Californian drought and climate change

Extreme weather catastrophes could be as much as three times more likely as the global climate warms, researchers from Stanford University warned this week. 

Using California as a case study, Noah Diffenbaugh and his colleagues, writing in Bulletin of the American Meteorological Society, show how as temperatures rise the risk of droughts becomes more likely. 

The 2013 California droughts were the driest since records began. A state-level "drought emergency" was declared by California Governor Jerry Brown, and all 58 counties of California were declared "natural disaster areas," with the economic costs estimated to be in the region of $2 billion. 

The droughts were caused by a large atmospheric ridge, a region of high pressure in the atmosphere, that prevented precipitation from reaching California, as well as Oregon and Washington.

"If you think of a large boulder in a stream," says Noah Diffenbaugh, one of the authors of the paper, "the water is going to be diverted around the boulder, and that's essentially what has been happening [in the atmosphere]." 

"As the air currents move across the Pacific, those that would normally make it to California have been blocked or steered northward." 

The atmospheric ridge responsible for the Californian droughts was extraordinary for its size and longevity, which lead to it being named the Ridiculously Resilient Ridge, by Daniel Swain a graduate student in Diffenbaugh's research group. 

To determine the role of climate change in the likelihood of the Californian droughts, the research group teamed up with Bala Rajaratnam, assistant professor of Statistics and of Environmental Earth System Science. Rajaratnam and his team used statistical techniques to analyse a variety of climate model simulations. They ran two sets of experiments, one where the greenhouse gas level in the atmosphere were set at today's values, and the other set to levels before the industrial revolution.

According to Diffenbaugh, the research shows with a 95% confidence "the global warming that has already happened, has at least tripled the probability of this extreme atmospheric condition, relative to the probability without the human contribution."

The blame for any individual climate event can never be fully attributed to climate change, but with an increased likelihood of extreme weather both in the US and around the globe we are going to need to learn to be better prepared.  

As Diffenbaugh puts it "there are a lot of opportunities to improve our resilience in the current climate and at the same time prepare for future events."

Click to listen to the full interview with Noah Diffenbaugh: