The risk of severe thunderstorms capable of producing extreme amounts of precipitation and tornadoes is expected to increase in frequency in the US, according to new research published this week in the journal Proceedings of the National Academy of Sciences. Weather disasters, including severe thunderstorms, caused more than $1 billion worth of economic losses in 2012. Now, scientists from Stanford University are reporting that global warming is expected to cause the conditions responsible for producing these types of storms to increase through most of the US over the next century. Noah Diffenbaugh, an associate professor of environmental Earth system science at the California-based university, and colleagues conducted a series of climate analyses which analyzed global warming’s affect on the two major factors responsible for thunderstorm formation – convective available potential energy (CAPE) and wind shear. CAPE is created when the air in the lower atmosphere warms, causing it to rise and carry moisture with it to higher altitudes. The atmosphere must contain a significant amount of CAPE in order for severe storms to form, the researchers explained. In addition, this phenomenon must also interact with strong vertical wind shear – essentially, strong moving wind currents which collect enough atmospheric energy and moisture to sustain a storm. Previously, climate experts have hypothesized that global warming would increase CAPE and decrease wind shear, though sparse data on both phenomena has limited their ability to make predictions about how these changes would alter thunderstorm frequency. However, Diffenbaugh’s team analyzed an experimental climate model, the Coupled Model Intercomparison Project (CMIP5), which demonstrated why these storms would become more common. “Although the climate model experiment does indicate an overall decrease in the average amount of wind shear, the researchers found that the bulk of that decrease occurs on days that produce levels of CAPE that are much lower than is normally seen during severe storms,” the Stanford researchers explained in a statement. “The net effect is that the increases in CAPE on other days drive increases in the occurrence of severe thunderstorm environments.” Diffenbaugh and his colleagues divided the US into regions that were approximately 60 miles on a side, and assessed the climate conditions that could occur over the next century. Their research demonstrated that the most noticeable changes would occur in the spring, with each of the regions experiencing an average of 2.5 additional storm days during that season each year by the late 21st century. The study authors also reported that sustained warming is likely to cause vigorous increases in the total number of autumn and winter storm days throughout large areas of the eastern US. Summertime storm frequency increased over the region as a whole, but those were the least robust throughout the region and across climate models. “We are looking at the conditions that produce severe events, which are relatively rare at present. For example, the changes during spring represent an increase of about 40 percent over the eastern U.S. by the late 21st century,” Diffenbaugh said, emphasizing that even one additional storm could be responsible for high levels of damage. “The severe thunderstorms we experience now can result in very high economic losses,” he added. “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.” Furthermore, these strong storms can also create conditions conducive to tornado formation, though the Stanford researchers stress caution in regards to drawing conclusions specifically about the effect of global warming on tornadoes. While Diffenbaugh said that they attempted to analyze atmospheric conditions associated with tornadoes, he noted that they do not expect global warming to cause a direct increase in those conditions.