Natural variability alone cannot explain the extreme weather pattern that has driven both the record-setting California drought and the cooler weather seen in the Midwest and East this winter, a major new study finds.
We’ve reported before that climate scientists had predicted a decade ago that warming-driven Arctic ice loss would lead to worsening drought in California. In particular, they predicted it would lead to a “blocking pattern” that would shift the jet stream (and the rain it could bring) away from the state — in this case a “Ridiculously Resilient Ridge” of high pressure.
A new study in Geophysical Research Letters (subs. req’d) takes the warming link to the California drought to the next level of understanding. It concludes, “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 NASA-funded study is behind a pay wall, but the brief news release, offers a simple explanation of what is going on. The research provides “evidence connecting the ampliﬁed wind patterns, consisting of a strong high pressure in the West and a deep low pressure in the East [labeled a ‘dipole’], to global warming.” Researchers have “uncovered evidence that can trace the ampliﬁcation of the dipole to human inﬂuences.”
As this figure shows, the amplitude of the dipole driving the extreme nature of the California drought is much higher than can be explained purely by natural causes, and greenhouse gases are needed to explain the difference. The release explains:
… it is important to note that the dipole is projected to intensify, which means more extreme future droughts for California. Historical data show that the dipole has been intensifying since the late 1970s. The intensiﬁed dipole can be accurately simulated using a new global climate model, which also simulates the level of greenhouse gases in the atmosphere. Simulations with only natural variability show a weakening dipole, which is opposite to what is currently being observed. Moreover, the occurrence of the dipole one year before an El Nino/La Nina event is becoming more common, which can only be reproduced in model simulations when greenhouse gases are introduced into the system
This research fits 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.”
The new study’s lead author, Dr. Simon Wang of the Utah Climate Center, told me in an email:
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.
I asked one of the country’s top climatologists, Dr. Michael Mann, what he thought of this new research. I’ll give him the final word:
We know that human-caused climate change has played a hand in the increases in many types of extreme weather impacting the U.S., including the more pronounced heat waves and droughts of recent summers, more devastating hurricanes and superstorms, and more widespread and intense wildfires.
This latest paper adds to the weight of evidence that climate change may be impacting weather in the U.S. in a more subtle way, altering the configuration of the jet stream in a way that disrupts patterns of rainfall and drought, in this case creating an unusually strong atmospheric “ridge” that pushed the jet stream to the north this winter along the west coast, yielding record drought in California, flooding in Washington State, and abnormal warmth in Alaska. The recent IPCC assessment downplays these sorts of connections, making it very conservative in its assessment of risk, and reminding us that uncertainty in the science seems to be cutting against us, not for us. It is a reason for action rather than inaction.