The deadly tornado outbreak from Arkansas to Illinois on the night of Dec. 10-11, 2021, was so unusual in duration and intensity, especially in December, So much so that many people, including the president of the United States, are asking what role climate change may have played — and whether tornadoes will become more common in a warming world.
Both questions are easier to ask than to answer, but research offers new clues.
I’m an atmospheric scientist who studies severe convective storms like tornadoes and the effects of climate change. This is what the science says so far.
Climate models can’t see tornadoes yet — but they can identify them
To understand how rising global temperatures will affect future climate, scientists use sophisticated computer models to describe the entire Earth system, from the sun’s energy inflow to how soil responds and everything in between, year to year and season. These models solve millions of equations around the world. Each calculation adds up to far more computing power than a desktop computer can handle.
To predict how earth’s climate will change by the end of the century, we currently have to use a wide range of scales. Think of it as a zoom function on your camera, looking at mountains in the distance. You can see the forest, but it’s hard to make out individual trees, and the cones on one of the trees are so small that you can’t see them even if you zoom in. Using climate models, the smaller the object, the harder it is to see.
Tornadoes and the severe storms that cause them are far below the typical size predicted by climate models.
Instead, what we can do is study the large-scale components that make conditions ripe for tornadoes to form.
Two key factors for severe storms are (1) energy driven by warm, moist air, which promotes strong updrafts, and (2) changes in wind speed and direction, called wind shear, which makes storms stronger and more persistent. The third component that is harder to identify is the trigger for storm formation, such as a very hot day, or perhaps a cold front. Without this ingredient, not all favorable conditions lead to severe storms or tornadoes, but the first two still make severe storms more likely.
By using these components to describe the likelihood of severe storms and tornadoes forming, climate models can tell us something about the changing risks.
How are storm conditions likely to change
Climate model projections for the United States suggest that by the end of the 21st century, the overall likelihood in favor of severe storms will increase. The main reason is that warmer temperatures are accompanied by more moisture in the atmosphere, increasing the likelihood of strong updrafts.
Rising global temperatures are driving major changes in seasons that we traditionally think of as rarely producing severe weather. A strong increase in warm, moist air in the fall, winter and early spring means there will be more days with favorable conditions for strong thunderstorms — and when those storms do occur, they have the potential to generate greater intensity.
Research shows frequency and intensity
In smaller areas, we can model thunderstorms in these future climates, which brings us closer to answering whether severe storms will form. Several studies have modeled changes in the frequency of severe storms to better understand this change to the environment.
Over the past few decades, we have seen evidence that weather conditions are shifting to more favorable conditions for severe storms in cooler seasons, while summer storms are becoming less likely to develop.
With tornadoes, things get trickier. Even with an accurate forecast the next day, there is no guarantee that a tornado will form. Only a small fraction of storms that arise in favorable conditions produce tornadoes.
Some simulations have explored what would happen if tornado outbreaks, or the storms that produce them, occurred during different levels of global warming. Projections suggest that more intense tornado storms may be more likely as global temperatures rise, though less intense than we would expect from the increase in available energy.
The impact of 1 degree warming
Much of what we know about how climate warming affects severe storms and tornadoes is regional, mainly in the United States. Not all parts of the world will see severe storm conditions change at the same rate.
In a recent study, my colleagues and I found that severe storm conditions increase more rapidly in the Northern Hemisphere, and more at higher latitudes. In the United States, our research suggests that for every 1 degree Celsius (1.8 degrees Fahrenheit) increase in temperature, favorable conditions can increase by 14-25% in spring, fall and winter, with the largest increase in winter. This is mainly driven by the increase in available energy due to higher temperatures. Remember, it’s about favorable conditions, not necessarily tornadoes.
How did that affect the December tornadoes?
To answer whether climate change affected the likelihood or intensity of tornadoes that erupted in December 2021, it’s still hard to attribute any single event like this to climate change. Short-term influences such as the El Nino-Southern Oscillation could also complicate the picture.
Of course, there are signs of a more stormy future direction, but how tornadoes behave is an open area of research.