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Global water cycle is revving up

Perhaps the most inconvenient thing about global warming is that mainly the poor will have to carry the can. While, for example, the amount of rainfall at wealthy and relatively freshwater-blessed mid and high latitudes is likely to increase as the climate warms, drought-prone regions such as the Sahel zone will likely get even drier.

Computer models used to simulate the effects of rising global temperature on the climate system at large do predict changes in the global water cycle whereby, metaphorically, the ‘rich get richer’ and the ‘poor get poorer’. An analysis published in the journal Science today of 50 years of ocean salinity data – an excellent indicator of a changing hydrological cycle – finds that existing models strongly underestimate the magnitude of the changes.

The laws of physics – namely vapour pressure dependence on temperature – dictate that a warm atmosphere can hold much more water vapour than a cold atmosphere. As global temperature rises, so will evaporation, atmospheric moisture content and precipitation. At the same time, global atmospheric circulation models suggest that the distribution of rainfall will change along a pattern that will dry subtropical areas and increase precipitation at higher latitudes.

As the world has already warmed around half a degree Celsius since 1950 the predicted changes should already be observable – except that there are not enough reliable measurements over land to confirm them without ambiguity. But in the oceans, which receive around 80 percent of global rainfall, their fingerprint is clearly visible.

Analysing some 1.7 million measurements of ocean salinity made between 1950 and 2000, Paul Durack a climate researcher at Lawrence Livermore National Laboratory in California and colleagues, found that relatively fresh ocean regions have got notably fresher, while regions already saltier than average got even saltier.

What’s more, the team found that the acceleration of the global water cycle proceeds much faster than current-generation climate models have been predicting. Changes since 1950 in ocean surface salinity suggest that the global water cycle speeds up at a rate of around 8% per each degree of warming – nearly double the average response seen in models. By 2100, when the world will likely have warmed another 2-3 degrees, the water cycle might intensify by 16-24 %, the scientists conclude.

What might that mean? Societies – not only in dry regions – are more vulnerable to changes in the water cycle than to temperature changes alone, experts point out. We would better not take it literally that the ‘rich get richer’ – even in moderate climates a speeding water cycle might bring about more heavy rain, floods and storms sooner than we think.

Figure: Surface salinity changes for 1950 to 2000. Red indicates regions becoming saltier, and blue regions becoming fresher. Paul Durack

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