By analyzing climate data dating to 1901, the authors of the study said average temperatures in California have increased by 2.5 degrees Fahrenheit over 113 years, and theorized that heat and dryness have intensified the state’s drought by up to 25 percent.
“A lot of people think that the amount of rain that falls out the sky is the only thing that matters,” said A. Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory who was the study’s lead author. “But warming changes the baseline amount of water that’s available to us, because it sends water back into the sky.”
Williams said California’s drought is natural, part of a trend that has bedeviled the Southwest since recorded time. It would have been destructive without global warming, he said. “But there’s no doubt that global warming has made it worse.”
California is in the fourth year of what officials are calling its most severe drought on record. Last year was the state’s warmest on record.
With no winter snowpack to recharge its rivers and little rain to replenish groundwater that helps irrigate crops, Gov. Jerry Brown (D) ordered a 25 percent cut statewide in urban water use in April 2014.
In the state’s agricultural regions, farmers and ranchers are drilling deeper into the ground to reach aquifers that are being drained for crop irrigation. The loss of groundwater is adding to arid conditions, scientists say.
As the state’s water supply in rural and metropolitan areas dips toward historic lows, Californians are hoping a giant El Niño weather pattern forming in the Pacific Ocean will bring a deluge of winter rain.
But a seasonal gusher would only delay the inevitable — a future of longer and more frequent droughts, Williams said.
A long list of climatologists support that conclusion. In February, researchers at NASA, Cornell and Columbia universities predicted that the Southwest will slip into a 30-year megadrought around 2050 if carbon emissions are not curtailed.
Megadroughts are periods of low precipitation and soil moisture loss that span generations, 10 times longer than a normal three-year drought.
North America’s last megadroughts were in medieval times, during the 12th and 13th centuries. Natural weather changes give droughts a 10 percent chance of forming at any time, but climate change driven by human activity will dramatically increase those chances.
A month after the NASA study was released, researchers at Stanford University published a report that said Californians will be well accustomed to drought by 2050, because steadily rising temperatures will offset normal rainfall and cause them about every year.
Stanford scientists relied on historical data to predict that average temperature increases would continue, quickly evaporating average precipitation.
Noah Diffenbaugh, author of the Stanford study, called Columbia’s report “a step forward,” saying it was based on firm data showing “that temperature makes it harder for drought to break, and increases the long-term risk.”
Amir AghaKouchak, a hydrology professor at the University of California at Irvine, also supported the study. Last year, he co-authored a report that said the state’s drought would be less intense without warming, but he did not link warming to human activity.
“Put simply, if temperatures weren’t so high, there was an 86 percent chance that the current drought would have been less extreme,” AghaKouchak said.
Williams said his team read the studies and sought to answer whether human-caused warming had an immediate impact. “We didn’t look at the future at all. This is a study about now,” he said.
The study, published in the journal Geophysical Research Letters, said that average temperatures in California have increased by 2.5 degrees Fahrenheit over 113 years. And, starting in the 1960s, temperature increases intensified with the introduction of more greenhouse gases from automobiles and other sources.
“When greenhouse gases accumulate, it’s like a bully showing up at your door to demand that you give it more and more every year,” Williams said. In California, that meant more moisture evaporating from rain and ground water sprayed on crops in farming regions such as the Central Valley.
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