California’s Extreme Drought, Explained
California’s Extreme Drought, Explained
California is experiencing the worst drought in its history, and the effects are being felt nationwide.
The severe California drought that has led the state to order cutbacks in water use may not have been set off by , scientists say, but global warming is making the situation worse.
“The drought is made of two components: not enough rain and too much heat,” said Michael Oppenheimer, a climate scientist at Princeton. “The rain deficit isn’t clearly connected to climate change, but the planetary warming has made it more likely that the weather would be hotter in California.”
Warmer temperatures worsen drought by causing more evaporation from reservoirs, rivers and soil. Scientists say that the warming trend makes it highly likely that California and other parts of the Western United States will have more severe droughts in the future.
“The 21st century for sure is being characterized by persistent, ubiquitous drought in the West,” said Deke Arndt, the chief of the climate monitoring branch of the National Climatic Data Center in Asheville, N.C. “The projection is for that to continue.”
The current drought, which began in 2011, is the worst in 120 years of climate record-keeping in the state, and some studies suggest it is the worst in more than a thousand years.
Recent research has blamed natural variability, rather than climate change caused by greenhouse gas emissions. Specifically, some scientists say that conditions in the Pacific Ocean have led to the formation of ridges of air off the West Coast that have kept storms from reaching the state.
While there is still debate about whether climate change has caused the lack of rain, there is less controversy about the role of warming temperatures. A recent study led by Noah Diffenbaugh, an associate professor in earth sciences at Stanford, found that in California over the past two decades, dry periods have more frequently overlapped with warm periods than in prior decades.
“It used to be that half the years were warm, and half were cool,” Dr. Diffenbaugh said. “Now we’re in a regime where most of the years are warm.”
Higher temperatures also reduce snowpack, now at record low levels. “In warmer conditions, precipitation falls as rain rather than snow,” Dr. Diffenbaugh said. “We saw that very clearly this winter.”
During major storms in December and February, warm conditions brought mostly rain and little snow to the Sierra Nevada.
Ordinarily, the snowpack naturally stores water, gradually melting into reservoirs and canals over the spring and summer. But with higher temperatures, Dr. Diffenbaugh said, what little snow there is melts sooner. “The water we have in the reservoirs now is essentially all we’re going to have until the start of the next rainy season,” he said.
Mr. Arndt said it was unlikely that there would be relief in the next six months. “It’s like a baseball lineup,” he said. “These are the wimpy hitters as far as their historical capacity to help out.” “Unless something dramatic happens very soon,” Mr. Arndt added, “we’re realistically looking to next fall as the next opportunity for putting a big dent in the drought.”