There's no debating the devastation of the two-mile-wide tornado that leveled much of Moore, Oklahoma, yesterday, May 20. The Oklahoma City suburb, situated right in the heart of Tornado Alley, suffered unimaginable loss of life and destruction: 24 deaths, including nine children, and an untold number of pulverized buildings, including schools and hospitals.
But, is this particular type of extreme weather caused by climate change?
Short answer: Depends on whom you ask and how you ask it.
While the head of the Intergovernmental Panel on Climate Change (IPCC), Rajenda Pachauri, says that this tornado cannot be pinned on climate change, some scientists are starting to connect the dots between global warming and stronger tornadoes.
The IPCC’s special report on managing extreme risks states: “There is low confidence in observed trends in small spatial-scale phenomena such as tornadoes and hail because of data inhomogeneities and inadequacies in monitoring systems.”
In laymen’s terms: We don’t have enough data to measure, since the United States only began keeping reliable tornado records in 1953, and Doppler radar wasn’t used for weather until the 1970s.
On the other hand: An increase in tornadoes is consistent with the warmer, wetter world created by climate change, and particularly large and unprecedented tornado events may represent the results of climate disruption.
“As far as climate change is concerned, there will likely be a greater clashing of cold air masses from the north with even warmer, even more humid air masses coming off the Gulf of Mexico—conditions that are favorable for breeding destructive storms,” says Michael Mann, climate scientist at Pennsylvania State University and author of The Hockey Stick and the Climate Wars.
“The wildcard is the shear—we don’t know with certainty whether that will increase or not in the key regions for tornado formation as a result of climate change,” Mann continues. “But if one factor is likely to be favorable, and the other is a wildcard, it’s still more likely that the product of the two factors will be favorable. Thus, if you’re a betting person—or the insurance or reinsurance industry for that matter—you’d probably go with a prediction of greater frequency and intensity of tornadoes as a result of human-caused climate change.”
Kevin Trenberth, head of the Climate Analysis Section at the USA National Center for Atmospheric Research, does the math: “The climate change effect is probably only a 5 to 10 percent effect in terms of the instability and subsequent rainfall, but it translates into up to a 33 percent effect in terms of damage. (It is highly nonlinear, for 10 percent it is 1.1 to the power of 3 = 1.33.) So there is a chain of events and climate change mainly affects the first link: the basic buoyancy of the air is increased. Whether that translates into a super-cell storm and one with a tornado is largely chance weather.”
By meteorological standards, the Moore, Oklahoma, tornado was a common, garden variety twister: warm humid air meeting a strong jet stream with strong winds, in the middle of Tornado Alley, during prime tornado season (May and June) and during the prime tornado hours (late afternoon).
The only abnormal thing about this tornado was its sheer size. The climate dice have indeed been loaded for bigger and badder supercells.