What causes a tornado?
Tornadoes are formed by a combination of atmospheric instability and wind shear. Instability occurs when warm, moist air is wedged under drier, cooler air aloft. This warm air rises, causing the intense updrafts and downdrafts seen in strong thunderstorms — the incubators of tornadoes. Wind shear refers to changes in wind direction and speed at different elevations in the atmosphere. The combination of instability and wind shear forms the circular air flow that generates a tornado.
Why are there so many in the U.S. Midwest?
The Central United States has an abundance of the ingredients necessary for tornado formation. During the spring, warm tropical air masses from the Gulf of Mexico collide with colder, drier air at higher altitudes to spawn intense thunderstorms. Thunderstorms also form into the summer and fall, as the region’s vast plains heat up air near the surface, causing atmospheric instability. These intense thunderstorms act as incubators for tornadoes. Tornadoes occur around the world but are most common in “Tornado Alley” of the Central United States.
Is there a link between climate change and tornadoes?
It is important to keep in mind that climate change has an impact on all weather events. The effect of the carbon dioxide in our atmosphere cannot be switched off – it influences our seemingly benign “everyday” or “normal” weather as well as the extreme events.
A specific link between tornadoes and climate change, however, is unclear. It is difficult to identify and diagnose trends in long-term records of tornadoes, since the population in many areas affected by tornadoes has grown (e.g., tornadoes in the early part of the 20th century may have occurred without anyone seeing them) and the technology used to observe tornadoes has improved (e.g., radars help us “see” tornadoes in ways that were not possible many decades ago).
How could climate change affect the frequency or intensity of tornadoes?
Researchers are working to better understand how the building blocks for tornadoes -- atmospheric instability and wind shear -- will respond to global warming. It is likely that a warmer, moister world would allow for more frequent instability. However, it is also likely that a warmer world would lessen chances for wind shear. Recent trends for these quantities in the Midwest during the spring are inconclusive. It is also possible that these changes could shift the timing of tornadoes or regions that are most likely to be hit.
Adding to the difficulty, tornadoes are too geographically small to be well simulated by climate models. Models can simulate some of the conditions that contribute to forming severe thunderstorms that often spawn tornadoes. Multiple studies (e.g., here and here) find the conditions that produce the most severe thunderstorms are likely to occur more often in the in a warmer world, even if the total number of thunderstorms decreases (because of fewer weak storms). However, this work does not conclusively tell us whether tornadoes should follow the same trend as their parent thunderstorms.