Tornado Outbreaks Are on the Rise, and Scientists Don’t Know Why

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Late Tuesday and early Wednesday morning, a dozen tornadoes tore through the southern part of the US, killing 5 people and destroying up to 20 buildings in one town alone in Alabama.

This cluster of tornadoes, all part of one large weather-event, constitute a “tornado outbreak.” These are the most damaging and harmful tornado-related events that can occur. And new research, published today in Science, has found that the frequency and intensity of these deadly tornado outbreaks has nearly doubled in the last 50 years and may continue to do so. Whether these trends are related to climate change or not is unclear, and raises the question as to whether or not global warming may be affecting weather events in ways we don’t understand.

Comprehending how the most extreme tornado outbreaks function and form is a not just a matter of public health, but also a matter of financial security. The first half of 2016 in the U.S. saw $8.5 billion in insured losses due to tornadoes and severe thunderstorms. So a team of climate scientists led by Michael K. Tippett of Columbia University analyzed the most intense events on two National Oceanic and Atmospheric Administration datasets of tornadoes and meteorological observations related to tornado outbreaks from 1965 to 2015.

They found that the worst outbreaks are containing more tornadoes, and worse, that those outbreaks are growing in intensity faster than the “regular sized” ones. But they can’t determine why this is happening.

Climate change is one guess, but the researchers haven’t found any conclusive signs that it is the culprit. Lead author Michael Tippett told Motherboard that climate models are good for predicting temperature rise, but they can’t say how weather will change. So scientists act like weather forecasters. They look for environments that are favorable to severe storms, and then see if climate change is likely to create more of those storm-friendly environments.

“If it’s climate change, well—we can expect more,” Tippett said. “That’s the bad news scenario. And the good news scenario is if it’s not climate change then we could go back to the way it was earlier. The 80s, let’s say.”

One of the main ingredients for tornadoes, thunderstorms and the like is something called convective available potential energy (CAPE), which refers to warm moist air near the ground surface that ultimately rises up into the atmosphere. Climate projections have said that increases in CAPE will cause more frequent storm-friendly environments, explained Tippett.

But despite the increase in the intensity of tornado outbreaks, “we don’t see these extreme CAPE environments changing” he said. “It’s either not climate change, or it’s something about climate change that we don’t understand.”

Tippett also pointed out that lots of aspects of climate change are unknown. Particularly with regards to weather. “We know temperature is going up, we know some things pretty sure, but the details of the weather—there’s a lot more uncertainty.” Ultimately, the frequency and intensity of tornado clusters in the United States—which are responsible for 79 percent of all tornado-related deaths—has doubled since 1965. And it would behoove us to find out why.

“The answer to the question can tell us what to expect in the future. That’s why we think it’s an important question,” said Tippett.

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