Using a large number of global climate model simulations, the study says the uptick in severe weather environments is “robust” and seen in most places in most seasons.
The most pronounced increases are projected for the central U.S. during spring but large increases also extend into the Northeast and back into the Plains. The lone decrease in severe thunderstorm environments is found in the central Plains during the summer.
The study finds the two critical ingredients necessary for severe thunderstorms will team up more frequently. The first ingredient is known as CAPE (Convective Available Potential Energy), which is simply a measure of heat energy, or the fuel available to storms. The second is wind shear or the turning of the wind with altitude, which helps storm spin and stay alive.
“[We] find that days with high convective available potential energy (CAPE) and strong low-level wind shear increase in occurrence, suggesting an increasing likelihood of atmospheric conditions that contribute to the most severe events, including tornadoes,” the study says.
Previous modeling studies had found mixed signals when simulating severe thunderstorms in a warming world. They found the added heat would boost CAPE, but that wind shear – driven by temperature contrasts – would decrease due to fewer and weaker intrusions of cold air.
This new study finds wind shear decreases overall, but not when the atmosphere is juiced and CAPE is high.
“…whereas expected decreases in mean wind shear have been used to argue for a negative influence of global warming on severe thunderstorms, we find that decreases in shear are in fact concentrated in days with low CAPE and therefore do not decrease the total occurrence of severe environments,” the study says.
Greg Carbin, the warning coordination meteorologist at the National Weather Service’s Storm Prediction Center and not involved in the study, praised the study but cautioned a favorable storm environment doesn’t always lead to more storms in the real world.
“It’s good work,” Carbin said. “[But] environments are not severe thunderstorms. And, while an increase in daily environments supportive of severe storms will more than likely relate to an increase in actual severe thunderstorm formation in the mean, predicting the number and variability of actual severe thunderstorms remains elusive.”
Harold Brooks, a researcher at the National Weather Service’s National Severe Storms Laboratory (NSSL), called the work “intriguing.”
The implications of more severe thunderstorm outbreaks are profound. In recent years, severe thunderstorm and tornado outbreaks have represented a major portion of the nation’s billion dollar weather disasters.
“The severe thunderstorms we experience now can result in very high economic losses,” said Stanford researcher Noah Diffenbaugh, the study’s lead author. “Sadly, we have many examples of cases where a single storm has had disastrous impact. So a 25 or 30 percent increase in the annual occurrence represents a substantial increase in the overall risk.”