Climate change is likely behind the devastating California drought, study finds

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California’s extended, catastrophic drought is “very likely” linked to climate change, Stanford scientists say in a new paper.

The reason is that the persistent area of high atmospheric pressure over the Pacific Ocean that has been blocking rain storms from reaching California was more likely to form in the presence of today’s levels of greenhouse gases, the team found. The scientists demonstrated this causal relationship by combining computer simulations and statistical analysis.

“Our research finds that extreme atmospheric high pressure in this region – which is strongly linked to unusually low precipitation in California – is much more likely to occur today than prior to the human emission of greenhouse gases that began during the Industrial Revolution in the 1800s,” said the research team leader Noah Diffenbaugh, an associate professor of environmental Earth system science at Stanford and a senior fellow at the Stanford Woods Institute for the Environment.

The team’s report was published this week in the Bulletin of the American Meteorological Society.

Scientists have known about the effect of what is known as a “blocking ridge” of air – called in this case the “Triple R” for “Ridiculously Resilient Ridge” — that has thwarted the California rains in 2013 and 2014. The Triple R has been particularly large and persistent and has “substantially altered atmospheric flow and kept California largely dry,” said Daniel Swain, a graduate student in Diffenbaugh’s lab and lead author of the study.

Effectively, this high air mass diverted the jet stream to the north, at times as far as to Alaska and the Arctic Ocean, keeping moisture away from California, and also, at times, from Oregon and Washington. (Watch the video to see how it diminished snow pack in the Pacific Northwest.)

The research team, which included experts in statistics and earth sciences from Stanford’s Woods Institute for the Environment, looked at the probabilities of such a stubborn ridge forming and found that it exceeded any similar event in the last 60 years; before which time there were not adequate atmospheric data for comparisons.

The statistical analysis found that a weather event like the Triple R was three times more likely to occur in the present climate as in the pre-industrial climate, when carbon dioxide in the atmosphere was much lower.

“We’ve demonstrated with high statistical confidence that the large-scale atmospheric conditions, similar to those associated with the Triple R, are far more likely to occur now than in the climate before we emitted large amounts of greenhouse gases,” said Bala Rajaratnam, an assistant professor of statistics and of environmental earth system science and a faculty member at the Woods Institute.

The California drought, in its third year, is being blamed for water shortages expected to cause more than $2 billion in agricultural losses, the loss of thousands of agricultural jobs as well as an increase in wildfires.

In 2013, most of the state experienced record light rainfall that was lower than any other year in recorded history. The agriculturally rich San Joaquin Valley received just 2.9 inches of rain in 2013, forcing growers to tap underground reserves.

Federal and state authorities have declared all of California to be a disaster area.

California Drought, recorded rainfall through 2013

(The Stanford study was supported by National Science Foundation CAREER Awards and grants from the National Institutes of Health, the Air Force Office of Research and the UPS Fund.)

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