U.S. to face an increased risk of severe thunderstorms

Stormy springs ahead
The analysis carved the United States into boxes that were roughly sixty miles on a side and assessed the climate conditions that could emerge over the next century. The analysis showed the biggest changes occurring in the spring season, with each box in the central United States experiencing about two-and-a-half additional storm days per spring by the late twenty-first century.

The researchers also reported that sustained global warming is likely to cause robust increases in storm days over large areas of the eastern United States not only in spring but also in winter and autumn. While the summer season also showed increases over the region as a whole, those increases were the least robust within the region and across the different climate models.

An additional few days of severe storm conditions might not seem like a large change, but Diffenbaugh emphasized that the projected increases are in fact substantial compared to the frequency of occurrence in the current climate.

“We are looking at the conditions that produce severe events, which are relatively rare at present,” Diffenbaugh said. “For example, the changes during spring represent an increase of about 40 percent over the eastern U.S. by the late twenty-first century.”

Diffenbaugh also emphasized even a single severe storm can cause very high levels of damage.

“The severe thunderstorms we experience now can result in very high economic losses,” Diffenbaugh said. “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.”

Potential for more tornadoes
Such storms also create conditions that can lead to tornado formation, although the researchers stress caution in drawing conclusions specifically about the effect of global warming on tornadoes.

“We have tried to analyze the atmospheric conditions that are associated with tornadoes,” Diffenbaugh said. “Although we do see that those conditions increase in occurrence in response to global warming, it is important to bear in mind that we are not resolving tornadoes in these experiments.”

The release notes that Diffenbaugh hopes to build on this research to improve the understanding of the atmospheric dynamics that lead to the development of severe thunderstorms, and to better incorporate those processes into climate models.

“These are rare but significant events,” Diffenbaugh said. “This new set of global climate model experiments has provided some important new insights. What we need to do next is develop ways to better represent the processes that produce individual storms in the real atmosphere.”

— Read more in Noah S. Diffenbaugh et al., “Robust increases in severe thunderstorm environments in response to greenhouse forcing,” Proceedings of the National Academy of Sciences (23 September 2013) (doi:10.1073/pnas.1307758110)