In unusual weather, a usual suspect
Experts examining climate change’s role in wild week
Hurricane-force gusts in nine states. Immense dust storms in Colorado and wind-fanned wildfires in Kansas. The firstever December tornadoes observed in Minnesota and western Iowa. Temperatures soaring past 70 degrees in cities like Omaha, Nebraska, and Des Moines, Iowa, a record.
The powerful storm that swept through the Midwest on Wednesday was extraordinary in many respects, with 100 million Americans under some form of weather alert.
“We’ve not seen that combination of heat and dry, followed by an extended period of strong wind,” said Grady Dixon, who teaches geosciences at Fort Hays State University in Kansas. “It was so unusual for the month of December — it has to make you ask questions.”
One key question is what role global warming might have played in fueling such an extreme weather event. Scientists cautioned that it could prove difficult, if not impossible, to untangle the precise links between this week’s storm and climate change, although it appeared to be helped in part by record warmth across much of the country.
And there is evidence that the United States can expect more unusual severe storms as the planet heats up, potentially striking in new places or at unexpected
times of the year. While some questions are difficult to answer — such as whether that will mean more tornadoes in the future — scientists say the risks of increasingly wild weather make it all the more urgent that cities and states take steps to protect people and property.
“We do expect an increase in favorable conditions for severe storms,” said John Allen, an associate professor of meteorology at Central Michigan University.
Two key ingredients for severe storms are warm, moist air creating updrafts and wind shear, a change in wind speed and direction that can allow storms to become stronger. But storms also need a trigger to form, such as a hot day or a cold front, which can be somewhat more unpredictable, Allen said.
Some studies have concluded that as global warming advances, driven by the burning of fossil fuels, favorable conditions for severe storms in the United States will increase this century. The increase is expected to be more pronounced during cooler seasons that don’t traditionally see as many thunderstorms, such as early spring, fall and winter, thanks to the presence of more warm, humid air during periods of higher wind shear.
It remains less certain as to whether those increasingly severe storms might lead to more tornadoes. These complex events are harder to model, and so far there doesn’t appear to be clear evidence that, for instance, tornadoes have changed in frequency or intensity over the past 40 to 60 years.
Even so, scientists have seen some evidence that tornado behavior seems to be shifting: In recent years tornadoes seem to be occurring in greater “clusters,” and the region known as tornado alley in the Great Plains, where most tornadoes occur, appears to be shifting eastward.
There’s also the possibility that derechos — groups of intense and fast-moving windstorms like those that caused havoc in the Midwest on Wednesday — may shift into new regions or even become more common as the planet warms, particularly during cooler seasons when they haven’t traditionally been as common.
Derechos “are primarily a summer phenomenon,” said Harold Brooks, a senior research scientist at the National Oceanic and Atmospheric Administration’s National Severe Storms Laboratory. “If you make it more summery, you might expect them to increase.”
The week’s ominous weather shocked even professional weather watchers. Zach Sharpe, who heads the Iowa Storm Chasing Network, said he had never experienced such bizarre weather in December.
Much of Wednesday was unseasonably warm — but once the storm front approached, a blast of cold air brought instantly freezing temperatures and 80 mph winds. “It was eerie to be chasing tornadoes 10 days before Christmas,” Sharpe said. “We were out in our vehicles, listening to ‘Jingle Bells,’ while tornado sirens were going off.”
Still, scientists said, this week’s storm was so unusual and had so many different forces behind it, including a strong jet stream moving across the central states, that it can be difficult to disentangle the effect of global warming compared with other factors like La Nina, an intermittent climate phenomenon in the Pacific Ocean that can influence winter storms.
For some scientists, however, arguing about the precise role of climate change is only part of the story. Whether or not scientists ultimately settle on an answer, they say, society should do as much as possible to reduce vulnerability to extreme weather.
“We might not know exactly how climate change is going to affect tornadoes going forward, but we do know that there are lot of things we can do to protect people today,” said Stephen Strader, a disaster scientist at Villanova University in Pennsylvania.