Is Climate Change to Blame for the Oklahoma Tornado?

Destroyed buildings and overturned cars left in the wake of the huge tornado that struck Moore, Oklahoma, near Oklahoma City, on May 20, 2013. Gene Blevins/LADailyNewsZuma

The story first appeared on the Guardian website and is reproduced here as part of the Climate Desk collaboration.

Global climate change and politics are linked to each other—for better or worse. No clearer was that the case than when Democratic Sen. Sheldon Whitehouse of Rhode Island gave an impassioned speech on global warming in the aftermath of Monday’s deadly Oklahoma tornado, and the conservative media ripped him. Whitehouse implied that at least part of the blame for the deadly tornado should be laid at the feet of climate change.

Is Whitehouse correct? It’s difficult to assign any one storm’s outcome to the possible effects of global climate change, and the science of tornadoes in particular makes it pretty much impossible to know whether Whitehouse is right.

Let’s start with the basics of what causes a tornado. A piece from my friend (and sometimes co-chatter) Andrew Freedman two years ago sets out the basics well.

First, you need warm, humid air for moisture. The past few days in Moore have featured temperatures in the upper 70s to low 80s, with relative humidity levels regularly hitting between 90 percent and 100 percent and rarely dropping below 70 percent.

Second, you need strong jet stream winds to provide lift. As this map from Weather Underground indicates, there were definitely some very strong jet stream winds on Monday in the Oklahoma region.

Image: Weather Underground

Third, you need strong wind shear (changing wind directions and/or speeds at different heights) to allow for full instability and lift. This mid-level wind shear map from the University of Wisconsin shows that there were 45 to 50 knot winds, right at the top of the scale, over Oklahoma on Monday.

Image: University of Wisconsin

Fourth, you need something to ignite the storm. In this case, a frontal boundary, as seen in this Weather Channel map, draped across central Oklahoma, did the trick.

Image: Weather Channel

The point is that all the normal ingredients were there that allowed an EF-4 tornado to spawn and strike. (Examination of the storm site may cause an upgrading to EF-5.) It happened in tornado alley, where warm moist air from the Gulf of Mexico often meets dry air from the north and Rocky Mountains for maximum instability. There wasn’t anything shocking about this from a meteorological perspective. It was, as a well-informed friend said, a “classic” look.

The long-term weather question is whether or not we’ll see more or less of these “classic” looks in our changing meteorological environment. It turns out that of all the weather phenomena, from droughts to hurricanes, tornadoes are the most complex to answer from a broader atmospheric trends point of view. The reason is that a warming world affects the factors that lead to tornadoes in different ways.

Climate change is supposed, among other things, to bring warmer and moister air to Earth. That, of course, would lead to more severe thunderstorms and probably more tornadoes. The issue is that global warming is also forecast to bring about less wind shear. This would allow hurricanes to form more easily, but it also would make it much harder for tornadoes to get the full about lift and instability that allow for your usual thunderstorm to grow in height and become a fully fledged tornado. Statistics over the past 50 years bear this out, as we’ve seen warmer and more moist air as well as less wind shear.