It’s official: climate change is behind California’s drought

The drought crippling California is by some measures the worst in the state's history. Credit: NOAA.
The drought crippling California is by some measures the worst in the state’s history. Credit: NOAA.

California’s unprecedented drought is likely the result of spewing too much carbon dioxide into the atmosphere. In other words, man-made climate change is at fault, according to an extensive new study.

Noah Diffenbaugh, a climate scientist at Stanford University, and colleagues used computer simulations and statistical techniques to show that a persistent region of high pressure over the Pacific Ocean “was much more likely to form in the presence of modern greenhouse gas concentrations,” according to a new release from the National Science Foundation.

“This isn’t a projection of 100 years in the future,” Diffenbaugh said in a statement. “This is an event that is more extreme than any in the observed record, and our research suggests that global warming is playing a role right now.”

Published n the Bulletin of the American Meteorological Society, the study is one of the most comprehensive to investigate the link between climate change and California’s drought, according NSF.

“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 emission of greenhouse gases that began during the Industrial Revolution in the 1800s,” Diffenbaugh said.

Combined with unusually hot temperatures and stagnant air, the lack of rain has triggered a dangerous increase in wildfires and air pollution across the state. It could trigger $2.2 billion in agricultural losses and some 17,000 season jobs could evaporate this year alone.

The drought can be immediately blamed on a tenacious “blocking ridge” over the northeastern Pacific – more popularly known as the Ridiculously Resilient Ridge, or Triple R. Such ridges disrupt wind patterns in the atmosphere.

“Winds respond to the spatial distribution of atmospheric pressure,” says Daniel Swain of Stanford, lead author of the paper. ”We have seen this amazingly persistent region of high pressure over the northeastern Pacific for many months, which has substantially altered atmospheric flow and kept California largely dry.”

The ridge is exceptional for both its size and longevity. ”At its peak in January 2014, the Triple R extended from the subtropical Pacific between California and Hawaii to the coast of the Arctic Ocean north of Alaska,” said Swain, who coined the term “ridiculously resilient ridge.”

The Triple R diverted the the jet stream far to the north, causing Pacific storms to bypass California, Oregon and Washington. Rain and snow normally destined for the West Coast were re-routed to Alaska and the Arctic Circle.

An important question, however, is whether climate change is behind this strange and debilitating weather. A number of environmental groups were quick to say that global warming was the likely culprit and now the study backs up these fears.

Here is more from the release:

The team first assessed the rarity of the Triple R in the context of the 20th century historical record. Analyzing the period since 1948, for which comprehensive atmospheric data are available, the researchers found that the persistence and intensity of the Triple R in 2013 were unrivaled by any previous event.

To more directly address the question of whether climate change played a role in the probability of the 2013 event, the team collaborated with scientist Bala Rajaratnam, also of Stanford. Rajaratnam applied advanced statistical techniques to a large suite of climate model simulations.

Using the Triple R as a benchmark, Rajaratnam compared geopotential heights–an atmospheric property related to pressure–between two sets of climate model experiments. One set mirrored the present climate, in which the atmosphere is growing increasingly warmer due to human emissions of carbon dioxide and other greenhouse gases.

In the other set of experiments, greenhouse gases were kept at a level similar to those that existed just prior to the Industrial Revolution. The researchers found that the extreme heights of the Triple R in 2013 were at least three times as likely to occur in the present climate as in the preindustrial climate.

They also found that such extreme values are consistently tied to unusually low precipitation in California, and to the formation of atmospheric ridges over the northeastern Pacific.”

Said Rajaratnam: “We’ve demonstrated with high statistical confidence that large-scale atmospheric conditions similar to those of the Triple R are far more likely to occur now than in the climate before we emitted large amounts of greenhouse gases.”



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