The historic rain and flooding in parts of Arkansas, Kentucky and other states caused by intense April thunderstorms was likelier and also more intense because of climate change.
That’s according to the World Weather Attribution project, a group of scientists who analyze major weather events for the effects of climate change.
From April 3 to April 6, torrential rain pounded the Southeast, causing flooding that put more than 70 million people under flood alerts, killed at least 15, swept away cars and derailed a train.
Researchers used climate models and historical data to analyze the storm system in eight states it tracked and found that it was about 9% more intense because of global warming and 40% likelier today than in a climate without global warming.
“We conclude that present warming of 1.3 degrees did amplify the extreme rainfall leading to flooding in this region,” said Ben Clarke, a researcher at Imperial College London, who helped author the report. “We know that a warmer atmosphere holds more moisture.”
The reference to 1.3 degrees is how much the world has warmed, in Celsius, since humanity began spewing greenhouse gases into the atmosphere as a result of the Industrial Revolution. It’s equivalent to about 2.3 degrees F.
Clarke said the group’s probability estimates are conservative. The researchers noted that a somewhat unusual meteorological setup contributed to the extreme rain.
Shel Winkley, a meteorologist with the nonprofit news organization Climate Central who contributed to the report, said the low pressure weather system producing the storms hit a ridge of high pressure and then stalled, which sent thunderstorms parading one after another over the same stretches of the Southeast and the Midwest.
“That front was the road for these storms to travel on and also the trigger mechanism … that allowed these thunderstorms to essentially pile up on already saturated soil,” Winkley said. “This is a very interesting event where weather and climate change collided together.”
Winkley said the National Weather Service issued the third-most severe weather warnings on record on April 2.
“By the end of that day, the National Weather Service had issued, across their different offices, 728 different severe thunderstorm warnings and tornado warnings combined,” Winkley said, adding that from April 3 to April 6, many locations got 6 to 12 inches of rain, with extremes upward of 16 inches.
After they analyzed the historic April rainfall, the researchers found that a storm system similar in scope and scale could be expected once every 100 years in today’s warmer climate.
Kentucky State Climatologist Jerald Brotzge, a professor at Western Kentucky University, who wasn’t involved in the research, said he’s often skeptical of such studies, which attribute large flooding events to climate change but don’t account for unique meteorological setups. But this research appeared solid, he said.
“It looks like they’ve done a pretty good job with it,” Brotzge said. “In this case, it was a stalled boundary, and the thunderstorms kept forming over the same area. They recognize that.”
Brotzge said his state, Kentucky, has warmed by about a degree Celsius (1.8 degrees F) over the past 130 years of recorded weather. The state has experienced heavier rainfall over time.
“Our annual rainfall has increased about 10%,” Brotzge said. “Our top 10 wettest years — five of those have occurred since 2011. 2011 remains our wettest year, and 2018 remains our second-wettest year. And this year, our January through April is our wettest start to the year.”
World Weather Attribution is a consortium of scientists who publish quickly produced analyses of climate change’s role in extreme events. Its methods are peer-reviewed, but specific analyses aren’t immediately reviewed. The group’s previous work on heat, wildfire and hurricane disasters has held up to outside academic scrutiny.
