New study quantifies global warming effect on California drought
FRISCO — Researchers say there’s new evidence that global warming will push the western U.S. into the driest conditions in at least the past 1,000 years, as higher temperatures exacerbate drought condition in the region.
The new study by scientists with Columbia University’s Lamont-Doherty Earth Observatory and Cornell University focused on the current California drought, showing that warmer temps drive moisture from plants and soil into the air. Warmer temps likely worsened the California drought by 25 percent, the scientists concluded in their paper, published in the journal Geophysical Research Letters.
The researchers outlined their findings in a press release.
“A lot of people think that the amount of rain that falls out the sky is the only thing that matters,” said lead author A. Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory. “But warming changes the baseline amount of water that’s available to us, because it sends water back into the sky.”
Analyzing monthly data spanning more than 100 years, the study found no long-term rainfall trend but showed average temperatures increasing by about 2.5 degrees Fahrenheit over the 114-year period (1901-2014), in step with building fossil-fuel emissions.
Natural weather variations have made California unusually hot over the last several years, so when rainfall declined in 2012, the air sucked the already scant moisture from the soil, trees and crops.
The study did not look directly at snow, but in the past, gradual melting of the high-mountain winter snowpack has helped water the lowlands in warm months. Now, melting has accelerated, or the snowpack has not formed at all, helping make warm months even dryer according to other researchers.
Last year, the U.S. National Oceanic and Atmospheric Administration sponsored a study that blamed the rain deficit on a persistent ridge of high-pressure air over the northeast Pacific, which has been blocking moisture-laden ocean air from reaching land.
Lamont-Doherty climatologist Richard Seager, who led that study (and coauthored the new one), said the blockage probably has nothing to do with global warming; normal weather patterns will eventually push away the obstacle, and rainfall will return.
In fact, most projections say that warming will eventually increase California’s rainfall a bit. But the new study says that evaporation will overpower any increase in rain, and then some. This means that by around the 2060s, more or less permanent drought will set in, interrupted only by the rainiest years. More intense rainfall is expected to come in short bursts, then disappear.
Many researchers believe that 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.”
Jonathan Overpeck, co-director of the Institute of the Environment at the University of Arizona, said, “It’s important to have quantitative estimates of how much human-caused warming is already making droughts more severe.” But, he said, “it’s troubling to know that human influence will continue to make droughts more severe until greenhouse gas emissions are cut back in a big way.”