Even though the findings suggest the drought mainly is the result of natural climate variability, the scientists added that the likelihood of a drought becoming acute is rising because of climate change.
Global warming from human emissions has most likely intensified California’s drought by 15 to 20 percent, scientists said Thursday, warning that future dry spells in the state are almost certain to be worse than this one as the world continues to heat up.
Even though the findings suggest the drought is primarily a consequence of natural climate variability, the scientists added that the likelihood of any drought becoming acute is rising because of climate change. The odds of California seeing droughts at the far end of the scale, like the current one that began in 2012, have roughly doubled in the past century, they said.
“This would be a drought no matter what,” said A. Park Williams, a climate scientist at the Lamont-Doherty Earth Observatory of Columbia University and the lead author of a paper published by the journal Geophysical Research Letters. “But it’s definitely made worse by global warming.”
The National Oceanic and Atmospheric Administration (NOAA) also reported Thursday that global temperatures in July had been the hottest for any month since record-keeping began in 1880, and that the first seven months of 2015 had also been the hottest such period ever. Heat waves on several continents this summer have killed thousands of people.
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July’s average temperature global was 61.86 degrees Fahrenheit, beating the previous global mark set in 1998 and 2010 by about one-seventh of a degree, according to NOAA. That’s a large margin for weather records, with previous monthly heat records broken by a 20th of a degree or less.
“It just reaffirms what we already know: that the Earth is warming,” said NOAA climate scientist Jake Crouch. “The warming is accelerating, and we’re really seeing it this year.”
Crouch and other scientists outside the government said the rising temperatures are caused by a combination of man-made climate change and a strong, near-record El Niño, a warming of the equatorial Pacific Ocean that alters weather worldwide for about a year.
Nine of the 10 hottest months on record have happened since 2005, according to NOAA.
The paper on the California drought echoes a growing body of research that has cited the effects of human emissions, but scientists not involved in the work described it as more thorough than any previous effort because it analyzed nearly every possible combination of data on temperature, rainfall, wind speed and other factors that could be influencing the severity of the drought.
The research, said David Lobell, a Stanford University climate scientist, is “probably the best I’ve seen on this question.”
The paper provides new scientific support for political leaders, including President Obama and Gov. Jerry Brown of California, who have cited human activity and the resulting global warming as a factor in the drought.
A report this week by researchers at the University of California, Davis, projected the drought would cost the California economy $2.7 billion this year. Much of that pain is being felt in the huge farming industry, which has been forced to idle a half-million acres and has seen valuable crops like almond trees and grape vines die.