One possible explanation for an increase in EF1+ tornadoes is that the Earth is warming, and warm temperatures increase the likelihood of thunderstorms during the cool seasons. Scientists have recently found that the number of days when at least one EF1+ tornado strikes the U.S. has decreased from 150 days in the 1970s to 100 days in the first decade of the 21st century.
The low-frequency climate change signal has already been imprinted on the air and sea surface temperatures. Global warming trends change the hydrological cycle, with more water vapor in the atmosphere. These changes may affect the amount of moisture absorbed by thunderstorms. And if climate change has a positive effect on tornado activity, we can expect a shift in severe weather seasons and regions.
Although tornadoes have not increased in recent decades, they have become more clustered, and multi-tornado outbreaks are becoming more common. Moreover, their power is rising. Researchers suspect that climate change is causing more tornadoes and thunderstorms to develop in the future.
Researchers at Purdue University have found that increasing greenhouse gases in the atmosphere may increase the number of days with severe thunderstorms in the south and east of the country. Doubling the number of greenhouse gases in the atmosphere could result in more significant numbers of days with severe thunderstorms. The scientists compared summer climate scenarios from 2072 to 2099 with the 1662-1989 record. The rise in the cap overwhelms a decrease in vertical wind shear, which leads to increased thunderstorm activity. They also predicted that the Quad Cities would experience slightly increased severe thunderstorms.
The impacts of climate change on tornado frequency and intensity are still being studied, but scientists have found several trends in the U.S., particularly in powerful tornadoes. Outbreaks with 30 or more tornadoes are increasing, as are the strength and density of clusters. The distribution of tornadoes has also shifted eastward, but these trends do not directly correlate with climate change. It remains to be seen what effect climate change has on tornado frequency and strength.