Was tornado Friday related to climate change?
WASHINGTON — The calendar said December but the warm moist air screamed of springtime. Add an eastbound storm front guided by a La Nina weather pattern into that mismatch and it spawned tornadoes that killed dozens over five U.S. states.
Tornadoes in December are unusual, but not unheard of. But the ferocity and path length of Friday night’s tornadoes likely put them in a category of their own, meteorologists say. One of the twisters — if it is confirmed to have been just one — likely broke a nearly 100-year-old record for how long a tornado stayed on the ground in a path of destruction, experts said.
Warm weather was a crucial ingredient in this tornado outbreak, but whether climate change as a factor is not quite as clear, meteorologists say.
Scientists say figuring out how climate change is affecting the frequency of tornadoes is complicated and their understanding is still evolving. But they do say the atmospheric conditions that give rise to such outbreaks are intensifying in the winter as the planet warms.
What causes a tornado?
Tornadoes are whirling, vertical air columns that form from thunderstorms and stretch to the ground. They travel with ferocious speed and lay waste to everything in their path.
Thunderstorms occur when denser, drier cold air is pushed over warmer, humid air, conditions scientists call atmospheric instability. As that happens, an updraft is created when the warm air rises. When winds vary in speed or direction at different altitudes — a condition known as wind shear — the updraft will start to spin.
These changes in winds produce the spin necessary for a tornado. For especially strong tornadoes, changes are needed in both the wind’s speed and direction.
“When considerable variation in wind is found over the lowest few thousand feet of the atmosphere, tornado-producing ‘supercell thunderstorms’ are possible,” said Paul Markowski, professor of meteorology at Pennsylvania State University.
How related is climate change to tornado outbreaks?
It’s complicated. Scientists are still trying to sort out the many conflicting factors about whether human-caused climate change is making tornadoes more common — or even more intense. About 1,200 twisters hit the U.S. each year — though that figure can vary — according to the NOAA National Severe Storms Laboratory. No other country sees as many.
Attributing a specific storm like Friday’s to the effects of climate change remains very challenging. Less than 10% of severe thunderstorms produce tornadoes, which makes drawing conclusions about climate change and the processes leading up to them tricky, said Harold Brooks, a tornado scientist at the National Severe Storms Laboratory.
The U.S. is likely to see more tornadoes occur in the winter, Brooks said, as national temperatures rise above the long-term average. Fewer events will take place in the summer, he said.
