全球变暖加剧了加州的旱情

A new study says that global warming has measurably worsened the ongoing California drought. While scientists largely agree that natural weather variations have caused a lack of rain, an emerging consensus says that rising temperatures may be making things worse by driving moisture from plants and soil into the air. The new study is the first to estimate how much worse: as much as a quarter. The findings suggest that within a few decades, continually increasing temperatures and resulting moisture losses will push California into even more persistent aridity. The study appears this week in the journal Geophysical Research Letters. "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."

The study adds to growing evidence that climate change is already bringing extreme weather to some regions. California is the world's eighth-largest economy, ahead of most countries, but many scientists think that the nice weather it is famous for may now be in the process of going away. The record-breaking drought is now in its fourth year; it is drying up wells, affecting major produce growers and feeding wildfires now sweeping over vast areas. 

The researchers analyzed multiple sets of month-by-month data from 1901 to 2014. They looked at precipitation, temperature, humidity, wind and other factors. They could find no long-term rainfall trend. But average temperatures have been creeping up--about 2.5 degrees Fahrenheit over the 114-year period, in step with building fossil-fuel emissions. Natural weather variations have made California unusually hot over the last several years; added to this was the background trend. Thus, when rainfall declined in 2012, the air sucked already scant moisture from soil, trees and crops harder than ever. 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.