Speculation is rife that a recent glut of killer tornadoes is a sign of rising temperatures. What’s the evidence?
LAST year was a particularly deadly year for tornadoes in the US, with the second highest death toll in 137 years. Already this year, major outbreaks have killed 63 people. All eyes are now on May, when the season usually peaks, amid talk in some quarters of another year of extremes.
The result is that discussion about human influences on severe thunderstorms and tornadoes has been reignited. Such discussion is not new. It seems that, following every major tornado event in the last decade, the influence of rising levels of atmospheric carbon dioxide has been questioned. Human impacts have been talked about for much longer than that, however. Three tornadoes that killed at least 90 people in 1953 led to speculation that atmospheric tests of nuclear weapons caused them.
But the speculation has been one-sided, with talk of human influences on tornadoes following disasters, but little about whether human influences have acted as a brake on their numbers after anomalously low activity.
A logical place to begin when examining whether the incidence and severity of tornadoes has changed in the US is the official record of reported tornadoes that dates back to 1950. Unfortunately, the way these reports are collected and the method of assessing damage, and hence tornado intensity, has changed over time and differs from place to place. This means the record is far from uniform. What it does reveal, though, is that from 1954 to 2003, anaverage of 14 more tornadoes per year have been reported, almost all in the weakest category, probably due to improved observation and reporting.
In addition, changes in how we assess tornado intensity mean that those prior to the mid-1970s were rated higher in intensity than those afterwards, especially those since 2000.
As a result, there is not much we can say about long-term patterns, except that the annual average count of all but the weakest tornadoes has not changed for more than half a century. Also, the timing of the tornado season hasn’t changed.
If the reports are inadequate for assessing the past, how can we use them to extrapolate into the future? Luckily there is another option. As advocated by a 2002 Intergovernmental Panel on Climate Change workshop, we can consider the atmospheric conditions in which the severe thunderstorms that can spawn tornadoes are most likely to form and see how they might alter.
In effect, we look at the problem like a weather forecaster, attempting to see if favourable conditions will change in frequency. This approach builds on 50 years of research intended to identify conditions in which severe storms are most likely to occur. Although many things have to come together in the right way to get an intense thunderstorm, there are two measures of the state of the atmosphere that can be used as indicators.
First, the Convective Available Potential Energy (CAPE) indicates instability in the atmosphere and hence the likelihood of big storms building. Second, vertical wind shear indicates the change in the wind direction from near the ground to several kilometres into the atmosphere. This shear helps organise storms and provides the source of rotation for the most severe thunderstorms, called supercells. Of the two measures, shear is the most important.
In general, CAPE and shear are a bit like two ends of a balance – when one is up the other tends to be down. One of the results of this is that tornadoes don’t happen in the US summer, when CAPE tends to be the largest, or winter, when shear is the largest. Instead they occur in spring and, to a lesser extent, autumn, when both can be fairly big.
To look into the effects of climate change on tornadoes, we can look to the average changes in CAPE and shear. From observations there is a suggestion that CAPE has increased and shear has decreased during the past half century, but the changes have not been statistically significant, mainly because of the extreme variability in both time and space.
The majority of the climate simulations that look at future changes support increases in CAPE and decreases in shear. Importantly, it is not just changes in the overall values that matter, but also whether they occur simultaneously.
The models suggest that high shear will not be associated with high CAPE any more than it is currently. And it takes a long time before any changes to the frequency of the occurrence of favourable conditions for the formation of these storms become statistically significant – 100 years in some cases.
The three simulations that have looked most closely at this, including one in which I was involved, agree that the increase in CAPE is probably more than enough to offset the decrease in shear for the production of severe thunderstorms.
Despite the resulting prediction of a rise in the number of days with favourable conditions for these storms, the extreme dependence of tornadoes on the shear makes it very difficult to tell if such a shift will translate into significant changes in tornado occurrence, up or down.
What is easier to say is that, given the history of tornado outbreaks, there will undoubtedly be more years with many bad storms, as well as years with relatively few. Damage and the threat to human life result from the unfortunate encounter of tornadoes with people, so regardless of the impact of climate change, it is certain there will be, at least occasionally, extremely damaging ones.
Equally clear is that those who continue to talk in certain terms of a future blighted by more severe tornadoes as a result of climate change are failing to heed all the available evidence.
Harold Brooks is a research meteorologist and head of the modelling, observation and analysis team at the National Oceanic and Atmospheric Administration’s National Severe Storms Laboratory in Oklahoma