Cleaner clouds are helping to slow the rate of global heating, a study shows. Falling levels of dust in the atmosphere are making clouds more reflective. This previously unidentified effect is not being taken into account by current climate models, which may mean they are slightly overestimating the projected rate of global heating.

Researchers analysed satellite observations of high clouds in the northern hemisphere and assessed the ratio of ice crystals to liquid droplets over the period 2008 to 2018. Their findings, published in Geophysical Research Letters, show that clouds have become less icy over that time. Weakening surface winds (due to climate change) have resulted in less dust being whipped up into the atmosphere over the last two decades. Dust particles help to seed ice crystals, so less dust means fewer ice crystals in clouds. The effect is not observed in southern hemisphere clouds because there is far less dust in the southern hemisphere atmosphere in the first place.

Liquid droplets reflect more sunlight than ice crystals do, and the findings suggest that the extra reflectivity from less icy clouds offsets about a quarter of the heating caused by other changes, such as an increase in high clouds, which trap more heat. It’s no reason to be complacent, but potentially this means global warming won’t happen quite as fast as current models predict.