Surprise! Global hydrological cycle is in social space #anthropocene

Complex systems often have elements of surprise.

Here is an important one, just published in Nature Climate Change. It links two factors – aerosol pollution and carbon emissions – to show how they have been balancing each other off. This helps to explain why the hydrological cycle was not as intense as initially expected under global warming scenarios. It is because aerosols muted some of the warming effects on the hydrological cycle. But as we have cleaned up our air (to some degree) the effects of warming gain momentum.

This is an important finding regarding how key drivers of the planet’s life support system (i.e. water) are inside the social space from within which we govern things like our aerosol emissions. Or, alternately, the space in which we fail to govern adequately, as in the case of carbon emissions.

Anthropogenic impact on Earth’s hydrological cycle

Peili Wu, Nikolaos Christidis & Peter Stott

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 flux. Changes in the hydrological cycle have a direct impact on droughts, floods, water resources and ecosystem services. Observed land precipitationand global river discharges do not show an increasing trend as might be expected in a warming world. 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 decadesand a further rapid increase in precipitation could be expected if the current trend continues.

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