Contraction of the World’s Storm-Cloud Zones the Primary Contributor to the 21st Century Increase in the Earth’s Sunlight Absorption

Observations of the Earth’s energy budget from the CERES instrument have shown that since the beginning of the 21st century the amount of sunlight absorbed by the Earth has been increasing at a rate of about 0.45 W/m2 per decade, caused primarily by a decrease in sunlight reflection by clouds. This increase is a main component of the increase in the Earth’s Energy Imbalance by about 0.5 W/m2 between the first and second decades of the century. We analyze the CERES radiative budget trends of the past 23 years, with the objective to separate the contribution to those trends of changes in the large-scale atmospheric circulation and in local-scale cloud controlling processes. Regimes of large cloud cover and strong cloud radiative cooling are defined in the low latitude and the high latitude zones, representing the tropical rainy zone and the midlatitude storm zones respectively, and the trends in the areal coverage of those regimes over the past 23 years are examined along with the trends in the cloud solar radiative effect within each regime. This allows the decomposition of the global solar cloud radiative trends into circulation induced changes and those induced by local-scale processes. The results show that the circulation component of the cloud radiative changes, which manifests itself as a contraction of the midlatitude storm zones and the tropical rainy zone, is the dominant term in the solar reflection trend causing decreased sunlight reflection of 0.37 W/m2 per decade. The discovery of this component provides a crucial missing piece in the puzzle of the 21st century increase of the Earth’s Energy Imbalance and points to the large effect that even small atmospheric circulation changes have on the Earth’s warming climate.