New analyses reveal that as the climate warms, the incidence of damaging thunderstorms in many parts of the United States—including the Midwest, Northeast, and Great Plains—is likely to increase. Wikimedia Commons/Postdlf
Researchers examine climate change models to resolve a key question about the effect of a warming climate on damaging storms.
October 8, 2013—Although severe thunderstorms are typically just 15 mi in diameter and approximately 30 minutes in duration, the accompanying flooding, powerful winds, and hail kill an average of more than 300 Americans every year, and cause several billion dollars in property damage.
A persistent question in climate change research has been what effect rising temperatures might have on the frequency and intensity of these storms. Researchers theorize that a warmer climate will create more days with what is called high convective available potential energy (CAPE), a measure of atmospheric buoyancy which is correlated to the storms. Scientists also theorize that a warmer climate will decrease another key element correlated to strong thunderstorms, vertical wind shear.
Recently, researchers at Stanford University and Purdue University analyzed output from a large suite of global climate models using the period from 1970-1999 as a baseline. The team then examined data from an emissions scenario that includes high population growth and high levels of greenhouse gas emissions. The scenario projects median global warming at 4.9° C above preindustrial levels.
The results of this research were published in the paper “Robust increases in severe thunderstorm environments in response to greenhouse forcing,” in the September 23 edition of the Proceedings of the National Academy of Sciences. The paper was authored by Noah S. Diffenbaugh, Ph.D., associate professor, and Martin Scherer, research assistant in the Department of Environmental Earth System Science and Woods Institute for the Environment at Stanford University, and Robert J. Trapp, Ph.D., an associate professor in the Department of Earth, Atmospheric and Planetary Sciences at Purdue University.
“We found that both theoretical expectations are confirmed by a large suite of climate model experiments,” Diffenbaugh says. “Climate models agree on decreasing mean seasonal wind shear and increasing mean seasonal CAPE.
“What’s new about this study—in addition to having access to a large suite of climate models—is we have been able to look at each individual day at the atmospheric conditions in each of the climate model experiments,” Diffenbaugh explains. “We have been able to analyze where ... the decrease in shear and the increases in CAPE are occurring.”
Global climate models indicate that the mean decreases in shear predicted by theory are concentrated in days with low CAPE, and thus already unlikely to spawn severe thunderstorms.
“So even though the shear does decrease as expected in these climate model experiments, the net result is an increase in the likelihood of days that have high CAPE and high shear,” Diffenbaugh says. “Further global warming is likely to lead to an increase in the number of days in which the atmospheric conditions are favorable for severe thunderstorm environments.”
The increase in days with severe thunderstorm environments projected by models for the years 2070-2099 varies by season and as well as region of the United States. The largest increase is seen in spring over the Midwest. Strong spring increases are also projected in the Northeast, the northern Great Plains, and the northern Midwest.
Summer will see the weakest changes, with small increases in most regions and a projected decrease in the central Great Plains, where projected decreases in shear are accompanied by only slight increases in CAPE.
The research indicates that changes in CAPE and days with severe thunderstorm environments begin to emerge before 2050 under the RCP8.5 scenario.
“At that point, the mean across the models is about a 20 percent increase in the occurrence over the eastern U.S. for the spring season,” Diffenbaugh says. “If we look out at the end of the 21st Century in these climate model experiments, then the mean across the models is about a 40 percent increase in the likelihood of severe thunderstorm conditions over the eastern U.S. as a region.”
Although the research indicates the number of days in which conditions will be favorable for severe thunderstorms, Diffenbaugh says this research doesn’t resolve the processes that cause severe thunderstorms or consider if the severity of the storms will change.
“We’ve been able to gain some important new insight about the influence of global warming on the atmospheric conditions that have created severe thunderstorms in the past,” Diffenbaugh says. “Because we are not resolving the processes that create storms in the real atmosphere, we are reaching the limits of what we are able to say from the approach we have used in this paper.”
The team has already begun to work on research to resolve those processes that “actually create real storms in the real atmosphere,” Diffenbaugh says. “We need to be able to better understand those storm scale processes. That’s work we have underway and we need to continue.”