What a Warming World Means for Deadly Twisters: Tornadoes and Climate Change

The deadly tornado outbreak that ripped through communities from Arkansas to Illinois on the night of December 10-11, 2021 was so unusual in terms of duration and strength, especially for December, that many people, including the US president, are wondering if climate change played a role - and whether tornadoes will become more common in a warming world.

Both questions are easier to ask than to answer, but new research is providing new insights.

I'm an atmospheric scientist who researches severe convective storms like tornadoes, as well as the effects of climate change. Here's what scientific research has revealed thus far.

Scientists use complex computer models to characterize the entire Earth system, from the Sun's energy streaming in to how the soil responds, and everything in between, year after year and season after season, to understand how rising global temperatures will affect the climate in the future.

On a global scale, these models solve millions of equations.

Each calculation accumulates, necessitating far more processing power than a desktop computer can provide.

We currently have to use a broad scale to project how the Earth's climate will change by the end of the century.

Consider the zoom feature on a camera when looking at a distant mountain.

The forest is visible, but individual trees are difficult to distinguish, and a pine cone in one of those trees is too small to see even when the image is blown up.

The smaller the object in a climate model, the more difficult it is to see it.

Tornadoes and the severe storms that produce them are much smaller than climate models can predict.

Instead, we can look at the large-scale factors that contribute to tornado formation.

(1) Energy driven by warm, moist air promoting strong updrafts, and (2) changing wind speed and direction, known as wind shear, which allows storms to become stronger and longer-lived, are two key ingredients for severe storms.

A third ingredient, which is more difficult to identify, is a storm-forming trigger, such as a particularly hot day or a cold front.

Although not every favorable environment leads to severe storms or tornadoes without this ingredient, the first two conditions make severe storms more likely.

Climate models can tell us something about the changing risk of severe storms and tornadoes by using these ingredients to characterize the likelihood of severe storms and tornadoes forming.

Climate model projections for the United States suggest that by the end of the century, the overall likelihood of favorable ingredients for severe storms will increase.

The main reason is that rising temperatures combined with rising atmospheric moisture increase the potential for strong updrafts.

Rising global temperatures are causing significant changes in seasons that we used to think only produced severe weather on rare occasions.

Stronger increases in warm humid air in the fall, winter, and early spring mean more days with favorable severe thunderstorm environments - and these storms have the potential to be more intense when they do occur.

We can simulate thunderstorms in these future climates over smaller areas, bringing us closer to determining whether severe storms will form.

To better understand this change in the environment, several studies have modeled changes in the frequency of intense storms.

In the last few decades, we've seen evidence of a shift toward conditions more favorable for severe storms in the cooler seasons, while the likelihood of storms forming in the summer has decreased.

When it comes to tornadoes, things get a little more complicated.

Even if the forecast for the next day is otherwise accurate, there is no guarantee that a tornado will form.

Only a small percentage of storms that form in a favorable environment produce tornadoes.

Several models have been run to see what would happen if a tornado outbreak or tornado-producing storm occurred at various levels of global warming.

Projections suggest that as global temperatures rise, stronger, tornado-producing storms will become more likely, though they will be weaker than we might expect given the increase in available energy.

Much of what we know about how climate change affects severe storms and tornadoes is limited to specific regions, primarily in the United States.

Changes in severe storm environments will not occur in all parts of the world at the same time.

In a recent study, my colleagues and I discovered that the rate of increase in severe storm environments will be higher in the Northern Hemisphere and at higher latitudes.

Our research suggests that in the United States, for every 1 degree Celsius (1.8 F) increase in temperature, a 14-25 percent increase in favorable environments is likely in spring, fall, and winter, with the greatest increase in winter.

This is primarily due to the increased energy available as a result of higher temperatures.

Remember, this is about favorable conditions, not necessarily tornadoes.

It's difficult to say whether climate change influenced the likelihood or intensity of tornadoes in the December 2021 outbreak because no single event like this can be attributed to climate change.

Shorter-term factors such as the El Nio-Southern Oscillation could further complicate the picture.

There are certainly signs pointing to a stormier future, but how this manifests in tornadoes is still a work in progress.

Central Michigan University's Associate Professor of Meteorology, John Allen

The Conversation has given permission to republish this article under a Creative Commons license.

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