Warming temperatures implicated in recent California droughts

http://www.eurekalert.org/pub_releases/2015-03/ssoe-wti022615.php

Public Release: 2-Mar-2015
Stanford's School of Earth, Energy & Environmental Sciences

California has experienced more frequent drought years in the last two decades than it has in the past several centuries. That observed uptick is primarily the result of rising temperatures in the region, which have climbed to record highs as a result of climate change, Stanford scientists say.

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The team found that the worst droughts in California have historically occurred when conditions were both dry and warm, and that global warming is increasing the probability that dry and warm years will coincide. The findings suggest that California could be entering an era when nearly every year that has low precipitation also has temperatures similar to or higher than 2013-2014, when the statewide average annual temperature was the warmest on record.

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Their analysis revealed that the years that were both warm and dry were about twice as likely to produce a severe drought as years that were cool and dry.

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In the past two decades, however, nearly all of the years in California have been either warm or hot.

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even though climate change to date has not substantially reduced yearly precipitation, human emissions are clearly implicated in California's statewide warming, and in the increased probability that dry years are also warm.

Assessment of climate model simulations that projected into the future also led the team to conclude that the risk of co-occurring warm and dry years will continue to increase in the coming decades. "We found that essentially all years are likely to be warm-or extremely warm-in California by the middle of the 21st century," said study coauthor Daniel Swain, who is also a graduate student in Diffenbaugh's lab. "This means that both drought frequency-and the potential intensity of those droughts which do occur-will likely increase as temperatures continue to rise."

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