The Weather Corner: Tornadoes wreak havoc across the Southern US. But the link between climate change and twisters is more complex than you think.

A supercell thunderstorm that turned into a tornado. Source: Flickr.

We’re bringing you exclusive content from our newsletter, The Forecast, right here on Medium. Sign up for our newsletter here. This story is from our feature called the Weather Corner, where we take a deep dive into weird weather around the world, from our April 3rd, 2021, newsletter.

Last month, a string of deadly tornadoes tore through the Southern United States as volatile weather called “supercells” swept across the region. At least six fatalities and an unknown number of injuries were reported across the affected states.

From March 17–18, National Weather Service offices confirmed 49 tornadoes Wednesday and Thursday that cut a combined path length of nearly 210 miles through nine Southern states. All were relatively low tornado intensity, and caused slight damage but no deaths — perhaps because of “a well-communicated forecast and because the twisters largely spared major population centers,” said the Washington Post. About a week later, March 25–26, another 24 more storms ripped through the region. Several of these were higher intensity levels — reaching 4 out of 5 on the scale, with at least one hitting a 5 out of 5 — and ripped up mobile homes, toppled large trees, cut off power for thousands of people, and caused the deaths of 6 people. For context, an average March in the entire US sees 75 total tornadoes. This March, we saw almost that same number concentrated across 4 days.

How did so many tornadoes form, and why were some so much worse than others? Tornadoes form from thunderstorms under certain conditions. Essentially, inside thunderstorm clouds, warm, humid air rises, while cool air, plus rain and hail, fall. That can cause spinning air currents inside the cloud. While these spinning currents start out horizontal, they can turn vertical and drop down from the cloud — becoming a tornado. These conditions are typical in supercells, which are the strongest type of thunderstorm, of which about 20% produce tornadoes.

Source: University Corporation for Atmospheric Research.

The South saw a set of supercell storms, which caused everything from baseball-sized hail to major flooding and ultimately turned into these tornadoes.

The National Weather Service warned about 30 million people in the South and Mid-Atlantic that they were under the threat of severe weather due to these violent storms. However, tornadoes are notoriously difficult to predict — scientists know how the storms are created, but it’s nearly impossible to predict where exactly a tornado will touch down.

Climate change’s effect on extreme weather events manifests in different ways. Some, such as increases in extreme heat events, reductions in extreme cold events, and increases in extreme precipitation events are easier to track. It is also easier to attribute the impact of climate change on these events. Others, such as the supercells that produce tornadoes, are much more difficult to unpack.

The role of climate change’s impact on the conditions for tornado formation is still an open question among the scientific community. Understanding whether or not climate change affects the frequency and intensity of tornadoes themselves is a challenge, because tornadoes themselves — despite their hellish reputation — are relatively short-lived and small in size compared to other extreme weather events.

Take hurricanes. Typical hurricanes are about 300 miles wide, whereas the biggest tornado ever recorded measured 4.2 kilometers (2.6 miles) wide. Additionally, the longest amount of time a hurricane lasted was three full days, compared to tornadoes, which can last from a few seconds to a few hours. These factors make them difficult to model in the climate simulations and model scenarios that scientists use to project the effects of climate change.

Rather, scientists can attempt to see how climate change might affect the individual weather conditions that support the development of supercell thunderstorms. According to National Geographic, these include:

  • the warm temperatures and moisture-holding capacity of the air
  • the instability of the atmosphere
  • how wind at different levels moves in different directions at different speeds (a phenomenon known as wind shear which impacts the formation of tornadoes)

Global warming does create a hotter atmosphere that holds more moisture, so there is more warm, moist air, increasing atmospheric instability. But scientists believe that wind shear might decrease as the planet warms. So it’s unclear how these factors might work together, and the overall changes that would lead to.

It’s easier, however, to see the change in the storms that tornadoes are born from. Climate change is directly linked to extreme rainfall, which has already increased over much of the central US, where many tornadoes have formed. Increased rainfall means more frequent and worse storms like supercells, but not directly more tornadoes.

Some of the year-to-year variability in tornado numbers is also influenced by El Niño and La Niña conditions. A 2017 paper found there are more US tornadoes in La Niña years, which we are currently in. We’ve written before about the link between these phenomena and climate change:

  • La Niña, the counterpart of El Niño, is characterized by lower-than-usual ocean surface temperatures in the Pacific near the Equator. It’s the result of shifting wind patterns in the atmosphere, where the jet stream curves and shifts north, and it happens every 3–5 years.
  • Climate change has altered some of the typical impacts of La Niña.
  • “La Niña tends to pull down global temperatures, but in recent years we have been warming the planet so fast, it’s like hitting a small speed bump at 80 mph — it barely even registers,” CNN meteorologist Brandon Miller said last year.

So how does climate change impact tornadoes? Well, scientists know that our climate change is impacting extreme weather across the world, including tornadoes. However, in terms of tornadoes, we don’t know exactly what that impact is.

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