Global warming may spread draught to third of Earth

One third of the world may be at increased risk of drought by 2100 as warmer temperatures wring more moisture from the soil, a new study has warned.
Increasing heat is expected to extend dry conditions to far more farmland and cities by the end of the century than changes in rainfall alone, researchers said.
Much of concern about future drought under global warming has focused on rainfall projections, but higher evaporation rates may also play an important role as warmer temperatures wring more moisture from the soil, even in some places where rainfall is forecasted to increase, they said.

The study is one of the first to use the latest climate simulations to model the effects of both changing rainfall and evaporation rates on future drought.
The study estimates that 12 per cent of land will be subject to drought by 2100 through rainfall changes alone; but the drying will spread to 30 per cent of land if higher evaporation rates from the added energy and humidity in the atmosphere is considered.
An increase in evaporative drying means that even regions expected to get more rain, including important wheat, corn and rice belts in the western US and southeastern China, will be at risk of drought. The study excluded Antarctica.

"We know from basic physics that warmer temperatures will help to dry things out," said the study's lead author, Benjamin Cook, a climate scientist at Columbia University and the NASA Goddard Institute for Space Studies.

In its latest climate report, the International Panel on Climate Change (IPCC) warns that soil moisture is expected to decline globally and that already dry regions will be at greater risk of agricultural drought, researchers said.
Using two drought metric formulations, the study authors analysed projections of both rainfall and evaporative demand from the collection of climate model simulations completed for the IPCC's 2013 climate report.

Both metrics agree that increased evaporative drying will probably tip marginally wet regions at mid-latitudes like the US Great Plains and a swath of southeastern China into aridity.
If precipitation were the only consideration, these great agricultural centres would not be considered at risk of drought, researchers said.
Researchers also said that dry zones in Central America, the Amazon and southern Africa will grow larger. In Europe, the summer aridity of Greece, Turkey, Italy and Spain is expected to extend farther north into continental Europe.
"For agriculture, the moisture balance in the soil is what really matters," said study coauthor Jason Smerdon.

"If rain increases slightly but temperatures also increase, drought is a potential consequence," said Smerdon.
The study was published in the journal Climate Dynamics.