Last updated March 4, 2019

Tornado Risk Increase

While global warming may increase the fuel available for tornadoes, it is not yet possible to determine whether and how climate change may be affecting tornado activity.

Long-term trends in tornadoes over the last century are unclear due to poor quality records. At the same time, climate modeling of tornado activity is not robust enough to confidently determine future trends, though existing modeling projects a greater frequency of the severe weather conditions in the U.S. that might generate more or more severe tornadoes.

This uncertainty translates as increased risk in a changing climate.

Climate science at a glance

  • Storm data indicates that the number of tornadoes per outbreak in the United States may be increasing and that weather extremes associated with severe thunderstorms are similarly on the rise.

  • Climate change has been proposed as contributing to these increases; however, the scientific literature is not yet robust enough to link changes in tornado activity to climate change.

  • Climate modeling of tornado activity is not robust enough to confidently determine future trends, though existing modeling projects a greater frequency of the severe weather conditions that might generate more or more severe tornadoes in the United States.

  • The 5 largest U.S. winter tornado outbreaks on record have all hit since 1999.

  • The uncertainty about the climate change connection translates as an increase risk due to climate change.


Background information

How do tornadoes form?

Tornadoes are convective storms, born from severe thunderstorms, that feed on warm, moist air from strong winds that change direction with altitude (known as wind shear).

The mechanics of tornado generation is still not perfectly understood. The available weather reporting data on past tornado activity is limited and makes it difficult to identify whether there have been any long-term trends in tornado frequency, intensity or range.

How might climate change affect tornadoes?

There are two ingredients that fuel severe storms that could spawn tornadoes: potential energy in the air and wind currents, or wind shear. 

The rising levels of greenhouse gases in the air add more energy to the climate system. There’s less consensus on how climate change will affect wind shear, though a 2013 study on the thunderstorm conditions that form tornadoes found that the impact was negligible.[1]

What do the climate models show?

Climate model projections indicate that convective available potential energy (CAPE)—a measure of the amount of energy available for convection—would increase in a warmer climate leading to more frequent environments favorable to severe thunderstorms in the United States.[1][2]

However, a December 2016 study links recent increases in the number of tornadoes per outbreak, not to CAPE, but to trends in storm relative helicity,[3] a quantity related to vertical wind shear previously identified as a factor in increased year-to-year variability of U.S. tornado numbers.[4]


US tornado trends

  • (Gensini and Brooks, 2018) found that over the past four decades, tornado frequency has increased over a large swath of the Midwest and Southeast and decreased in portions of the central and southern Great Plains, a region traditionally associated with Tornado Alley.[5]
  • (Childs et al. 2018) found that the months of November to February are seeing an increase in average tornado activity, with a shift away from the Southern Plains and a ramp-up over the favored terrain of “Dixie Alley,” including Arkansas, Louisiana, Mississippi, Alabama, and Tennessee.[6]
  • (Agee and Larson, 2016) looked into what impact a warming trend over the U.S. in recent decades might have had on tornadoes, and found a new maximum center for tornadic storms emerging in the Deep South - the rise of the so-called "Dixie Alley."[7]
  • (Tippett and Cohen, 2016) found that while the total number of tornadoes rated F/EF1 and higher each year hasn’t increased, the average number per outbreak has, rising from about 10 to about 15 since the 1950s.[8]
  • (Trapp et al. 2009 and Diffenbaugh et al. 2013) consider climate model projections which show that convective available potential energy (CAPE)—a measure of the amount of energy available for convection—would increase in a warmer climate leading to more frequent environments favorable to severe thunderstorms in the United States.[1][2]

Quotes

Changes in tornado activity could be caused by global warming, but our usual tools, the observational record and computer models, are not up to the task of answering this question yet.[9]

Michael Tippett, a climate and weather researcher at Columbia University's School of Applied Science and Engineering and Columbia's Data Science Institute

The scientific community has thought a great deal about how the frequency of future weather and climate extremes may change in a warming climate. The simplest change to understand is a shift of the entire distribution, but increases in variability, or variance, are possible as well. With tornadoes, we’re seeing both of those mechanisms at play.[9]

Michael Tippett, a professor of applied mathematics at Columbia University

Tornadoes are the kind of extreme event where we have the least confidence in our ability to attribute the odds or characteristics of individual events to an influence of global warming.[10]

Noah Diffenbaugh, professor of earth system science at Stanford University

We do have strong evidence that at the large scale global warming is likely to increase the atmospheric environments that create the kind of severe thunderstorm that produces tornadoes.[10]

Noah Diffenbaugh, professor of earth system science at Stanford University