Climate scientists have been trying to work out why new computer models have begun projecting a potentially much hotter future as CO2 levels rise. A new analysis gives our best idea yet – it seems to be to do with clouds. Ahead of the next major UN climate science panel reports in 2021, researchers have found their sixth generation of climate models show a much wider range for the future temperature than before, up from 1.5 to 4.5 C to 1.8 to 5.6 C. Those estimates are for when "equilibrium climate sensitivity" (ECS) occurs, a theoretical point when the climate system comes into equilibrium after CO2 levels have doubled. "There is definitely not one single common cause. But quite a lot of the models at the high end have introduced new, more sophisticated models of clouds and aerosols. That does seem to be the driver of the new, higher sensitivity," says Catherine Senior at the UK's Met Office.