Polluted clouds at air pollution hot spots help to better understand anthropogenic impacts on Earth’s climate

It is unclear to what extent the cooling effect of anthropogenic air pollution particles, known as aerosols, counteract the warming effect of greenhouse gases. In particular, it is uncertain how strong the cooling effect caused by aerosol-induced changes in cloud properties is. Clouds and precipitation can form in the Earth's atmosphere thanks to aerosols. However, cloud thickness, coverage, and lifetime may be perturbed when anthropogenic activities add additional aerosols to clouds, leading to the formation of more numerous, but smaller droplets. We show that it is possible to quantify these perturbations by comparing the properties of polluted clouds at air pollution hotspots with those of nearby unpolluted clouds. There are large-scale polluted cloud areas around the world that are hundreds of kilometres in size, yet surrounded by distinctively less polluted cloud areas. We show that such strong anthropogenic cloud perturbations occur intermittently and only under favourable meteorological conditions. We challenge the assumption of a unidirectional increase in cloud thickness in current climate models and show that, on average, cloud thickness does not increase in response to aerosols. This suggests that the cooling effect of anthropogenic aerosols on Earth's climate may not be as strong as previously thought. Our results will ultimately lead to more reliable climate projections.