"We found that increasing amounts of greenhouse gases in the atmosphere will likely lead to an increase in the frequency of severe thunderstorm conditions in the US," team leader Robert Trapp told environmentalresearchweb.

The researchers used climate models to simulate the atmosphere. Weather forecasters routinely use similar models to look for potential hot spots where conditions like wind shear and "storm energy" are large. Trapp and co-workers took the same approach except they looked at much longer time periods – from the middle to the latter part of the 21st century.

Large storm energy and wind shear values in localised areas produce increased rainfall, hailstones and destructive surface winds and tornadoes. The team found that the number of days that favour the formation of such severe weather conditions could more than double by the end of this century in regions like the east and southeast US, which includes the cities of New York and Atlanta.

The models also showed that the increase in stormy conditions would occur during a typical storm season. "This increases seasonal extremes, as opposed to more storms spread throughout the year," said Noah Diffenbaugh of Purdue's Climate Change Research Center.

Such increases in storm severity would be seen for temperature increases of 2 to 6°C, which are predicted by the Intergovernmental Panel on Climate Change based on current greenhouse gas emission rates. This new work shows that a few degrees of global warming could make severe events much more common than they are today, added Diffenbaugh.

"The damaging winds, hail and tornadoes associated with severe thunderstorms have a significant impact on society," explained Trapp. "In 2006, such thunderstorms caused more than $2.5 billion property and crop damage. An additional $2 billion was lost to damage from flash floods, which often accompany severe thunderstorms. Any change in thunderstorm frequency will thus increase this economic impact, not to mention the hazard to life."

The effect on severe thunderstorm frequency would be less with lower greenhouse gas emissions, suggest the researchers.

The conclusions in the current study were based on the weather conditions that favour severe thunderstorm formation and not on the occurrence of individual storms themselves. This is because the climate models used cannot represent or resolve single thunderstorms. The researchers hope to remove this limitation in the next stage of their work, which will involve higher resolution "cloud resolving" models. "These will give us more detail about the storms, such as their likelihood to spawn a tornado," said Trapp.

The work was reported in PNAS.