Global Warming to Cause Increase in US Severe Storms, Researchers Report

A new climate analysis led by scientists at Stanford University suggests that severe thunderstorms in the United States are likely to increase over the next century as a result of global warming.

According the the researchers, severe thunderstorms are one of the primary causes of catastrophic losses in the US, with damage from destructive rainfall, hail and tornadoes resulting in billions of dollars in damages. Last year alone, 11 weather-related events in the US exceeded the $1 billion threshold in damages, and seven of those events were related to severe thunderstorms, the researchers report, adding that the likelihood of similarly severe storms is predicted to increase as time goes by.

Noah Diffenbaugh, an associate professor of environmental Earth system science at Stanford, led the research team's effort to produce what Stanford calls the "most comprehensive projections of severe storm conditions for the next century."

Historically, data on the atmospheric conditions which cause severe thunderstorms has been sparse, therefore limiting scientists' ability to project the long-term effects of global warming on storm frequency.

To make predictions, Diffenbaugh and his team first identified two essential components of in the cocktail of thunderstorm generation. The first is referred to as CAPE -- or convective available potential energy. CAPE is generated as the low air in the atmosphere warms; the warm air rises, carrying moisture with it to higher altitudes.

The second component is a strong vertical wind shear, which combined with CAPE will organize the the atmospheric energy and moisture so that it can sustain a storm.

Previous research had hypothesized that global warming will increase CAPE, but cause an overall decrease in wind shear, which led to uncertainty about the effect of global warming. The new model suggests that wind sheer will still decrease, but the bulk of the decrease will happen on days that produce levels of CAPE much lower than typically seen in severe storms. The researchers contend that the net effect is that an increase in CAPE on other days will drive an increase in severe thunderstorm conditions.

"We're seeing that global warming produces more days with high CAPE and sufficient shear to form severe thunderstorms," said Diffenbaugh.

The model showed the biggest increase in storms occurring in the spring season. The model mapped out the US in 60 square mile sections, and each section in the central US is projected to see about 2.5 additional storm days per spring by the late 21st century. The data also showed an increase in storm days in the Eastern US not only in spring, but in winter and fall as well.

Diffenbaugh said that a few more stormy days may not seem like much, but compared to the frequency of storms in the current climate, the increase is substantial.

"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 21st century."

He also emphasized that just one powerful storm has the potential to cause catastrophic losses, and that the increase in stormy days "represents a substantial increase in the overall risk."

"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."

Diffenbaugh and his colleagues' research is published in the Proceedings of the National Academy of Sciences.