Extensive Decline of Reflective Clouds over the North Atlantic and Northeast Pacific from Aerosol Reductions

Over the past several decades, the proportion of solar radiation reflected back into space has declined, accelerating the accumulation of heat within the Earth system. Satellite observations provide compelling evidence for the loss of reflective marine clouds and rising sea surface temperatures in the Northern Hemisphere. Natural climate variability is unlikely to be the primary cause of this cloud reflectivity decrease, which is poorly understood. Here we show that the marine cloud reflectivity, as measured by the shortwave cloud radiative effect, decreased on average by 2.8 +/- 1.2% per decade in the combined North Atlantic and Northeast Pacific regions between 2003 and 2022. The majority of the Earth System Models we analyzed simulated a cloud reflectivity decrease that is significantly less than observed in these regions. Our simulations using an updated aerosol-climate model show that reductions in sulfur dioxide and other air pollutants accounted for 69% (range 55 to 85%) of the decrease through aerosol-cloud interactions, consistent with the observed aerosol optical depth and cloud droplet number trends. These emission reductions are projected to persist over the next few decades, which raises the prospect of a continuing cloud reflectivity decrease and warming enhancement in these regions and globally.