New climate analyses led by Stanford University scientists indicate that global warming is likely to cause a robust increase in the conditions that produce severe thunderstorms across much of the country over the next century.
Severe thunderstorms are one of the primary causes of catastrophic losses in the United States and often exhibit the conditions that generate heavy rainfall, damaging winds, hail, and tornadoes.
Sparse historical data describing the atmospheric conditions that cause severe thunderstorms has limited scientists' ability to project the long term effects of global warming on storm frequency. But, using a complex ensemble of physics-based climate models, researchers led by Noah S. Diffenbaugh, an associate professor of environmental earth system science at Stanford, have produced the most comprehensive projections of severe storm conditions for the next century.
Scientists have identified the two main ingredients involved in generating a severe thunderstorm. The first was that the atmosphere must contain a significant amount of what scientists call convective available potential energy (“CAPE”), created as the air in the low atmosphere warms. The warm air rises, carrying with it moisture to higher altitudes.
To transform into a severe thunderstorm, CAPE also must interact with strong vertical wind shear – essentially a moving wind current that organizes the atmospheric energy and moisture such that it can sustain a storm.
Climate researchers have previously hypothesized that global warming will increase CAPE and cause an overall decrease in wind shear, which created uncertainty about the net effect.
The new climate model experiment that Dr. Diffenbaugh and his co-authors, Martin Scherer of Stanford and Robert J. Trapp of Purdue, called the Coupled Model Intercomparison Project (“CMIP5”), confirmed these competing effects, but in a different way than previously believed.
Although the climate model experiment indicated an overall decrease in the average amount of wind shear, the researchers found that the bulk of that decrease occurred on days that produced levels of CAPE that were much lower than normally was seen during severe storms.
The net effect was that the increases in CAPE on other days drove increases in the occurrence of severe thunderstorm environments.
“We're seeing that global warming produces more days with high CAPE and sufficient shear to form severe thunderstorms,” said Dr. Diffenbaugh, who is also a senior fellow at the Stanford Woods Institute for the Environment.
Stormy Springs Ahead
The analysis carved the United States into boxes that were roughly 60 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 21st century.
In particularly disturbing news for insurers and policyholders, the study found that sustained global warming was 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 Dr. Diffenbaugh emphasized that the projected increases were 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,” Dr. Diffenbaugh said. “For example, the changes during spring represent an increase of about 40 percent over the eastern U.S. by the late 21st century.”
Dr. Diffenbaugh also emphasized even a single severe storm could cause very high levels of damage. “The severe thunderstorms we experience now can result in very high economic losses,” Dr. 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 created conditions that could lead to tornado formation, although the researchers stressed 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,” Dr. 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.”
Dr. Diffenbaugh said that he 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,” Dr. 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.”
Victoria Spears
Victoria Prussen Spears is senior vice president of Meyerowitz Communications Inc., a law firm marketing communications consulting company. A graduate of Sarah Lawrence College and Brooklyn Law School, Ms. Spears was an attorney at a leading New York City law firm before joining Meyerowitz Communications. She may be contacted at [email protected].