How climate change robs California of scant water supplies

A new study is the first to put numbers to the idea that increasing heat drives moisture from the ground, intensifying drought conditions in places like California.

Jim Gensheimer/San Jose Mercury News/AP/File
The dried up bed of the Guadalupe River in San Jose, Calif., July 17. Climatologists say that climate change has worsened California's historic drought, in a new study published Thursday in the journal Geophysical Research Letters.

Global warming has measurably worsened the ongoing California drought, according to a new study published Thursday in the journal Geophysical Research Letters.

Scientists had already acknowledged a correlation between climate change and drought, but this new study is the first to put real numbers to how much worse drought effects are on the proportion of the drought caused by global warming. The paper estimates 8 to 27 percent, but lead author A. Park Williams, a bioclimatologist at Columbia University's Lamont-Doherty Earth Observatory, said that somewhere in the middle – probably 15 to 20 percent – is a more likely estimate.

"A lot of people think that the amount of rain that falls out the sky is the only thing that matters," said Mr. Williams. "But warming changes the baseline amount of water that's available to us, because it sends water back into the sky."

The findings suggest that within a few decades continually increasing temperatures and resulting moisture losses will push California into even more persistent dry conditions, according to the researchers, who add that the rising temperatures may be making things worse by driving moisture from plants and soil into the air.

In a state where rivers are running dry, wildfires are raging, and a lack of groundwater is making the land sink at an estimated rate of two feet annually, news that climate change is contributing to the conditions is likely no surprise to Californians, who have endured severe drought for four years.

The researchers say they analyzed multiple sets of month-by-month data from 1901 to 2014, looking at data on precipitation, temperature, humidity, and wind, among other weather factors. Average temperatures in the region have steady ticked upward – about 2.5 degrees Fahrenheit over the 114-year period, which the authors say is in line with a rise in fossil fuel emissions. They conclude that when rainfall declined in 2012, the air sapped already low moisture from soil, trees, and crops more dramatically than before, making 2014 the harshest drought season on record.

The study is not the first to assert global warming's role in the planet's increasingly dry conditions, the authors from Columbia acknowledge, citing a paper by scientists from Lamont-Doherty and Cornell University, published this February, which warned that climate change will push much of the central and western United States into the driest period for at least 1,000 years, as The Christian Science Monitor reported at the time. The researchers also included a March study out of Stanford University that stated California droughts have been intensified by higher temperatures.

Many researchers have claimed rain will resume as early as this winter. "When this happens, the danger is that it will lull people into thinking that everything is now OK, back to normal," said Williams. "But as time goes on, precipitation will be less able to make up for the intensified warmth. People will have to adapt to a new normal."

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