The following essay is reprinted with permission from The conversation, an online publication covering the latest research.
The deadly tornado outbreak that swept through communities from Arkansas to Illinois on the night of December 10-11, 2021, was so unusual in its duration and strength, especially for December, that many people, y including the American president, are ask what role climate change could have played– and if tornadoes will become more frequent in a warming world.
Both questions are easier to ask than to answer, but research offers new clues.
I am a atmospheric scientist which studies severe convection storms such as tornadoes and the influences of climate change. Here’s what scientific research shows so far.
Climate models cannot see tornadoes yet, but they can recognize tornado conditions
To understand how rising global temperatures will affect the climate in the future, scientists use complex computer models that characterize the entire Earth system, from the flow of solar energy to the reaction of the ground and everything in between. year to year and season to season. These models solve millions of equations on a global scale. Each calculation adds up, requiring much more computing power than a desktop computer can handle.
To project how the Earth’s climate will change until the end of the century, we currently have to use a large scale. Think of it like the zoom function of a camera looking at a mountain in the distance. You can see the forest, but the individual trees are harder to distinguish, and a pine cone in one of these trees is too small to be seen even when you zoom in the image. With climate models, the smaller the object, the harder it is to see.
The tornadoes and the severe storms that create them are well below the typical scale that climate models can predict.
What we can do instead is look at the large-scale ingredients that create the conditions for tornadoes to form.
Two key ingredients for severe storms are (1) the energy entrained by warm, humid air favoring strong updrafts, and (2) the change in wind speed and direction, known as wind shear, which allows storms to become stronger and more durable. A third ingredient, which is more difficult to identify, is a trigger for the formation of thunderstorms, such as a very hot day, or perhaps a cold front. Without this ingredient, not all favorable environments lead to severe storms or tornadoes, but the first two conditions still make severe storms more likely.
By using these ingredients to characterize the likelihood of severe storms and tornadoes forming, climate models can tell us how risk is changing.
How storm conditions are likely to change
Climate model projections for the United States suggest that the likelihood of favorable ingredients for severe thunderstorms will increase by the end of the 21st century. The main reason is that warming temperatures accompanied by increased humidity in the atmosphere increases the potential for strong updrafts.
Rising global temperatures are causing significant changes to the seasons that we traditionally view as rarely producing severe weather events. Stronger increases in hot and humid air in autumn, winter and early spring means there will be more days with environments conducive to severe thunderstorms – and when those storms do occur, they have the potential to be more intense.
What studies show about frequency and intensity
Over smaller areas, we can simulate thunderstorms in these future climates, which brings us closer to the question of whether severe storms will form. Several studies have modeled changes to the frequency of severe thunderstorms to better understand this evolution of the environment.
We are already seeing changes in recent decades towards more favorable conditions for severe storms during cooler seasons, as the summer probability of storm formation decreases.
For tornadoes, things get complicated. Even in an otherwise accurate forecast for the next day, there is no guarantee that a tornado will form. Only a small fraction of storms produced in a favorable environment will produce a tornado.
Several simulations explored what would happen if a tornado outbreak or a storm producing a tornado has occurred at different levels of global warming. Projections suggest that stronger storms producing tornadoes may be more likely as global temperatures rise, although they strengthen less than might be expected with increased energy availability.
The impact of 1 degree of warming
Much of what we know about how global warming influences severe storms and tornadoes is regional, primarily in the United States. Not all parts of the world will see the changes in severe storm environments at the same rate.
In one recent study, my colleagues and I have found that the rate of increase in severe storm environments will be higher in the northern hemisphere and increase more at higher latitudes. In the United States, our research suggests that for every degree Celsius (1.8 F) of temperature increase, a 14-25% increase in favorable environments is likely in spring, fall, and winter, with the most strong increase in winter. This is mainly due to the increase in the energy available due to the higher temperatures. Keep in mind that these are favorable environments, not necessarily tornadoes.
What does this say about the December tornadoes?
To answer whether climate change has influenced the likelihood or intensity of tornadoes in the December 2021 epidemic, it remains difficult to attribute a single event like this to climate change. Shorter-term influences such as El Niño-Southern Oscillation can also complicate the picture.
There are certainly signs pointing to a more stormy future, but how this manifests itself for tornadoes is an open area of research.