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Aerosol-cloud interaction is the most uncertain mechanism of anthropogenic radiative forcing of Earth's climate, and aerosol-induced cloud water changes are particularly poorly constrained in climate models. By combining satellite retrievals of volcano and ship tracks in stratocumulus clouds, we compile a unique observational data set and confirm that liquid water path (LWP) responses to aerosols are bidirectional, and on average the increases in LWP are closely compensated by the decreases. Moreover, the meteorological parameters controlling the LWP responses are strikingly similar between the volcano and ship tracks. In stark contrast to observations, there are substantial unidirectional increases in LWP in the Hadley Centre climate model, because the model accounts only for the decreased precipitation efficiency and not for the enhanced entrainment drying. If the LWP increases in the model were compensated by the decreases as the observations suggest, its indirect aerosol radiative forcing in stratocumulus regions would decrease by 45%.

Plain Language Summary It remains unclear how much of the global warming induced by greenhouse gases is offset by aerosols because the effect of aerosol particles on clouds is the most uncertain mechanism of forcing of Earth's climate by human activities. Cloud water responses to aerosols are especially uncertain. Here we compare the properties of low marine clouds impacted by volcanic and ship emissions with the properties of the nearby unpolluted clouds in order to increase the understanding of aerosol impacts on clouds. Clouds impacted by emissions from volcanoes and ships lose or gain water depending on meteorological conditions, but on average the amount of water does not change much in the polluted clouds. These observations disagree with the systematic increases in cloud water in response to aerosols simulated by the Hadley Centre climate model. This model, like other contemporary climate models, only accounts for cloud water increases that result from decreased precipitation efficiency and does not account for the enhanced drying in polluted clouds. Our results suggest that the ability of aerosols to offset global warming might be overestimated. The observational constraints derived here on aerosol-induced cloud water changes would ultimately translate into reduced uncertainties in projections of the future climate.