Climate change is expected to worsen water shortages, including short-lived, hard-to-predict flash droughts.

New research in the journal PNAS proposes a global early warning system that could help farmers save their crops.

Flash droughts take anywhere from less than a week to a month to develop. Some last weeks or months; others morph into longer, more typical droughts.

"It's really hard to forecast because the onset is just so fast. And, also for that reason, it's really hard to prepare for," said senior author Guiling Wang of the University of Connecticut. "That's why, when we see the potential of having an early warning of up to two months, we consider it a big deal."

By monitoring the rate of change of a satellite-based measurement of photosynthesis called solar-induced chlorophyll fluorescence, the authors found they could predict flash drought onset and recovery by two or more weeks.

The technique works best in grasslands and croplands, where there's a close feedback between land and sky.

"I think in the Southwest, in places where there is grass or crops, it should work. But in places where there is just bare land, you don't have vegetation to give you that same signal," said Wang.

The satellite data are widely available and cover the globe, which means the system could help developing nations that don't have access to other kinds of data.