Anthropogenic impact on Earth’s hydrological cycle

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The impact of climate change on the global hydrological cycle is unclear, with land precipitation and river discharges not increasing as expected. This discrepancy is investigated and tropospheric aerosols are found to have weakened the hydrological cycle between the 1950s and 1980s. The increase in greenhouse gases since the 1980s strengthened the cycle, indicating a further increase in precipitation if the current trend continues. The global hydrological cycle is a key component of Earth’s climate system. A significant amount of the energy the Earth receives from the Sun is redistributed around the world by the hydrological cycle in the form of latent heat flux1. Changes in the hydrological cycle have a direct impact on droughts, floods, water resources and ecosystem services. Observed land precipitation2,3,4 and global river discharges5 do not show an increasing trend as might be expected in a warming world6,7,8,9,10,11. Here we show that this apparent discrepancy can be resolved when the effects of tropospheric aerosols are considered. Analysing state-of-the-art climate model simulations, we find for the first time that there was a detectable weakening of the hydrological cycle between the 1950s and the 1980s, attributable to increased anthropogenic aerosols, after which the hydrological cycle recovered as a result of increasing greenhouse gas concentrations. The net result of these two counter-acting effects is an insignificant trend in the global hydrological cycle, but the individual influence of each is substantial. Reductions in air pollution have already shown an intensification in the past two decades12,13,14 and a further rapid increase in precipitation could be expected if the current trend continues.