LOS ANGELES — If there were any doubts that global warming has exacerbated California’s four-year drought, a new study puts them to rest by quantifying the impact for the first time: Rising temperatures are worsening the drought by up to 25 percent.
While natural weather cycles are largely responsible for the historic drought, man-made emissions of greenhouse gases are intensifying the severity of the drought by raising air temperatures, according to a report by the Earth Institute at Columbia University.
Because warmer air evaporates moisture faster, “Rising temperatures mean we have to get more rain just to break even,” said A. Park Williams, the lead author of the report, which appears today in the journal Geophysical Research Letters. “Each raindrop and each snowflake is a little less valuable.”
The researchers analyzed monthly data on precipitation, temperature, humidity, wind and other factors from 1901 to 2014 and concluded that average temperatures in California climbed about 2.5 degrees Fahrenheit in that period.
When rainfall declined in 2012, the warmer air sucked the little moisture left in the soil, trees and crops at a faster rate because temperatures are higher.
At least 20 percent of the severity of the drought is caused by global warming, and 80 percent is due to natural variability, said Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory.
“So global warming makes the drought 25 percent worse than it would be in the absence of global warming,” he said.
California is hoping that a strong El Niño will bring much-needed rain to the parched region this fall and winter. But even if there are heavy rainfalls, higher temperatures will evaporate a larger portion of precipitation, leaving crops still thirsty, Williams said.
“As time goes on, precipitation will be less able to make up for the intensified warmth,” he said. “It’s just like a puddle evaporates faster on a warm day than on a cool day. … People will have to adapt to a new normal.”
The drought is expected to cost California’s agricultural economy $1.84 billion and more than 10,000 jobs this year, according to a report by the University of California at Davis Center for Watershed Sciences out this week.
“We can no longer deny the growing evidence that the consequences of climate change are here now and not something to anticipate in the future,” said Peter Gleick, the director of the Pacific Institute, a nonprofit global water research and policy think tank. “There is a growing scientific consensus that the California drought has been influenced by human-caused climate change. … This is the first paper to actually quantify what those effects are.”
California Gov. Jerry Brown said in a statement that the new findings “now make it crystal clear that climate change is already affecting California and the Southwest in the form of higher temperatures and a more devastating drought."
Brown called on Republicans and "foot-dragging corporations and other deniers to wake up and take sensible action before it’s too late.”
Earlier this year, Brown issued an executive order to reduce greenhouse gas emissions in California 40 percent below 1990 levels by 2030.
There have been numerous studies on the impact of climate change. A study earlier this year warned that much of the central and western United States is headed for its driest period in at least 1,000 years.
“One of the things that some of us have been arguing for a long time is that rising temperatures mean we have to get more rain just to break even,” Gleick said. “Temperature increases make it harder for us to catch up with the severe impact of the drought.”