Global Carbon Budget 2019

Friedlingstein, Pierre; Jones, Matthew W.; O'Sullivan, Michael; Andrew, Robbie M.; Hauck, Judith; Peters, Glen P.; Peters, Wouter; Pongratz, Julia; Sitch, Stephen; Le Quéré, Corinne; Bakker, Dorothee C. E.; Canadell, Josep G.; Ciais, Philippe; Jackson, Robert B.; Anthoni, Peter; Barbero, Leticia; Bastos, Ana; Bastrikov, Vladislav; Becker, Meike; Bopp, Laurent; Buitenhuis, Erik; Chandra, Naveen; Chevallier, Frédéric; Chini, Louise P.; Currie, Kim I.; Feely, Richard A.; Gehlen, Marion; Gilfillan, Dennis; Gkritzalis, Thanos; Goll, Daniel S.; Gruber, Nicolas; Gutekunst, Sören; Harris, Ian; Haverd, Vanessa; Houghton, Richard A.; Hurtt, George; Ilyina, Tatiana; Jain, Atul K.; Joetzjer, Emilie; Kaplan, Jed O.; Kato, Etsushi; Klein Goldewijk, Kees; Korsbakken, Jan Ivar; Landschützer, Peter; Lauvset, Siv K.; Lefèvre, Nathalie; Lenton, Andrew; Lienert, Sebastian; Lombardozzi, Danica; Marland, Gregg; McGuire, Patrick C.; Melton, Joe R.; Metzl, Nicolas; Munro, David R.; Nabel, Julia E. M. S.; Nakaoka, Shin-Ichiro; Neill, Craig; Omar, Abdirahman M.; Ono, Tsuneo; Peregon, Anna; Pierrot, Denis; Poulter, Benjamin; Rehder, Gregor; Resplandy, Laure; Robertson, Eddy; Rödenbeck, Christian; Séférian, Roland; Schwinger, Jörg; Smith, Naomi; Tans, Pieter P.; Tian, Hanqin; Tilbrook, Bronte; Tubiello, Francesco N.; van der Werf, Guido R.; Wiltshire, Andrew J.; Zaehle, Sönke

doi:https://doi.org/10.5194/essd-11-1783-2019

Articles | Volume 11, issue 4

Earth Syst. Sci. Data, 11, 1783–1838, 2019

https://doi.org/10.5194/essd-11-1783-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Earth Syst. Sci. Data, 11, 1783–1838, 2019

https://doi.org/10.5194/essd-11-1783-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 11, issue 4

Data description paper 04 Dec 2019

Data description paper | 04 Dec 2019

Global Carbon Budget 2019

Pierre Friedlingstein et al.

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Final revised paper (published on 04 Dec 2019)
Preprint (discussion started on 10 Oct 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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EC1: 'Editor Comment', David Carlson, 10 Oct 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Pierre Friedlingstein on behalf of the Authors (10 Oct 2019) Author's response

ED: Referee Nomination & Report Request started (10 Oct 2019) by David Carlson

RR by H. Damon Matthews (21 Oct 2019)

Suggestions for revision or reasons for rejection

This is a solid update to the annual global carbon budget dataset. I have only a few minor comments:

1) There is an increasingly problematic terminology confusion that has arisen as a result of the phrase "carbon budget" being used for two completely different concepts -- i.e. the current use (current sources and sinks of CO2) and the "emissions budget" use (where "carbon budget" refers to the total or remaining allowable cumulative emissions for a given climate target). A nice solution to this problem would be to adopt here the phrase "global carbon cycle budget" here rather than "global carbon budget" -- this would serve to differentiate more clearly from the "remaining carbon budget" which is increasingly widely used as a term but is not at all the same thing as what is reported in the current paper.

2) Along the same lines, it would be very helpful to clearly distinguish between what "carbon budget" means in this paper and the allowable emissions use of "carbon budget" elsewhere in the literature. I had a colleague recently refer to Corine Le Quere as *the* world expert on calculating remaining carbon budgets ... which does of course speak highly of her reputation, but is also not really accurate since this would be the other kind of carbon budget. So it would help to provide this distinction at the outset of the paper (which would be easier if you would consider adopting "global carbon cycle budget" ...)

3) On lines 158-159 the authors refer to 1850 as the base year for historical simulations in CMIP6, and 1870 as the base year for cumulative emissions in AR5 based on the availability of global temperature data. It would be nice to update this: the upcoming IPCC report will use 1850-1900 as a consistent base year for both model simulations and historical temperature data, so it would make sense to align the estimate of historical cumulative emissions in Table 8 with this new standard base period -- i.e. either use the mid-point (1875, rather than 1870), or subtract the mean annual emissions for 1850-1900 from the sum since 1850 so as to calculate the total relative to this base period.

4) I am not sure about the statement on lines 1287-1291 regarding the recommendation to calculate the remaining carbon budget as an anomaly from present-day. I understand where this is coming from, but I would argue that the uncertainty in historical cumulative emissions is also relevant to the uncertainty in the remaining budget, and discarding this does tend to artificially deflate the uncertainty in the remaining budget estimate.

