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Global warming has made California's drought worse, say scientists

Scientists say global warming has likely contributed to California's megadrought. According to the researchers, increasing heat is driving moisture from the ground. 

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    Gino Celli, who relies on senior water rights to water his crops, inspects a wheat field nearing harvest on his farm near Stockton, Calif., May 18, 2015.
    Rich Pedroncelli/AP/File
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Climate change has worsened California’s historic drought, according to a study released Thursday. 

The study's authors say the drought is a mostly natural event, but they blamed human-induced global warming for increasing its intensity by up to 20 percent.

The study, published in the journal Geophysical Research Letters, found that climate change due to human emissions of carbon dioxide and other greenhouse gases is the cause of between 8 to 27 percent of the drought conditions between 2012 and 2014 and between 5 to 18 percent in 2014.

"A lot of people think that the amount of rain that falls out the sky is the only thing that matters," said lead author A. Park Williams, a bioclimatologist at Columbia University's Lamont-Doherty Earth Observatory, in a statement. "But warming changes the baseline amount of water that's available to us, because it sends water back into the sky."

To figure out how much global warming contributed to California’s drought, the researchers analyzed month-by-month climate data from 1901 to 2014 to look for changes in precipitation, wind, temperature, and humidity. The study found that average temperatures have creept up – about 2.5 degrees Fahrenheit over the 114-year period, right in step with growing fossil-fuel emissions.

The researchers say for California, which has been unusually hot over the last several years when rainfall decreased in 2012, the air sucked already scant moisture from soil, trees, and crops harder than ever.

The National Oceanic and Atmospheric Administration forecasts a giant El Niño, raising the prospect of long and heavy rains that could relieve California’s four-year dry spell. “When this happens, the danger is that it will lull people into thinking that everything is now OK, back to normal,” said Williams. “But as time goes on, precipitation will be less able to make up for the intensified warmth. People will have to adapt to a new normal.”

While scientists have made such assertions before, this is first study to estimate how much worse. A paper by scientists from Lamont-Doherty and Cornell University, published in February, warned that climate change will push much of the central and western United States into the driest period for at least 1,000 years. A March study by Stanford University said that California droughts have been intensified by higher temperatures, and gives similar warnings for the future.

"Of course low precipitation is a prerequisite for drought, but less rain and snowfall alone don't ensure a drought will happen. It really matters if the lack of precipitation happens during a warm or cool year,” climatologist Noah Diffenbaugh who led the Stanford research said. "We've seen the effects of record heat on snow and soil moisture this year in California, and we know from this new research that climate change is increasing the probability of those warm and dry conditions occurring together."

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