A tornado, albeit a weak one, touched down in New York City last Thursday, in the College Point neighborhood of Queens. A few days earlier, a stronger tornado was spotted near the town of Douglas, in central Massachusetts. And a few days later a whirlwind ripped through nearby Webster, displacing dozens of people from their damaged homes.
The storms were far from the region in the middle of the country known as Tornado Alley, where the bulk of the nation’s tornadoes occur. In a summer already marked by simmering heat that researchers have linked to global warming, is climate change also making tornadoes more common in places where they once were infrequent?
Though individual weather events are distinct from the more broadly changing climate, global warming does influence weather patterns. Still, any link between climate change and the frequency of tornadoes is far from straightforward, according to researchers.
“Tornadoes are the kind of extreme event where we have the least confidence in our ability to attribute the odds or characteristics of individual events to an influence of global warming,” said Noah Diffenbaugh, a professor of earth system science at Stanford University.
The reason, in part, has to do with the limited data available on tornadoes versus other types of weather phenomena. When it comes to extreme heat, for example, researchers can mine independent temperature records that stretch back more than a century. This historical record helps them assess, as they have this summer, that a heat wave is in keeping with human-driven changes to the climate, rather than just natural variation.
Tornado research, by contrast, does not benefit from a long data record. Until the 1990s tornado data was primarily based on someone spotting a tornado and reporting it to the National Weather Service. Since many tornadoes are small and last only a few minutes, some of them have presumably gone unnoticed.
“If a tornado happens and no one was there to observe it, then it wasn’t counted,” Diffenbaugh said.
In more recent years, increases in population and the rise of social media have made it more likely that tornadoes are spotted. This may lead the public to believe that there has been an uptick in tornadoes, when what is at play is increasing awareness. There have, after all, been reports of tornadoes hitting New York City as far back as 1880.
The development of a comprehensive Doppler weather radar system helped overcome many observation gaps by the 1990s, but modern technology cannot make up for the limited records of the past. This makes it harder to find trends in tornado data than in temperature data, said Michael Tippett, a professor of applied mathematics at Columbia University.
However, Tippett and his colleagues have published research on tornado outbreaks — tornadoes that occur in a bunch — and they did find a trend. “Those bunches are getting bigger,” said Tippett.
Though it is not possible to quantify to what degree, if any, climate change played a role in the tornadoes in New York and Massachusetts, researchers have some inkling into how climate change will affect tornadoes more broadly.
They do not yet have the historical data or computing power to simulate tornadoes themselves. But they can simulate the broader, changing climate to see how it will affect the underlying conditions that create thunderstorms, which set the stage for tornadoes.
Researchers know that there are two ingredients that fuel severe storms that could spawn tornadoes: potential energy in the air and wind currents, or wind shear. The rising levels of greenhouse gases in the air add more energy to the climate system, Diffenbaugh said. There is less consensus on how climate change will affect wind shear, though a 2013 study on the thunderstorm conditions that form tornadoes found that the impact was negligible.
Overall, “We do have strong evidence that at the large scale that global warming is likely to increase the atmospheric environments that create the kind of severe thunderstorm that produces tornadoes,” Diffenbaugh said.
When scientists run climate models assuming global average temperatures of one degree Celsius (two degrees Fahrenheit) higher than preindustrial levels — where the Earth currently stands — some show an uptick in tornado frequency, but others do not. That disagreement, however, fades away at two degrees Celsius of warming, the threshold that the Paris climate agreement is intended to avoid. All the models agree that the frequency of tornadoes will increase by that point.
And climate change may already be affecting tornadoes, Diffenbaugh said. “It’s just that we can’t distinguish the signal from the noise,” he said.