BY JOE ROMM
POSTED ON SEPTEMBER 29, 2014 AT 2:59 PM
NSF: “The drought crippling California is by some measures the worst in the state’s history.” CREDIT: NOAA
A Stanford study funded by the National Science Foundation (NSF) confirms a growing body of research that finds “The atmospheric conditions associated with the unprecedented drought in California are very likely linked to human-caused climate change.”
The NSF news release, headlined, “Cause of California drought linked to climate change,” explains:
Climate scientist Noah Diffenbaugh of Stanford University and colleagues used a novel combination of computer simulations and statistical techniques to show that a persistent region of high atmospheric pressure over the Pacific Ocean–one that diverted storms away from California–was much more likely to form in the presence of modern greenhouse gas concentrations.
Unprecedented droughts often combine a reduction in precipitation with higher temperatures that increase evaporation, leaving soil parched. As the NSF notes in this case, “Combined with unusually warm temperatures and stagnant air conditions, the lack of precipitation has triggered a dangerous increase in wildfires and incidents of air pollution across the state.”
We know, of course, that global warming is making heat waves longer and stronger and more frequent, which in turn makes droughts worse everywhere. But climate change is also causing reduced precipitation in many regions, such as the Mediterranean and southwestern United States. This double whammy from carbon pollution means we’ll be seeing more and more dangerous record droughts.
California’s 3-year drought has reached epic proportions. The L.A. Times reported last week, “Drought has 14 communities on the brink of waterlessness.”
Here’s the most recent Drought Monitor for the state:
So what is the proximate cause of the reduced precipitation over California? New studies suggest that increases in sea surface temperatures are not the cause of the drying. The NSF study, however, explains:
Scientists agree that the immediate cause of the drought is a particularly tenacious “blocking ridge” over the northeastern Pacific — popularly known as the Ridiculously Resilient Ridge, or “Triple R” — that prevented winter storms from reaching California during the 2013 and 2014 rainy seasons.
Blocking ridges are regions of high atmospheric pressure that disrupt typical wind patterns in the atmosphere.
The NSF study analyzed “the period since 1948, for which comprehensive atmospheric data are available.” Researchers “found that the persistence and intensity of the Triple R in 2013 were unrivaled by any previous event.” Stanford’s Bala Rajaratnam then “applied advanced statistical techniques to a large suite of climate model simulations.”
Finally, researchers looked at two sets of models — one set that duplicated the current climate, in which carbon pollution is warming the atmosphere, and the other set in which carbon pollution levels were comparable to those just before 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.
Prof. Rajaratnam concluded, “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.”
This matches the finding in an April study that “there is a traceable anthropogenic warming footprint in the enormous intensity of the anomalous ridge during winter 2013-14, the associated drought and its intensity.” The lead author of that study, Dr. Simon Wang of the Utah Climate Center, told me in an email earlier this year, “I personally think that the debate over global warming leading to stronger blocking has passed. The ongoing challenge is how we predict WHEN and WHERE those blocking will happen and affect WHICH region.”
Indeed, as I’ve reported, scientists a decade ago not only predicted the loss of Arctic ice would dry out California, they also precisely predicted the specific, unprecedented change in the jet stream that has in fact caused the unprecedented nature of the California drought.
In fact, a growing body of evidence — documented by Senior Weather Channel meteorologist Stu Ostro and others — that “global warming is increasing the atmosphere’s thickness, leading to stronger and more persistent ridges of high pressure, which in turn are a key to temperature, rainfall, and snowfall extremes and topsy-turvy weather patterns like we’ve had in recent years.”
Bottom Line: Human activity has made droughts longer and stronger in many places, including California. If we continue on our current path of unrestricted carbon pollution, we will be sharply increasing the chances of civilization-threatening mega-droughts here and abroad.