Southeast Tornadoes and Climate Change

According to the National Oceanic and Atmospheric Administration (NOAA), the 25-28 April 2011 Tornado Outbreak killed at least 318 people in the southeastern United States.  They estimate that there were 305 tornadoes between 8 am 25 April and 8 am 28 April, although efforts are still ongoing to reconcile eyewitness reports and storm surveys.

Rotation tracks detected by NWS Doppler radars on 27 April.
Source: NOAA/National Weather Service.

There has been some discussion about possible linkages between climate change and the tornado outbreak, as nearly almost always happens following a severe weather event.  The reality is that it is extremely difficult to establish a clear linkage between anthropogenic climate change and trends in rare events such as the 25-28 April 2011 tornado outbreak.  There are a number of reasons for this.  First, these events are rare.  The conditions needed for such an extreme outbreak come together only every decade or two.  Second, the number and intensity of tornadoes during these events has always be difficult determine and large uncertainties exist.  Even with modern technology and communications, we are still working to get an accurate account for the 25-28 April 2011 event.  Finally, when looking at fatalities and damage it is essential to consider societal aspects of the problem.  Today we have a world-class tornado warning system, but also a much higher population density.  In other words, careful analysis is needed just to understand one event, let alone long term trends.

Dr. Martin Hoerling of the Earth Systems Research Laboratory recently posted a preliminary discussion of some of the issues above and an assessment of the event in terms of climate trends since 1979.  He highlights the following conclusions from the IPCC and US Climate Change Synthesis Report, which echo some of the concerns above:

"There is insufficient evidence to determine whether trends exist in...small-scale phenomena such as tornadoes, hail, lightning and dust-storms" - IPCC AR4

"The data used to examine changes in the frequency and severity of tornadoes and severe thunderstorms are inadequate to make definitive statements about actual changes" - US Climate Change Synthesis Report SAP 3.3

These statements reflect the current scientific consensus on the topic.  The 25-28 April event was extraordinary, but individual events of this type result from intrinsic weather variability.  Whether or not trends in the frequency or intensity of such events exist and are related to anthropogenic climate change is an issue that requires further research.