Global warming caused by human emissions has most likely intensified the drought in California by roughly 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 that 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 suffering droughts at the far end of the scale, like the current one that began in 2012, have roughly doubled over the past century, they said.
"This would be a drought no matter what," said A. Park Williams, a climate scientist at Columbia University and the lead author of a paper published by the journal Geophysical Research Letters. "It would be a fairly bad drought no matter what. But it's definitely made worse by global warming."
The paper echoes a growing body of research that has come to similar conclusions, 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 B. Lobell, a Stanford University climate scientist, is "probably the best I've seen on this question."
The paper provides fresh scientific support for political leaders, including President Obama and Gov. Jerry Brown of California, who have cited human emissions and the resulting global warming as a substantial factor in the drought. They have been attacked for that stance by Republicans who question the science of climate change. Mr. Obama, in turn, has mocked the Republicans for attitudes he once described as reminiscent of the Flat Earth Society.
"We have a real challenge in California," Mr. Brown said this month as he visited firefighters battling a blaze near Clear Lake that has burned thousands of acres. "Unlike the East, where climate change seems to be adding more storms, here in California and the Southwest it's more dryness."
A report this week by researchers at the University of California, Davis, projected that the drought would cost the California economy some $2.7 billion this year. Much of that pain is being felt in the state's 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.
Farmers will spend about $590 million extra this year just to pump groundwater from rapidly falling water tables, the report estimated.
Reservoirs have dwindled, and mandatory restrictions imposed this year by the governor have upended routines throughout the state, forcing some people to let their lawns die and to alter habits as personal as showering. At the same time, the drought has redoubled the political resolve in California to find ways to limit greenhouse gases, a task at which the state is already ahead of the rest of the country.
As climate scientists analyze the origins of the drought, they have been tackling two related questions: What caused the dearth of rain and snow that began in 2012? And, regardless of the cause, how have the effects been influenced by global warming?
The immediate reason for the drought is clear enough: For more than three years, a persistent ridge of high pressure in the western Pacific Ocean has blocked storms from reaching California in the winter, when the state typically gets most of its moisture. That pattern closely resembles past California droughts.
Some scientists have argued that the ocean and atmospheric factors that produced the ridge have become somewhat more likely because of global warming, but others have disputed that, and the matter remains unresolved.
On the question of the effects, scientists have been much clearer.
Rising temperatures dry the soil faster and cause more rapid evaporation from streams and reservoirs, so they did not need any research to tell them that the drought was probably worse because of the warming trend over the past century - the same is true for pretty much any drought in the world. The challenge has been to quantify how worse the California drought has become because of human emissions.
The group led by Dr. Williams concluded that human-caused climate change was responsible for between 8 percent and 27 percent of the deficit in soil moisture that California experienced from 2012 to 2014.
The group led by Dr. Williams concluded that human-caused climate change was responsible for between 8 percent and 27 percent of the deficit in soil moisture that California experienced from 2012 to 2014.
But, in an interview, Dr. Williams said the low number was derived from a method that did not take into account the way that global warming had sped up since the 1970s. That led him and his colleagues to conclude that climate change was most likely responsible for about 15 to 20 percent of the moisture deficit.
Since 1895, California has warmed by a little more than 2 degrees Fahrenheit. That increase sounds small, but as an average over an entire state in all seasons, scientists say, it is a large number. The warmer air can hold more water vapor, and the result is that however much rain or snow falls in a given year, the atmosphere will draw it out of the soil more aggressively.
Dr. Williams calculated that the warmer atmosphere over California is able to absorb about 8.5 trillion more gallons of water in a typical year than would have been the case in the cooler atmosphere at the end of the 19th century. The air does not always manage to soak up that much, however, because evaporation slows as the soils dry out.
How much more California will warm depends on how high global emissions of greenhouse gases are allowed to go, but scientists say efforts to control the problem have been so ineffective that they cannot rule out another five or six degrees of warming over the state in this century. That much warming would probably turn even modest rainfall deficits into record-shattering droughts.
"The whole water system that we have in California was designed for the old climate," said Noah S. Diffenbaugh, a climate scientist at Stanford who edits the journal that published the new paper. "Just from the temperature change, we're in a new climate. The water system wasn't built for the climate that we have now."