Modeling shows global warming could increase number of severe storms 24 (UPI) -- Global warming may impact weather, possibly causing more severe thunderstorms, which could mean billions of dollars in damages, a U.S. researcher said. To examine how global warming could impact atmospheric conditions, researchers led by Stanford University's Noah Diffenbaugh used a complex array of physics-based climate models. Last year, 11 weather disasters in the United State topped the billion-dollar threshold in economic losses, Science World Report reported Tuesday. The researchers identified two main ingredients to generate a thunderstorm. First, the atmosphere must contain a significant amount of what scientists call convective available potential energy, or CAPE, created as the air in the low atmosphere warms. Second, CAPE must interact with strong vertical wind shear, basically a moving wind current that organizes atmospheric energy and moisture so a storm can be sustained The new model indicates an overall decrease in the amount of wind shear, yet most of that decrease occurs on days producing levels of CAPE much lower than normally seen during severe storms, researchers said. The net effect is that the increases in CAPE on other days pushes increases in the occurrence of severe thunderstorm environments. Researchers found the biggest changes with global warming occurred during spring, Science World Report said. At that time, the United States would experience about 2 1/2 additional storm days by the late 21st century, the modeling indicated. "We're seeing that global warming produces more days with high CAPE and sufficient shear to form severe thunderstorms," said Diffenbaugh, with Stanford's School of Earth Sciences. The researchers also discovered that sustained warming likely would cause strong increases in storm days over a large portion of the eastern United States in fall and winter as well. The summer season showed increases over the region as a whole. "The severe thunderstorms we experience now can result in very high economic losses," said Diffenbaugh. Sadly, we have many examples of cases where a single storm has had disastrous impact. So a 25- to 30 percent increase in the annual occurrence represents a substantial increase in the overall risk.