Tornadoes have been recorded all over the world, but the United States experiences around a thousand of them each year, which is far more than anywhere else on the planet. Most of these occur in “Tornado Alley,” an area of the Great Plains region, where the atmospheric conditions are just right for massive, tornado-spawning thunderstorms. The resulting tornadoes leave a trail of destruction in their wake, often with deadly consequences.
A Changing Climate
Scientists agree that the climate is changing, and humans are responsible. The burning of fossil fuels, such as coal, oil, and gas, releases huge amounts of carbon dioxide (CO2) into the atmosphere every year, which is leading to a rise in global temperatures, known as global warming.
Global warming is just one symptom of the larger problem of climate change. Climate change has also caused an increase in extreme weather events all over the world. “Extreme weather events” is a catch-all term for a variety of very different weather phenomena, some of which are easier to attribute to climate change than others. For example, scientists can say with a high degree of certainty that a warming planet will lead to more severe droughts in some areas and heavier rainfall in others. Unfortunately, other weather events, such as tornadoes, are much harder for climatologists to predict.
Tornadoes Are Changing
Predicting whether climate change will have an effect on the frequency and power of tornadoes is a challenge.
For all their destructive fury, tornadoes are relatively small when compared to some other extreme weather events. Hurricanes, for example, can span hundreds of miles, whereas the biggest tornado ever recorded measured 4.2 kilometers (2.6 miles) wide. They are also very short lived, lasting from a few seconds to a few hours as opposed to days or weeks at a time. This makes them very difficult to model in the climate simulations that scientists use to project the effects of climate change.
Instead, scientists must attempt to predict how climate change might affect the individual weather “ingredients” that support the development of supercell thunderstorms (the type that produce tornadoes). These weather ingredients are:
- warm, moist air;
- an unstable atmosphere; and
- wind at different levels moving in different directions at different speeds, a phenomenon known as wind shear.
As global temperatures rise, the hotter atmosphere is able to hold more moisture. This increases atmospheric instability, a vital supercell ingredient. On the other hand, as the planet warms, wind shear (another vital ingredient) is likely to decrease. These two forces work against each other, and it is difficult to anticipate which might have a greater impact on tornado formation.
Some studies predict that climate change could provide the opportunity for more severe thunderstorms to form. However, this does not necessarily mean that more tornadoes will occur, especially in light of the fact that only about 20 percent of supercell thunderstorms produce tornadoes. To complicate things further, no one fully understands how tornadoes are formed.
Climate simulations can help scientists predict what effect climate change might have in the future. They can also examine official records to see if there have been any changes in frequency and strength of tornadoes over time. Unfortunately, in the United States, tornado records only date back to the 1950s.
At first glance, there appears to have been an increase in tornadoes since these records began, but that is not the full story. It was not until the early to mid-1990s that an extensive Doppler radar network was established in the United States for the detection of tornadoes. Until then, records relied on eyewitnesses to report tornado sightings, which means that if no one saw a tornado, it would not appear on weather records. This makes it hard for researchers to spot any long-term trends because the data is skewed by an increased detection of small tornadoes and tornadoes in sparsely populated areas after Doppler radar networks were introduced.
In fact, when you remove small tornadoes from the record, the data does not suggest any long-term increase in tornado frequency. If anything, there may be a slight decline in the number of very strong tornado events. However, other research has found evidence of an increase in tornado power.
While there have been no long-term trends in thefrequency of tornadoes, there have been changes in tornado patterns in recent years. Research has shown that there are fewer days with at least one tornado but more days with over thirty, even as the total number of tornadoes per year has remained relatively stable. In other words, tornado events are becoming more clustered.
There is also evidence to suggest that tornado patterns have shifted geographically. The number of tornadoes in the states that make up Tornado Alley are falling, while tornado events have been on the rise in the states of Mississippi, Alabama, Arkansas, Missouri, Illinois, Indiana, Tennessee, and Kentucky.
There is speculation that some of these changes are linked to climate change and its effect on the jet stream. Tornado outbreaks have also coincided with rising ocean temperatures. But no one can say for certain that climate change is a contributing factor in these events. It is very hard to tease out which changes are down to climate change and which changes might be caused by interaction with natural climate fluctuations such as El Niño.
Is Climate Change to Blame?
The fourth National Climate Assessment summarizes the complicated relationship between tornadoes and climate change: “Some types of extreme weather (e.g. Rainfall and extreme heat) can be directly attributed global warming. Other types of extreme weather, such as Tornadoes, are also exhibiting changes which may be linked to climate change, but scientific understanding isn’t detailed enough to project direction and magnitude of future change.” In other words, we still have a lot to learn about how climate change might affect tornadoes.
One thing we know for certain is that we live in a warmer, wetter world thanks to climate change, and this is likely to have an effect on extreme weather events, including tornadoes. Unfortunately, in the case of one of nature’s most violent storms, we cannot yet predict what that effect might be.