LAST year was a particularly deadly year for tornadoes in the US, with the second highest death toll in 137 years. Already this year, major outbreaks have killed 63 people. All eyes are now on May, when the season usually peaks, amid talk in some quarters of another year of extremes. The result is that discussion about human influences on severe thunderstorms and tornadoes has been reignited. Such discussion is not new. It seems that, following every major tornado event in the last decade, the influence of rising levels of atmospheric carbon dioxide has been questioned. Human impacts have been talked about for much longer than that, however. Three tornadoes that killed at least 90 people in 1953 led to speculation that atmospheric tests of nuclear weapons caused them. But the speculation has been one-sided, with talk of human influences on tornadoes following disasters, but little about whether human influences have acted as a brake on their numbers after anomalously low activity.