In an email interview with ThinkProgress, Dr. Kevin Trenberth, one of the world’s top climate scientists, who has been exploring for years how greenhouse pollution influences extreme weather, said he believes that it is “irresponsible not to mention climate change” in the context of these extreme tornadoes. Trenberth, head of the Climate Analysis Section of the National Center for Atmospheric Research, added that the scientific understanding of how polluting our atmosphere with billions of tons of greenhouse gases affects tornadic activity is still ongoing:
It is irresponsible not to mention climate change. … The environment in which all of these storms and the tornadoes are occurring has changed from human influences (global warming). Tornadoes come from thunderstorms in a wind shear environment. This occurs east of the Rockies more than anywhere else in the world. The wind shear is from southerly (SE, S or SW) flow from the Gulf overlaid by westerlies aloft that have come over the Rockies. That wind shear can be converted to rotation. The basic driver of thunderstorms is the instability in the atmosphere: warm moist air at low levels with drier air aloft. With global warming the low level air is warm and moister and there is more energy available to fuel all of these storms and increase the buoyancy of the air so that thunderstorms are strong. There is no clear research on changes in shear related to global warming. On average the low level air is 1 deg F and 4 percent moister than in the 1970s.
Climate scientist Michael Mann, director of the Earth System Science Center at Pennsylvania State University, explains further that “climate change is present in every single meteorological event”:
The fact remains that there is 4 percent more water vapor–and associated additional moist energy–available both to power individual storms and to produce intense rainfall from them. Climate change is present in every single meteorological event, in that these events are occurring within a baseline atmospheric environment that has shifted in favor of more intense weather events.
Climate scientist Gavin Schmidt, climate modeller at the NASA Goddard Institute for Space Studies, concurred:
It is a truism to say that everything has been affected by climate change so far and therefore this latest outbreak must in some sense have been affected, but attribution is hard and the further down the chain the causality is supposed to go, the harder this is. For heat waves it is easier, for statistics on precipitation intensity it easier – there are multiple levels of good modelling, theory and observations to back it up. But we have much less to go on with tornadoes.