Trends in subtropical marine low-level cloud cover from 2007 to 2021

Marine low-level clouds play an important role in the Earth's energy budget. They reflect large amounts of incoming solar radiation that would otherwise heat the Earth's surface. Areas of persistent low-level stratocumulus clouds cover the subtropical eastern oceans where lower-tropospheric stability is high. Based on spaceborne lidar measurements, which allow to precisely identify clouds at different altitudes, we find that low-level cloud cover of Peruvian, Namibian, and Californian stratocumulus decreased by between 0.05 and 0.10 per decade between 2007 and 2021. A seasonally resolved analysis gives even stronger trends for those seasons in which low-level cloud cover is highest. No trend is found for Australian and Canarian stratocumulus. The decrease in low-level cloud cover is strongly tied to increasing sea-surface temperatures. Our analysis suggests statistical significant reductions in albedo for Namibian and Californian stratocumulus of 0.11 and 0.05, respectively, per 0.10 decrease in low-level cloud cover. Such changes will have a direct impact in the Earth's energy budget by reducing the cooling effect of major marine stratocumulus sheets.