What, if anything, is climate change doing to tornadoes?
TORNADOES CAN, individually, be unpredictable to the point of caprice. In general, though, their behaviour follows particular rules. They spawn in places where big slabs of cold air can move in on top of warmer air below on a large scale, with strong winds headed in different directions at different altitudes. The Great Plains of the United States, where cold air from the Rockies can pour down over warm air from the Gulf of Mexico, are pretty much the best place in the world for this to happen. That is why America sees a lot more tornadoes than other places. There is a seasonal predictability, too; Americas tornadoes tend to be a springtime phenomenon, because at that time the centre and south of the country can be pretty warm even as wintry weather is still able to blow in from the north and west. In the south-east of the country, though, there is a second bump in frequency in the autumn. More distant factors also add a certain statistical predictability. In La Nina years, during which the trade winds in the Pacific blow strongly and the western Pacific gets anomalously warm, changes to the jet stream make the south warmer and encourage low-pressure storm systems east of the Rockies, which adds up to a greater tornado risk. 2011, a very bad tornado year in the south-east, was also a La Nina year. The terrible damage done by tornadoes in Americas south-east on the night of December 10th-11th fits with some of this predictability. Late tornadoes are more likely in the south-east, though tornadoes this late and this destructive are very much a rarity. And 2021 has been a La Nina year, as 2022 is expected to be, though this did not make the spring tornado season particularly destructive; it was, in fact, unusually placid, wrong-footing predictions. One result of the carnage has been a rush to say whether there is another predictability at play: that provided by a warming world. It is certainly true that, in the area hit by the tornadoes as in most of the rest of the country, this has been a remarkably warm autumn. The Gulf of Mexico was far warmer than usual for the time of year in both October and November. It is also true that, in its latest assessment of the physical science of climate change, the Intergovernmental Panel on Climate Change (IPCC) found that Climate models consistently project environmental changes that would support an increase in the frequency and intensity of severe thunderstorms that combine tornadoes, hail, and winds. For example Convective available potential energy, a measure of how much trouble a particular bit of atmosphere can store up, is expected to rise along with the temperature. But that does not mean that Deanne Criswell, head of Americas Federal Emergency Management Agency, is right when she says that this sort of weather will be the new normal. Most strikingly, although the world has already seen around 1.1-1.2C of warming, American tornadoes have not in fact become more common; there is no discernible trend in their frequency. Despite the fact that various underlying factors suggest a trend might be expected, the IPCC feels that predictions of such a trend cannot be made with any confidence. Other aspects of the data do show trendsbut here, too, a direct link to climate change is missing. Tornado seasons are becoming more variable, both from year to year and day by day within a given year. Given that the long-term averages are staying the same, this means more pronounced extremes of activity and inactivity both day to day and year to year. More worryingly, tornados are moving east. The number of tornado days has been falling in Colorado, Kansas, Oklahoma and Texas; to the east it has been rising, most notably in Alabama, Arkansas, Mississippi and Tennessee. Bob Henson, a meteorologist who writes for the Eye on the Storm blog published by the Yale Centre for Environmental Communication, notes that a decade of higher temperatures in the south-west and an ever warmer Gulf of Mexico could be factors, but whether they are indeed causal remains to be seen. Whatever the reason, though, the trend is disturbing. The south-east already faces higher tornado risks than the Great Plains do, and is more likely to see night-time cyclones, which are harder to track and harder to spread warnings of (since people tend to be asleep). Increasing incidence there, and a move farther east into areas where people are less prepared, is likely to make things worse. Understanding the reasons for the trend might well be helpful. But you do not need to understand it to act on it. And you do not need to think tornadoes are becoming more common to think that mitigating climate change should remain an overarching global imperative.