|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.
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. "