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.
Throughout March, a string of deadly tornadoes tore through the Southern United States as volatile weather phenomena 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, a National Weather Service-confirmed 49 tornadoes 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. However, about a week later on March 25 and 26, another 24 more twisters ripped through the region. Many 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, falls. 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 towards land — becoming a tornado.
These conditions are typical in supercell thunderstorms, which are the strongest type of thunderstorm. About 20% of superstorm thunderstorms produce tornadoes.
The South saw a set of supercell storms, which caused wild weather from baseball-sized hail to major flooding. Ultimately, these turned into tornadoes.
Some tornadoes then can become more dangerous than others because of stronger winds or because of their path — if the location is more populated or developed — or a combination of both.
At the time, 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.
So how does climate change play in? Climate change’s effect on extreme weather events manifests in different ways. Some of these events, such as increases in extreme heat events or extreme precipitation events, are easy enough to track and 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 already 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.
Now, compare that 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 modelling scenarios that scientists use to project and understand the effects of climate change on the atmosphere and weather.
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 indeed create a hotter atmosphere that holds more moisture, so there is more warm, moist air, increasing atmospheric instability. But some 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 this would lead to.
It’s easier, on the other hand, to see the change in the storms that tornadoes are borne from. Climate change is directly linked to extreme rainfall, which has already increased over much of the central US, where many tornadoes have formed. According to research, since 1991, the amount of rain falling in very heavy precipitation events has risen, with the greatest increase in the Northeast, Midwest, and upper Great Plains, where it’s more than 30% above the 1901–1960 average. 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 still don’t know exactly what that impact is.