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RR by Albertus J. (Han) Dolman (22 Oct 2019)

Suggestions for revision or reasons for rejection

The authors are, once again, to be congratulated on putting together this comprehensive framework for evaluating the historical and the current global carbon budget. I have a few minor comments, as there are no major changes in methodology compared to last year’s budget.

L 61. I would suggest to move line 61-l 65 closer to the end of the abstract, after line 72. That makes the reading a bit more logical. To be honest, I am not quite sure of the added value of the projections. They are somewhat relevant, but maybe a bit out of place in the paper and would benefit from a separate paper explain more about the details and problems involved.

L 107. Insert “fully” before accounted

L449. Suggest that only in H&N peat burning and drainage are added. When reading line 456 one notices that this is also the case for BLUE. I would suggest to phrase this not as a discrepancy but as a similarity, so: ”In both H& Blue….”

L 458. delete “also”

l 742. In my understanding el-Nino is an atmosphere-ocean interaction, with a dominant variability of the order of a few years, rather than less than one year. I may be wrong but I do not quite understand this.

L 935. I know you have a lot of figures, but I would prefer B4 to be in the main textt, as it shows the history of the paper, and importantly the progress in calculating the budget.

L977. It is a pity that one of the reasons why one would prefer to use a DGVM (not so much for calculating land use change effects), for estimating the response of the sink to climate and CO2, does not clarify this issue. Is there not a clever way to separate these two effects? If not currently existing, what would be the way forward?

L993. While honest, it is disturbing that DGVMs cannot do well in terms of NEE and some of the other carbon variables. Last year I made a remark on lack of progress in inversions. In this case I would also like to see a stronger statement on their failings and maybe their way forward. The fact that they exist is in itself no good reason to use them.

L979-l1001. It would be nice if the uncertainties mentioned here would be made comparable, for instance by giving bias and RSME for all three of them in similar units.

L1023. factor>factors

L1143. There is increasing literature on the impact of land management, not just forest management. I would expand this statement by either adding a sentence on other land management or just delete the word forest.

L1147. Not sure the standard reply of getting more observations will help here. We have gotten a lot more in recent years. Maybe there still is a lack of understanding that needs to be clarified.

L 1287-1291. Inunderstand the caution the authors find appropriate, but lines 1289-1291 are now incomprehensible. It is also the question why this cannot be used in this way? It would be nice to see the current results being so strong that they could be used!

Another suggestion is to move the conclusions forward to after the discussion and end the paper with data availability. This would make the paper more readable for the majority of the authors who do not wish to dive into the data sources.

One final remark. It would help in reviewing the paper, if changes compared to last year are highlighted. That helps concentrating reviewers of a 130-page paper.

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RR by Anonymous Referee #3 (27 Oct 2019)

Suggestions for revision or reasons for rejection

Verbatim comments from additional (third) reviewer:

"The global carbon budget 2019 provides an updated synthesis of multiple data streams and modelling results to assess the traditional global carbon budget and its uncertainty over various time scales. The text is well written and the overall methodology, also the changes with respect to previous editions, are defendable. Small comments on the present text are given below.

I have two fundamental comments, which are probably more important for a next edition of the budget, since this version is anyway to be published very soon under time constraints.

1) Who will read this paper? As it stands, it is primarily a reference for scientists who are interested in the details of where the datasets come from and how they were compiled and synthesised into the global numbers presented here. As such it is quite comprehensive and complete, and thus adequate for a data paper in ESSD. However, the global carbon budget is also of relevance for stakeholders as the authors of this paper allude to. For this readership the paper is way too complicated and detailed. Non-specialists and non-carbon cycle scientists will get the impression that understanding the global carbon balance is a very complex undertaking. Even the key information of the whole paper (except for Figure 2), Tables 5 and 6 are now very complicated - compare this e.g. with the global carbon budget table in the first assessment report of IPCC (1992, p. 13). The numbers given there are almost the same as those in the present Table 6 for the decade 1980-89 (within their or the present uncertainties). And at that time no sophisticated process models were used to establish the global budget. Thus the concept of the global carbon cycle must be fundamentally quite simple. I think a terse kind of “policymaker summary” summarising the key messages/numbers would make very much sense. And much of the present detailed analysis/synthesis/model intercomparison and model evaluation material could go into an appendix as reference and documentation.

2) In-line with the comment above, I believe the present methodology is way too much based on comprehensive models. Observational constraints for the budget or individual terms are only used as (coarse) evaluation of the complex process-based models. There are several non-model based approaches (e.g. isotopes, oxygen, bottom-up inventories, flux estimates etc.) which could provide independent constraints on the global budget. And it would also better highlight the value of observational data. The present model evaluation does this to some extent, but is not readily visible.
Also, complex model results are often for the non-specialists, especially for the skeptics (!), (a) suspicious, (b) can not be reproduced and (c) provide no insight. In climate science we have a whole hierarchy from complex high-end climate models over EMICS to conceptual simple models, which all are useful in their domain and which complement each other. Why not here? Ultimately the global carbon budget consists of just 4 independent time series that need to be explained and understood. I think it would make the message of the paper here much more strong - also to the non-specialist.

