Storm data indicates that the number of tornadoes per outbreak in the United States may be increasing and that weather extremes associated with severe thunderstorms are similarly on the rise. Climate change has been proposed as contributing to these increases; however, the scientific literature is not yet robust enough to link changes in tornado activity to climate change.
Climate model projections indicate that convective available potential energy (CAPE)—a measure of the amount of energy available for convection—would increase in a warmer climate leading to more frequent environments favorable to severe thunderstorms in the United States.
However, a December 2016 study links recent increases in the number of tornadoes per outbreak, not to CAPE, but to trends in storm relative helicity, a quantity related to vertical wind shear previously identified as a factor in increased year-to-year variability of US tornado numbers.
The 5 largest U.S. winter tornado outbreaks on record have all hit since 1999 and the uncertainty about the climate change connection translates as an increase risk due to climate change.