| 12 Jan 2026
Asymmetry in carbon cycle feedbacks and transient climate response under positive and negative CO2 emissions
Abstract. Most emissions scenarios consistent with limiting warming to well below 2 °C above pre-industrial levels rely on carbon dioxide removal to offset residual positive emissions or achieve net-negative emissions. While carbon cycle and climate metrics are well quantified for positive CO2 emissions, applying the same metrics under negative emissions may over- or underestimate the effectiveness of carbon dioxide removal. This study uses an Earth system model to investigate the asymmetry in carbon cycle feedbacks and climate response under positive and negative CO2 emissions. To this end, symmetric concentration-driven simulations are initialized from a state at equilibrium with twice the preindustrial CO2 concentration and run in biogeochemically coupled, radiatively coupled and fully coupled modes. Our results suggest that land and ocean carbon cycle feedbacks are asymmetric. Compared to their respective magnitudes under positive emissions, the concentration-carbon feedback is larger, whereas, the climate-carbon feedback is smaller under negative emissions. Asymmetries in land carbon cycle feedbacks arise from the saturation of the CO2 fertilization effect and asymmetric temperature and soil respiration responses. Asymmetries in ocean carbon cycle feedbacks are driven by non-linear responses to CO2 and temperature change, as well as asymmetric ocean circulation responses. Asymmetries in carbon cycle feedbacks propagate onto asymmetry in the Transient Climate Response to Cumulative CO2 Emissions: a negative CO2 emission results in greater global mean temperature change than a CO2 emission of the same magnitude. Our study highlights the need to quantify metrics under negative emissions as reliance on metrics derived from positive emission scenarios may result in inaccurate quantification of the climate response under net negative CO2 emissions.
Review of “Asymmetry in carbon cycle feedbacks and transient climate response under positive and negative CO2 emissions” by Chimuka and Zickfeld.
This is a clear and well written manuscript that explores the symmetry of response of the carbon cycle under positive or negative emissions of CO2. This is an important question, and one of increasing interest as both science and policy are required to think more about aspects of overshoot and reversibility. Much more is known about the climate system in a steadily warming world than one where CO2 removal leads to cooling. This manuscript contributes to a research agenda to better address this issue.
The paper uses the UVic earth system model of intermediate complexity. This is a model widely used in the literature and suited to exploring this question. While the results from this model are interesting, and the analysis sheds light on processes involved, I find the value of the paper is more in terms of testing an experimental design which could be adopted more widely by other models. We know that such responses are model-dependent and even the sign of behaviour of terms such as ZEC can vary from model to model. So it is not so much that these results are definitive, but that they lead the way for other models to follow and for a wider research initiative into symmetry and reversibility.
I have some minor comments which I list below which I hope the authors find useful to clarify some points.
I do, though, have a major comment about the design and intent of the paper. The more I think about this question, the more I realise that there is a difference between “symmetry” and “reversibility”. It may feel nuanced, but I believe it is important.
The difference is that “symmetry” - as defined in this study - is asking the question “do we see the same response if we go upwards from a starting point or downwards from that same point?”, whereas “reversibility” would ask “do we see the same response going up from one point to another as we do going back down from the second point back to the first?”
Previous studies of reversibility (e.g. Boucher et al 2012, or CDRMIP simulations) have addressed the question of “going back down again”, BUT they have been contaminated by the problem that they have very large legacy effects on the way down because the simulation follows immediately from very strong upwards forcing and is not in steady state. The current authors know this well and discuss it in detail in Chimuka et al (2023). The present study therefore represents a novel experiment design and analysis by equilibrating at a higher level (here 2xCO2), thus avoiding the legacy response in the negative emissions phase. I think this is vital and a very nice experimental design.
However, if the question of interest is whether or not we can recover a previous climate state by the same path in reverse (i.e. does TCRR = TCRE?) then I am not sure that the current experiment answers exactly that question. The asymmetry found in this paper may be because the system response differs above 2xCO2 from below. For example, if TCRE is state-dependent (e.g. lets assume the TCRE gradient gets less steep at higher warming), then we would expect TCRE measured above 2xCO2 to be lower than TCRE measured up to 2xCO2. This would manifest as an asymmetry about 2xCO2 – that’s true. But it does not necessarily imply that reversibility is along different lines. TCRR and TCRE could still be identical to each other. Does that make sense?
A more direct answer to the reversibility question would be to compare your ramp-down experiments here with the ramp-up from 1xCO2 up to 2xCO2. So you can measure a TCRE going from 1xCO2 up to 2xCO2, and then a TCRR going back down (but from an equilibrated state at 2xCO2). I think this then answers the reversibility question.
I think you must have such experiments, but I realise that requesting them (and new analysis) to be added here is a big task. As the manuscript stands it does indeed answer the stated question regarding “symmetry”. But my challenge to the authors is whether or not that is really the target question? Given that work like this will undoubtedly be influential in experimental design for CMIP (CMIP7 and beyond), then making sure that we address the correct questions is as important as making sure we design the experiments to answer them. One option is to keep the present manuscript and be clear that the question asked is on symmetry but does not necessarily reflect reversibility. This opens up the opportunity for a follow-on paper on reversibility. Or a second option would be to add to the current study an analysis of positive vs negative emissions up vs down between the same points.
I appreciate a request for extra analysis is never welcome, but I hope the authors see this a constructive suggestion.
Chris Jones
Minor comments/suggestions: