Does Disabling Cloud Radiative Feedbacks Change Spatial Patterns of Surface Greenhouse Warming and Cooling?

Abstract

The processes controlling idealized warming and cooling patterns are examined in 150 year-long fully coupled Community Earth System Model version 1 (CESM1) experiments under abrupt CO2 forcing. By simulation end, 2xCO2 global warming was 20% larger than 0.5xCO2 global cooling. Not only was the absolute global effective radiative forcing ∼10% larger for 2xCO2 than for 0.5xCO2, global feedbacks were also less negative for 2xCO2 than for 0.5xCO2. Specifically, more positive shortwave cloud feedbacks led to more 2xCO2 global warming than 0.5xCO2 global cooling. Over high latitude oceans, differences between 2xCO2 warming and 0.5xCO2 cooling were amplified by familiar linked positive surface albedo and lapse rate feedbacks associated with sea ice change. At low latitudes, 2xCO2 warming exceeded 0.5xCO2 cooling almost everywhere. Tropical Pacific cloud feedbacks amplified: 1) more fast warming than fast cooling in the west, 2) slow pattern differences between 2xCO2 warming and 0.5xCO2 cooling in the east. Motivated to quantify cloud influence, a companion suite of experiments were run without cloud radiative feedbacks. Disabling cloud radiative feedbacks reduced the effective radiative forcing and surface temperature responses for both 2xCO2 and 0.5xCO2. Notably, 20% more global warming than global cooling occurred regardless of whether cloud feedbacks were enabled or disabled. This surprising consistency resulted from the cloud influence on non-cloud feedbacks and circulation. With the exception of the Tropical Pacific, disabling cloud feedbacks did little to change surface temperature response patterns including the large high-latitude responses driven by non-cloud feedbacks. The findings provide new insights into the regional processes controlling the response to greenhouse gas forcing, especially for clouds.

On the Effect of Historical SST Patterns on Radiative Feedback

“…Note that while our focus is on the atmospheric response to a given SST pattern, causality can work in both directions. For example, cloud feedback has been shown to have an impact on the pattern of tropical Pacific SST changes in models (Chalmers et al., 2022).…”

Asymmetric Warming/Cooling Response to CO2 Increase/Decrease Mainly Due To Non‐Logarithmic Forcing, Not Feedbacks

“…Specifically, CO 2 is varied from 1/8× to 8×PI values, to test the CO 2 symmetry of the climate system response to comparable increased and decreased CO 2 . While we are not the first ones to perform such symmetric CO 2 runs (Chalmers et al., 2022; Colman & McAvaney, 2009; Hansen et al., 2005; Russell et al., 2013), here we explore (a) a larger CO 2 range than previously considered, (b) we do so using two different fully coupled climate models and, most importantly, (c) we perform the experiments with both abrupt and transient CO 2 runs.…”

“…The question thus remains: is the climate system response symmetric across a broad range of positive (warm) and negative (cold) CO 2 forcings? The question of symmetry was examined recently by Chalmers et al (2022), who compared 1/2× and 2×CO 2 simulations performed with the CESM1-CAM5 model, and found that global surface temperatures warm 20% more than they cool. Roughly 50% of this asymmetry was shown to derive from an asymmetry in CO 2 radiative forcing; the rest was associated with differences in feedbacks which, interestingly, were found not to be related to clouds.…”

Causes of the weak emergent constraint on climate sensitivity at the Last Glacial Maximum

“…However, when comparing halving and doubling of CO2 from pre-industrial conditions, either little differences of global λ cl were found in the Australian BRMC model (e.g. Colman and McAvaney, 2009), or weaker λ cl in abrupt0p5xCO2 experiment than abrupt2xCO2 experiment in CESM1.2 (Chalmers et al, 2022).…”