Over the last several years, the U.S. has not experienced an overall increase in tornadoes. Instead more twisters are grouping together, according to decades’ worth of information gathered by the NOAA (National Oceanic and Atmospheric Administration.) If the trend continues the U.S. could see less tornadoes each year, but more grouped tornadoes that can do real damage.

Scientists are concerned that the warmer climate might affect the frequency or the intensity of the twisters that plague the most of the U.S. But it has not been clear how those will be obvious. “We know that tornadoes form when there is lots of energy available for thunderstorms and when there is lots of wind shear,” stated NOAA tornado researcher Harold Brooks.

Wind shear is the change of the winds speed or direction. Global warming is increasing the energy for storms to form, but it also could decrease wind shear reported smithsonianmag.com.

“In effect, there is a low probability of a day having a tornado, but if a day does have a tornado, there is a much higher chance of having many tornadoes,” reporter the NOAA researchers .