PNNL

Abstract

Earth’s energy imbalance exhibits a robust positive trend over the past two decades, reaching a record value in 2023. Both the trend and year-to-year anomalies are primarily attributable to decreased reflection of sunlight by low-level clouds, which is pronounced over the Northern Hemisphere oceans. However, the causes of these low-cloud anomalies and their implications for future Earth system evolution are unknown, leading to speculation that the cloud feedback may be larger than expected. Here we quantify the meteorological factors that have driven interannual cloud-radiative anomalies, several of which aligned to cause the extreme global value observed in 2023. These meteorological variations are superposed on a background of decreasing sulfate aerosol concentrations, leading to a robust trend of reduced cloud reflection over the past 22 years. Constraints on cloud feedback and aerosol forcing implied by these observations support previous analyses and reaffirm an equilibrium climate sensitivity near 3?C with a likely (66% confidence) range of 2.7 to 4.1?C. Hence recent observations do not imply that a stronger-than-expected cloud feedback is emerging in nature or that future warming has been underestimated.