Meteorologists have blamed a persistent high-pressure system over the Pacific Ocean for California’s record-breaking drought. Now Stanford University scientists for the first time have linked that weather pattern to climate change.
“The extreme atmospheric conditions we were seeing steered storms that would typically reach California to the north towards Canada and Alaska,” said Noah Diffenbaugh, an associate professor of environmental earth system science at Stanford and coauthor of the new study.
Scientists are calling that high-pressure system a “ridiculously resilient ridge”—a Triple R. In computer simulations, Diffenbaugh calculated the probability of its formation before humans began emitting greenhouse gases.
“We found that the probability of a ridge forming like that is three times more likely in the current climate than it was pre–Industrial Revolution,” said Diffenbaugh.
That means there’s a good chance that carbon emissions have intensified California’s drought.
“In using these advanced statistical techniques to combine climate observations with model simulations, we've been able to better understand the ongoing drought in California,” Diffenbaugh said in a statement. “This isn't a projection of 100 years in the future. 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.”
The high-pressure system that formed during the 2013–2014 rainy season was an unusually stubborn blocking ridge. Blocking ridges periodically form along the North Pacific Coast, disrupting the typical wind patterns that produce storms in California. But the Triple R was unusual in 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," Daniel Swain, a coauthor of the study, said in a video explaining the phenomenon.
While it dissipated briefly during the summer months of 2013, it returned even stronger by fall 2013 and persisted through much of the winter rainy season.
Stanford released its report as the Bulletin of the American Meteorological Society published multiple studies that examined 16 of the world’s most severe weather events of 2013. Temperatures hit record highs in several countries last year, and scientists found more evidence that the world’s growing number of heat waves is due to human-induced climate change.
But the findings in three studies of California’s three-year dry spell were not so definitive. The reports’ authors could not determine a clear connection between human-caused climate change and the drought.