Observational Assessment of Changes in Earth’s Energy Imbalance Since 2000

Observations from the Clouds and the Earth’s Radiant Energy System (CERES) show a marked increase in Earth’s energy imbalance (EEI) since 2000. At the same time, we’ve seen marked changes in numerous geophysical variables that influence EEI. While observations alone cannot quantify the anthropogenic and natural contributions to changes in these quantities, they can provide insight into how changes in different components of the climate system have led to the observed EEI trend. Using additional data from MODIS, CALIPSO, Cloudsat, and reanalysis, we find the increase in EEI to be due to decreased reflection by clouds and sea-ice, which cause a pronounced increase absorbed solar radiation (ASR), and a decrease in outgoing longwave radiation (OLR) due to increases in trace gases and water vapor. The ASR increases are largest over the subtropics and mid-latitudes in regions with decreases in low and middle cloud fraction, which likely occur in response to observed increases in sea-surface temperature (SST) in those locations. We diagnose the SST changes by performing an ocean mixed layer energy budget analysis at regional, hemispheric, and global scales using TOA and surface radiation observations from CERES, SST and temperature/humidity fields from ERA-5, and ocean mixed layer depth from ocean reanalysis. This analysis suggests that heating of the mixed layer and the subsequent increase in SST stems from ocean mixing/advection rather than from surface forcing.