Global and Regional Climate Feedbacks in Response to Uniform Warming and Cooling

We compare the radiative feedbacks in an ensemble of global climate models under uniform 4 K warming and cooling of sea surface temperatures. The global-mean net feedback is less stabilizing under warming in all nine models. This results primarily from less stabilizing water vapor and extra-tropical shortwave cloud feedbacks, partially offset by more stabilizing tropical cloud feedbacks. The zonal-mean feedbacks are also robust across the ensemble. In the extra-tropics, the less stabilizing shortwave cloud feedback under warming is associated with further poleward migration of the mean Southern Hemisphere jet in some models. However, additional experiments with an aquaplanet version of the HadGEM3 model suggest that the asymmetry of the jet shift is not driving the asymmetry in the cloud feedbacks at these latitudes. In the tropics, the stronger water vapor feedback under warming is offset by weaker shortwave cloud feedbacks. The result is that the ensemble spread in the differences between the global feedbacks under warming and cooling is mainly driven by their differences in the tropics. The spatial distribution of the feedbacks largely reflects the zonal-mean behavior, although there is considerable model spread in the regional cloud feedbacks, particularly in the tropical shortwave cloud feedback. Comparison with CO2- and solar-forced coupled experiments suggests that the global-mean longwave cloud feedback is nearly invariant to warming and cooling, regardless of the nature of the forcing. The shortwave cloud feedback is generally more positive under warming in the coupled models, consistent with the uniform SST perturbation experiments. A longstanding question in climate science is whether the study of cooler past climates can help us to understand future climate change in response to increasing CO2 and other greenhouse gases. Answering this question is difficult because when the Earth was much colder the climate itself was quite different from today's. This means that we cannot be sure that feedbacks in the climate system operated as they do now, or as they might do in the future. To simplify the problem we have examined climate model experiments in which the surface temperature of the oceans is alternately warmed and cooled by a fixed amount relative to the present day. This provides us with a baseline for understanding the differences between more realistic scenarios of past and future climates. We highlight the role of clouds and atmospheric water vapor in determing these differences and the relative importance of the tropics compared to higher latitudes. The global climate feedback is robustly less stabilizing under warming compared to coolingThis is driven by water vapor and extra-tropical shortwave cloud feedbacks, partially offset by more stabilizing tropical cloud feedbacksSimplified climate model experiments are a valuable tool to help us understand climate feedbacks in colder and warmer climates