A Comparison of Regional Climate Changes Under Idealized Zero-Emissions and Exceedance-and-Return Experiments

Abstract. We examine the behavior of ten Earth system models under a set of idealized CO₂ emissions-driven simulations to look at regional temperature and precipitation changes in response to either an abrupt cessation of emissions or an idealized exceedance-and-return pathway, in which emissions linearly ramp down from positive to negative emissions. This allows a comparison of the regional climate at three different time points, all of which have the same level of cumulative CO₂ emissions and therefore the same expected global warming amount. I.e., this lets us ask how regional climates would differ if we first emit, and then remove, a given amount of CO₂, as compared to a case where the CO₂ past a certain amount hadn't been emitted, and the coupled carbon-climate system had been left to freely evolve over the same length of time. We find consistently higher global precipitation for a given global temperature after zero emissions in both pathways, with a larger increase under the exceedance-and-return pathway than the zero-emissions pathway. Averaging over land and ocean domains, we find cooling over land relative to ocean, which is also stronger under exceedance-and-return than zero-emissions, and for about half of the models a difference in the relationship between land-ocean temperature contrast and global mean temperature change than occurs under positive emissions. We find low agreement across different models in the regional patterns of both temperature and precipitation change in all scenarios. For most of the models, there is a strong gridcell-level correlation between the temperature changes in the exceedance-and-return and zero-emissions responses, such that roughly half of the regional variance in temperature change at a given location that occurs under the exceedance-and-return pathway also occurs under zero emissions. Because only a single realisation of each experiment is available, this shared fraction is a lower bound on the common forced response: internal variability, which is uncorrelated between the two experiments, acts to reduce the apparent correspondence rather than inflate it. However, for some models there is low or negative correlation between the regional climate changes in the two cases, indicating a potential for fundamentally different regional climate change under exceedance-and-return than under zero emissions. Regional precipitation changes are more different between the exceedance-and-return and zero-emissions responses for all models.