Minor comments:

p. 5 l 57: write “fossil emissions”
p. 5 l 59: The canonical factor converting ppm to GtC is 2.124. If so, 2.4 ppm/yr = 5.1 GtC/yr. Where does the small (0.01ppm) uncertainty for the decadal CO2 increase come from?
p.6 l 90: Indicate the year of the beginning of the industrial era
p. 7 l 115: Where does the factor 1 ppm = 2.124 GtC come from?
p. 21 l 533: Why are only positive E_LUC model output retained? Does this mean that the entire data of a model with E_LUC<0 during the 1990s is discarded? Or are negative values set to zero? This procedure is not described clearly.
p. 22 l 583 and Table 1: AR5 used as conversion factor 2.12 PgC/ppm, compiled in Prather et al., 2012.
p. l 943: The grow rates in %/yr are not shown in Figure 5.
p. 37 l 1009: This sentence could be challenged: Formally, the models were selected to fall within some consensus interval in 1990s, the models are driven by atmospheric CO2 and clearly, development was not “blind” to the global carbon budget constraint.
p 39 l 1081: Why is much of the material in section 3.2.3 not already included in section 3.1? Section 3.2 focuses on the last decade, while the key Figure in section 3.2.3 (fig 4) shows the entire classical budget broken into the three latitude bands. I understand that most of the observational constraints are available more for the recent decade(s), but much of the arguments put forward here relate to the time frame since the 1980’s (e.g. the discussion of the land budgets of the northern extratropics vs the tropics). I’d move section 3.2.3 up into section 3.1.
p 46 l 1287: This last paragraph is difficult to understand for the un-initated, since it moves beyond pure carbon cycle science. I’d restrict to say here (in a carbon cycle data paper) that there are remaining uncertainties in the cumulative budgets. Everything related to scenarios, remaining carbon budget, and the time frames for which it is to be calculated is nor really the topic of this paper.
p 93, Table 3: Am astonished: up to the budget in 2016 the terrestrial sink was estimated as a residual; the DGVM models were used as a corroboration. "

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ED: Publish subject to technical corrections (28 Oct 2019) by David Carlson

Good product, good preparations. Publication recommended by all three reviewers. Suggest you evaluate and respond to comments in two categories: what you can and have fixed/changed in this version and what valid comments you incorporate for future authors and future versions.

A few editorial comments follow:

Lines 207, 208 and 237, 238 and 1468: Andrew et al Cement Budget latest version should have reached publication status in ESSD. Change to ‘in press’ or full as-published reference? Certainly by proof stage?

Line 1189, parentheses wrong? I think you want “(range of 1.8 to 3.7%) published in Le Quéré et al. (2018b) based on national emissions”? Proofreaders will not catch this so you need to ensure you have punctuated it correctly for your intent.

I typically check references, because reviewers typically do not. But what reviewer/editor can evaluate 35 pages of references? I think reviewers found all references as expected in the narrative, but probably publishers need new editing software to check the output of your bibliographic software to confirm the full reference list? Machines checking other machines?

Order of appendices will sort itself. The suggestion by one reviewer to move conclusion section ahead of data availability section and then to finish the narrative with that data availability section seems useful. Many ESSD products follow that sequence.

All reviewers worry about the text in lines 1287-1291. I tend to agree. I see two issues. First, the emerging confusion between this careful carbon source/sink accounting, which you / we properly label as a ‘global carbon budget’ while - as one reviewer confirms - the much-less careful (one might even say ‘casual’ or ‘incautious’?) public and press use the term ‘carbon budget’ to refer to remaining carbon emissions. The research community follows your terminology: what you refer to as “permissible emissions for a given climate stabilization target” in line 97 and as the “”remaining carbon budget”” at lines 1288-1289. The carbon community will need to deal with this growing confusion. You tread carefully here, appreciated, but you may need to deal even more explicitly with ‘remaining carbon’ in the future. Second, and more closely related to the text at 1287-1291, as I read it you have introduced confusion rather than clarity: you urge caution at using cumulative emissions for any purpose but then you seem to encourage remaining carbon estimates by a (relatively) straightforward extrapolation from present day values into the future? You have several options here: a) delete these two sentences; b) keep only the first sentence, perhaps appended to the previous paragraph; or c) revise these sentences to convey a clearer message?

Lines 1405, 1406

In prior versions: “a growing understanding of and improved capacity to anticipate the evolution of the carbon cycle in the future”

In this version: “a better understanding of the causes of climate change, and an improved capacity to anticipate the evolution of the carbon cycle in the future”

The new phrase introduces a new hesitancy? You really do not intend such apparent confusion? (Indeed, why then do we have GGP and annual GCB?). I think you mean “better quantification of” or “better accounting of” in place of ‘better understanding’?

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AR by Pierre Friedlingstein on behalf of the Authors (08 Nov 2019) Author's response Manuscript

Short summary

The Global Carbon Budget 2019 describes the data sets and methodology used to quantify the emissions of carbon dioxide and their partitioning among the atmosphere, land, and ocean. These living data are updated every year to provide the highest transparency and traceability in the reporting of CO₂, the key driver of climate change.