When do you hope a drought will last as long as possible? When it’s a tornado drought—and a historic tornado drought is exactly what the U.S. experienced between May 2012 and April 2013. During that 12-month period the National Oceanic and Atmospheric Administration (NOAA) estimated that just 197 tornadoes hit the U.S. rated EF1 or stronger. (Tornadoes are ranked on a scale of EF0 to EF5, with sustained gusts between 65 and 85 mph for the lowest ranking and above 200 mph for the highest ranking cyclone.) Going back to 1954, which is about when decent records on tornado hits began being kept, this is the fewest number of tornadoes to hit the U.S. over a one-year period. The previous low for a 12-month consecutive period? 247, between June 1991 and May 1992, which shows just how unusual the last 12 months have been.
Consider that drought over. A massive, mile-wide supercell tornado ripped through the suburbs of Oklahoma City, destroying homes, schools and other buildings. The tornado was on the ground for some 40 minutes, according to the National Weather Service (NWS), and police reported that an occupied elementary school was in the path of the cyclone. Early estimates had winds on the ground near 200 mph, which would have made the cyclone an F4 or higher. Witnesses said the damage was like something out of an atomic bomb strike, and there are at least 24 people dead, including many young children, with a toll that could eventually be far higher. Nor is the Oklahoma City cyclone the only one to strike—more tornadoes hit the area, and last week at least 10 twisters struck north-central Texas, killing at least six people and injuring dozens more.
Right now rescuers are doing their best to sift through the wreckage amid hopes that Moore, Oklahoma—ground zero for the cyclones today—doesn’t become another Joplin, the Missouri town that was essentially flattened in a May 2011 tornado that killed over 150 people. But it’s impossible to avoid wondering in this year of strange weather what impact climate change may have had on this monstrously powerful tornado.
The truth is, however, there’s no clear answer. A 2012 U.N. Intergovernmental Panel on Climate Change report on global warming and extreme weather concluded that it was likely that man-made carbon emissions were already to stronger and longer-lasting heat waves—much like the ones experienced through the U.S. last year, which went down as the hottest year for the continental U.S. on record. There’s also confidence that carbon emissions and other greenhouse gases are leading to more extreme rainfall events—like, say, the superstorm that flooded the East Coast last fall—and more intense droughts, like the one that is still suffocating much of the Midwest. As hotter air can hold more moisture, climate change seems likely to create the fuel for heaving hurricanes and other storms.
But when it comes to the connection between climate change and tornadoes, the connection is cloudy at best. First our historical data on the frequency and strength of the tornadoes is sketchy, especially as we go back further in the past. That’s partially a matter of numbers. A couple dozen tropical storms might hit the U.S. per year, but hundreds of tornadoes touch down annually, some for just a few moments. As a result, there’s little discernable trend in the number and strength of tornadoes over the past 60 years. There appear to be more weak tornadoes these days, but that’s likely a result of the NWS detecting cyclones that would have been missed in the pre-satellite days.
There’s also the fact that climate change seems to have a contradictory effect on the physical factors that drive tornadoes. Tornadoes occur when warm moist air at low levels and cold, drier air above collides with vertical wind shear. Global warming is expected to increase the available energy in a storm system, which is one reason why climate change should drive heavier rainfall. But it also appears to blunt vertical wind shear.
As NOAA scientist Harold Brooks told Marlene Cimons of Climate Nexus:
The energy goes up, and the shear goes down. Thus, we have one ingredient expected to become more favorable and another expected to become less favorable.
Dr. Marshall Shepherd, the president of the American Meteorological Society and someone who is not shy about taking on climate skeptics, put it this way in a tweet:
Climate change is real issue but can we not dilute the issue with that right now. No links to tornadoes.
Tornadoes are the most American of natural disasters. Cyclones of the sort that struck Moore really only occur in the U.S., along that Midwest belt where warm, moist air from the Gulf of Mexico meets colder, drier air pushed down from the north and channeled by the Rocky Mountains. While our weather forecasters can predict when tornado conditions will occur—and residents in this area of Oklahoma knew in advance that conditions were dangerous—a direct tornado warning was in effect 16 minutes before the cyclone touched down. That’s much better than what we could do in the past—but as the rising death toll shows, it’s still not fast enough for some people. And the very complex local processes that make tornadoes so hard to predict in the moment make them that much more difficult to fit into the long term models of climate science. The people of Moore, Oklahoma were victims of weather, and bad, bad luck.