Recent tornadoes have ravaged the midwestern United States, demolishing buildings and killing at least 90 people.
“I don’t think we’ll have seen damage at this scale, ever,” Kentucky Gov. Andy Beshear said.
But damage on this scale from tornados will almost certainly be seen again, because climate change will likely increase the frequency and severity of tornadoes, according to climate scientists.
While the causal link between climate change and tornadoes is less well established than it is for other extreme weather events such as wildfires and hurricanes, climate change creates the conditions that are most conducive to tornadoes.
“The strongest statement that can probably be made about tornadoes and climate change so far is that there really isn’t a clear historical signal, but that there is increasingly strong evidence from climate model simulations that tornado-producing environments may occur more frequently in the future, particularly in winter,” Daniel Swain, a UCLA climate scientist told Yahoo News.
The main reason for the relative uncertainty about the effect of climate change on tornadoes is that tornadoes are relatively uncommon globally and therefore not as well understood.
“It’s really difficult to get data because we do not have good records of tornadoes going back very far,” Swain said. “Unlike thermometers or rain gauges, we don’t have a long record.”
Moreover, Swain noted, “tornadoes are really sparse. Every weather station on Earth experiences some downpours and a heat wave or two every year. The average weather station on Earth is never hit by a tornado in its entire existence.”
So whether tornadoes are increasing in frequency the way that hurricanes and heat waves are remains unclear. Many more tornadoes were recorded in the last 20 years than in the previous two decades, but that may be largely due to more tracking.
Nonetheless, Swain said, “you can approach it sort of indirectly, by asking about atmospheric environments that are favorable tornado development, or severe tornado development. When you ask tornado scientists, they’ll say, ‘We know what these ingredients are that produce tornadoes, so we can look at those ingredients, how are those ingredients changing?’”
And those environments that lead to tornadoes are exactly the kind of environments that are becoming more prevalent because of global warming.
“All tornadoes come from thunderstorms,” Swain pointed out, and thunderstorms are made more likely and more severe on average by warmer weather and the higher rates of evaporation it causes.
“With each degree of warming, or even every half degree of warming — it’s surprisingly striking even for relatively small amounts of warming — the thermodynamic environment becomes more favorable for tornadoes,” Swain said.
The recent tornadoes are therefore connected to the unusual heat wave that has engulfed much of the country.
In particular, severe tornadoes may become more common because of climate change in the southeastern United States in winter.
“The most robust increases in these tornado-producing environments seem most likely to happen in what used to be cooler times of the year,” Swain said. “In winter, historically, you didn’t have a lot of heat and humidity.”
“So what seems to be happening, the most plausible connection to climate change, is that in these cooler season months, you have a lot of warming and additional moisture in the atmosphere. And it is worth noting that immediately before the tornadoes this week, there were tons of record-high temperatures and record-high dew-point levels in the atmosphere being set.
The Gulf of Mexico is experiencing record warm temperatures for this time of year. So there’s a huge pool of unusually warm water just to the south of this region that experienced these anomalous tornadoes. So that would be very consistent with the main mechanism people have proposed as to why climate change would influence tornadoes, if it did.”
This reasoning is bolstered by recent research. A study published last month in the academic journal Earth’s Future modeled the effects of warming on the creation of “convective environments” that can lead to tornadoes. The researchers found that each degree Celsius of average warming increases the frequency of connective environments by 5 percent to 20 percent, with much larger changes at higher latitudes, especially in the Northern Hemisphere.
Experts caution, however, that direct links between tornadoes and climate change remain unproved.
Echoing Swain’s analysis, John T. Allen, a professor of meteorology at Central Michigan University, wrote in a USA Today op-ed piece: "Climate projections for the late 21st century have suggested that the conditions favorable to the development of the severe storms that produce tornadoes will increase over North America, and the impact could be greatest in the winter and fall.”
But meteorologist Harold Brook of the National Oceanic and Atmospheric Administration told the BBC that while the increasingly common tornado clusters “clearly implies that the patterns of the atmosphere have changed. ... That may be related to climate change — but we cannot make a full conclusion.”
One scientist compared climate change’s influence on tornadoes to the relationship between fog and car crashes. “You might see that, for example, when it’s foggy, there’s more crashes, but you don’t say the fog caused the crashes,” James Elsner, a professor of geography at Florida State University told the Hill. “If you think about climate change as the fog, we can’t really say it caused the accident, but it contributes.”