some introductory text.

P50, Line 24 – P51, Line 15

This section should be edited for content and clarity. (Role of Carbon—where, for what?)

P50, Line 25-27

The first couple of sentences are vague. See Cole, 2013.

P51, Line 16-21

Some background discussion about the Earth system would be helpful here: for instance, give % marine, %land, % terrestrial, % freshwater, % ice.

P51, Line 16-21

Missing from the system discussion and from Figure 1.1 are feedbacks; this should be included.

P51, Line 38-40

Add temporal reference for slow carbon cycles and geologic reservoirs.

P52, Line 4-11

Mention spatial extent of carbon stored in soils, permafrost, etc.

Page 52, Line 29

The Southern Ocean is the largest region of carbon sink (e.g. Gruber et al., 2009). It is more diffuse than the North Atlantic, but acts over a much larger area. It should be mentioned here for

completeness. Also, “top-down” estimates of the North American CO₂ sink are tied to the highly uncertain magnitude of the Southern Ocean sink.

Page 52, Line 31

While small interannual variability of the ocean is what Wanninkhof et al., 2013 suggests, newer information suggests that the variability is likely substantially larger than previously thought (Landschützer et al., 2015).

P52, Line 37-38

This reads as if El Nino of 2016 is the only driver.

P53, Line 4

Add time frame

P53, Line 7

Why “note?”

P53, Line 36

Many of these natural processes are anthropogenically influenced or are a results of feedbacks.

P54, Line 6

Since 1870? From when to 1870? Is the time frame 1870-2014?

P54, Line 7-11

Need references and or a synthesis figure to support this assertion.

P54, Line 27-28

It seems odd to include the reference on how to avoid emissions here.

P54, Line 29-32

Name major sources of OH, to provide more context for this sentence.

P54, Line 22-32

This paragraph mixes emissions and reduction of emissions strategies and processes.

P55, Line 20

What time frame is used for the cumulative emissions discussion?

P55, Line 33-39

The accounting is not clear as written. North American emissions (from fossil fuel burning and cement production only—not inland waters?) are being compared to the terrestrial sink in North America?

P56, Line 3-4

Where is the boundary vis-à-vis ocean uptake? Is any part of that considered a North American sink? This is unclear.

P56, Line 16

The text would benefit from a figure showing feedbacks and how they may interact to influence the future carbon cycle.

P56, Line 25-27

As is pointed out elsewhere in the document, the situation is not quite as direct as CO₂ causing a direct fertilization effect.

P56, Line 32-35

This paragraph is rather superficial. In particular, deeper discussion of the likely future response of the ocean sink is needed, as it is not as simple as suggested here. There is substantial uncertainty in future uptake, as a function of ocean circulation, warming, chemical changes (Lovenduski et al., 2016; Randerson et al., 2015). The sensitivity of the ocean sink to emission pathways needs more study. The idea of a continually growing sink in the ocean only applies under a high emission trajectory. If a low emission trajectory is taken, the ocean should outgas carbon, particularly in the subtropics where waters with high anthropogenic carbon content are circulating in the upper ocean (DeVries et al., 2017).

P56, Line 4

The title for this subsection is mismatched with content.

P57, Line 26

Safe for what?

P59, Line 4-5

There are too many “furthermores” used here.

P60, Line 14

This should be estimates of cumulative carbon emissions, correct?

P61, Line 22-26

It would be helpful to see a summary figure showing the economic and emissions data.

P61, Line 35-39

This is a vague paragraph.

P62, Line 15-17

What about emissions from inland waters?

Page 62, Line 15-26

The presentation here suggest equal uncertainty in the land and ocean sinks, which is not the case (Le Quéré et al., 2016; 2017). In addition, the approach to estimation of the ocean sink is mis-represented. The cumulative ocean sink is best constrained using ocean interior data (DeVries, 2014; Khatiwala et al., 2009; 2013; Sabine et al., 2004; Sabine and Tanhua, 2010), surface ocean pCO₂ data can provide independent confirmation of the magnitude of the mean sink (Landschützer et al., 2013; 2014; 2015; 2016; Takahashi et al., 2009). There remains substantial uncertainty with respect to interannual variability in the global carbon cycle, with models believed to strongly underestimate the actual

variability (Landschützer et al., 2015). These models are quantitatively tied to the estimates from interior ocean data and surface ocean pCO₂.

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