A pair of tornadoes seen near Pilger, Nebraska, in June 2014. (Image: Eric Anderson/AP)

Tornadoes that come in bunches are on the rise in the United States, according to a new study. Though it might be tempting to blame climate change, scientists aren’t entirely sure what’s causing this troubling trend.

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From 1965 to 2015, the frequency of tornado outbreaks—that is, six or more tornadoes that occur in close succession—has increased in the continental United States, according to a new study published in the journal Science. Alarmingly, these extreme weather clusters have caused nearly 80 percent of tornado-related fatalities between 1972 and 2010.

The Columbia Engineering scientists who conducted this research say tornado outbreaks are not just increasing in frequency over time, they’re also increasing in terms of severity. The scientists are at a loss to explain why this is happening, and say further research is warranted.

Tornado damage in Kokomo, Indiana, August 24, 2015. (Image: Nicole Misencik/WTHR 13 via AP)

For the study, the researchers looked at trends in the severity of tornado outbreaks, with severity measured by the number of tornadoes per outbreak. Weather-related factors were measured over the course of the time period studied, but none could be connected to climate change.

This study is raising important questions about tornadoes, and a potential, but still unproven link, to climate change, which is also expected to influence the frequency, nature, and severity of thunderstorms.

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“The fact that we don’t see the presently understood meteorological signature of global warming in changing [tornado] outbreak statistics leaves two possibilities: either the recent increases are not due to a warming climate, or a warming climate has implications for tornado activity that we don’t understand,” noted study lead author Michael Tippett in a statement. “This is an unexpected finding.”

Tippet, along with his colleagues Chiara Lepore and Joel Cohen, looked at two NOAA datasets, one containing tornado reports, the other observation-based estimates of meteorological data linked to tornado outbreaks. This, unlike previous research efforts, allowed the researchers to perform an independent check of the tornado reports to see if they overlap with the extreme meteorological environments associated with severe thunderstorms, and to see if climate change is responsible for these trends.

Numbers of tornadoes per outbreak. (Image: Tippett et al., 2016/Science)

Looking at the time period from 1965 to 2015, Tippett’s team found that the frequency of outbreaks with multiple tornadoes is on the rise—and it’s increasing faster for more extreme outbreaks. The frequency of tornadoes during the most extreme outbreaks have doubled, increasing from 40 in 1965 to nearly 80 in 2015. It’s important to note that average tornado strength has not increased over the course of the past half-century.

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Importantly, this data was adjusted for differences in weather monitoring. Over the past 50 years, more tornadoes have been reported because more people are reporting weaker tornadoes. Accordingly, the researchers excluded any tornado below a F0 rating (like hurricanes, tornadoes are ranked along a scale, from F0 to F5). Even after these adjustment, the researchers still found a significant increase in both the frequency and intensity of the outbreaks during this period. “In addition to tornado reports, we considered the meteorological environments favorable to tornadoes, and they have become more extreme too,” Tippett told Gizmodo.

A tornado near Elk Mountain, west of Laramie Wyoming on the June 15, 2015. The tornado passed over mostly rural areas of the county, lasting over 20 minutes. (Image: John Allen/Central Michigan University)

Indeed, weather patterns have changed in the US in the past 50 years, and in a way that’s more tornado-friendly. Given the onset of human-induced climate change, the researchers naturally assumed that a meteorological phenomenon known as convective available potential energy (CAPE) was to blame. Cape is a measure of the propensity of air to rise, and a weather trend often associated warm moist air near the surface. As Tippett pointed out, “Tornadoes like CAPE.” Weather models have shown that CAPE will increase as the climate warms, leading to more severe thunderstorms in the United States. That said, the researchers found that the meteorological trends driving the recent spate of tornado clusters were not due to increasing CAPE.

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Instead, the researchers noticed trends in the appearance of something called storm helicity, a measure of vertical wind shear, which is how wind speed and direction vary with altitude. “Tornados like vertical wind shear,” Tippett told Gizmodo, “and extreme values of storm helicity do show upward trends like the tornado reports.” All this said, trends in storm helicity are not projected to increase under climate change. Hence the mystery of what’s causing the rise in tornado outbreaks.

Extreme environments: Measures of convective available potential energy (CAPE) could not be linked to tornado outbreaks, but the same could not be said for rates of Sheer Relative Helicity (SRH). CAPE has been associated with climate change, but SRH has not. (Image: Tippett et al., 2016/Science)

“Changes in CAPE can’t explain the change,” said Harold Brooks, a senior scientist at the NOAA who was not involved with the study. “It seems that changes in shear are more important, but we don’t yet understand why those have happened and if they’re related to global warming.”

So what’s driving the rise of sheer helicity, and by consequence the frequency of tornado clusters, if it’s not climate change? One possible answer lies in a lesser-known weather phenomenon known as the Atlantic Multidecadal Oscillation (AMO), a pattern of sea surface temperature variability in the North Atlantic. “We can’t rule it out, but the evidence for it being an important factor is not compelling,” says Tippett. More data is required to link AMO to the sudden spate of tornado clusters in the US. To that end, Tippett’s team is going to investigate where these weather trends are happening, and when during the year.

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“What’s pushing this rise in extreme outbreaks is far from obvious in the present state of climate science,” says study co-author Joel Cohen. “Viewing the thousands of tornadoes that have been reliably recorded in the US over the past half century or so as a population has permitted us to ask new questions and discover new, important changes in outbreaks of these tornadoes.”

[Science]