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-yr-long fully coupled Community Earth System Model, version 1 (CESM1), experiments under abrupt CO2forcing. By simulation end, 2 x CO2global warming was 20% larger than 0.5 3 CO2global cooling. Not only was the absolute global effective radiative forcing ∼10% larger for 2 x CO2than for 0.5 3 CO2, global feedbacks were also less negative for 2 x CO2than for 0.5 3 CO2. Specifically, more positive shortwave cloud feedbacks led to more 2 x CO2global warming than 0.5 3 CO2global cooling. Over high-latitude oceans, differences between 2 x CO2warming and 0.5 3 CO2cooling were amplified by familiar linked positive surface albedo and lapse rate feedbacks associated with sea ice change. At low latitudes, 2 x CO2warming exceeded 0.5 3 CO2cooling almost everywhere. Tropical Pacific cloud feedbacks amplified the following: 1) more fast warming than fast cooling in the west, and 2) slow pattern differences between 2 x CO2warming and 0.5 3 CO2cooling in the east. Motivated to quantify cloud influence, a companion suite of experiments was run without cloud radiative feedbacks. Disabling cloud radiative feedbacks reduced the effective radiative forcing and surface temperature responses for both 2 x CO2and 0.5 3 CO2. 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.