|Review of: Assessment of negative and positive CO2 emissions on global warming metrics using a large ensemble Earth system model simulations|
Vakilifard et al.
The revised version of the paper is greatly improved from the original version and most of my critiques have been satisfactorily addressed. There are a few remaining issues that need to be addressed before publication but these can be addressed with more careful explanations and wording of the paper. Once these issues are addressed I believe that the paper will be ready for publication.
(1) While improved from the original version of the paper the use of ZEC is still not quite correct. ZEC is 'Zero Emission Commitment', however in your experiments emissions of non-CO2 greenhouse gases and aerosols continue for the period where ZEC is assessed. To avoid confusions and apples to oranges comparisons I recommend defining an 'effective Zero Emissions Commitment' eZEC analogous to eTRCE used in the paper. By clearly defining such a metric future research and reviews will not so easily get tripped up by inconsistent definitions of ZEC.
(2) Organization of the paper is a bit weird. Results basically begin at line 200, part-way through the Methods section.
Abstract: There is not enough context in the abstract to clearly understand what Lines 41 to 42 and 45 to 46 mean. I recommend re-writing the abstract to either add context or remove these sentences.
Line 53: Add a citation to the text of the Paris Agreement.
Line 87: Add a comma after 'biosphere'
Line 100: 86 is not really a 'large' ensemble. Similar studies have used 250 or 1000 model variants (e.g. Steinacher & Joos 2016, MacDougall et al. 2017). Maybe just say 'an ensemble'
Line 101: You should add a sentence to acknowledge the very high uncertainty in land-use reconstructions prior to about 1800 CE. Although the 850 CE start date was used by Eby et al. 2013 and other models intercomparison and studies, it is now well known that the land-use reconstructions used for forcing those experiments was based a very poor population estimate data-sets and Eurocentric conceptualizations of land-use. See Koch et al 2019, for a review of the problems in the Americas (the reconstruction used by Eby et al is designated P08 in Koch et al 2019).
Line 103: Add 'climate' after pre-industrial
Line 181: How did you account for the transition from historical RCP datasets ending in 2005 to SSP 2.6 beginning in 2015? Are there any discontinuities in the forcing data-sets?
Figure 1: Add CE after year for clarity. Model years and years BP are also commonly used in ESM studies.
Figure 2: Gray lines with a mean value in black or another colour on-top would be clearer than the rainbow shown.
Line 254 and 255: You are conflating the natural world and your model here. In your model the ocean is the only major energy sink, while it is in the natural world that the ocean takes up ~90\% of heat.
Line 273: Also cite Koven et al. 2022
Figure 7: Be clear that these are cumulative not instantaneous fractions.
Line 425: Cite MacDougall et al. 2017 here, which had a similar result for climate sensitivity.
Line 476: Reword for clarity. When a nation-state no longer wants to abide by a treaty they 'leave' it. Thus the wording here is very confusing. Revise to specifically mention temperature targets.
Eby M, Weaver AJ, Alexander K, Zickfeld K, Abe-Ouchi A, Cimatoribus AA, Crespin E, Drijfhout SS, Edwards NR, Eliseev AV, Feulner G. Historical and idealized climate model experiments: an intercomparison of Earth system models of intermediate complexity. Climate of the Past. 2013 May 16;9(3):1111-40.
Koch A, Brierley C, Maslin MM, Lewis SL. Earth system impacts of the European arrival and Great Dying in the Americas after 1492. Quaternary Science Reviews. 2019 Mar 1;207:13-36.
Koven CD, Arora VK, Cadule P, Fisher RA, Jones CD, Lawrence DM, Lewis J, Lindsay K, Mathesius S, Meinshausen M, Mills M. Multi-century dynamics of the climate and carbon cycle under both high and net negative emissions scenarios. Earth System Dynamics. 2022 May 16;13(2):885-909.
MacDougall AH, Swart NC, Knutti R. The uncertainty in the transient climate response to cumulative CO2 emissions arising from the uncertainty in physical climate parameters. Journal of Climate. 2017 Jan 15;30(2):813-27.
Steinacher M, Joos F. Transient Earth system responses to cumulative carbon dioxide emissions: linearities, uncertainties, and probabilities in an observation-constrained model ensemble. Biogeosciences. 2016 Feb 23;13(4):1071-103.
This contribution assesses the benefits of negative emissions / CO2 removal technologies deployment for future climate states using an ensemble of intermediate complexity earth system model results. The authors use effective transient climate response to cumulative CO2 emissions (eTCRE) and zero emissions commitment (ZEC) as metrics to quantify these impacts. The authors find that thermal dependance and airborne fraction of CO2 contribute almost equally to the uncertainty in eTCRE, which is in contrast with recent analysis of the CMIP6 ensemble. Additionally, the authors find that negative emissions deployment can help avoid continued warming after net-zero emissions are reached. The manuscript is clear and well-written and the analysis appears free of errors. However I have several recommendations aimed at increasing the impact and clarity of this work, which are detailed below.
The authors use the RCP 4.5 medium-level mitigation scenario as a benchmark to assess the future climate response to negative CO2 emissions. However much of the discussion of prospective large-scale negative emissions deployment in the recent literature focuses on their use towards limiting end-of-century warming to well-below 2 C, more consistent with RCP 2.6 or RCP 1.9. Although even the “medium” mitigation scenario may seem optimistic relative to the present real-world trajectory, using one or both of these forcing scenarios representing even deeper levels of mitigation could increase the impact of this work. I recommend the authors run similar analysis on one or both of these deeper mitigation scenarios, even as a sensitivity case. This could allow the modeling community, policymakers, and other stakeholders insight into what a “best case” scenario might look like in terms of transient climate response and committed warming.
In the final paragraph of the conclusions section the authors refer to the need for negative emissions technologies that have naturally long storage times. In the main body of the manuscript it would be helpful to describe exactly what types of carbon removal technologies are represented in the models used to develop the ensemble. Long-lived and permanent storage such as direct air capture or enhanced rock weathering referred to in the conclusions? Or biospheric such as afforestation? Or is the representation of negative emissions agnostic as to the source in the models? Any biospheric contribution to the negative emissions and potential feedbacks or the limitations in representing them should be identified in the discussion around land carbon.
The authors should be more precise in differentiating point source carbon capture from carbon capture from the atmosphere for negative emissions. Throughout the manuscript the more generic “carbon capture” or “carbon capture and storage” is used. While readers might infer from context that this is referring to negative emissions, this term should be clearly defined at every use to avoid the possibility of misinterpretation.