Climate change induced by human activities apparently leads to many conditions that affect people and the natural world negatively. Negative aspects of climate change dominate the media, and understandably so, as some negative outcomes are likely to be painful. Many activists emphasize the negative aspects because they might lead people to take greater action. Yet, informed people should realize that not everything gets worse. Recently, Bill Nye tweeted "More severe weather. More suffering. Let's all take climate change seriously." I strongly agree with the need to take climate change seriously. Yet, his focus on severe weather may be backwards! During the tornado season, we all see the pictures, and they seem to create the feeling that this year is always worse than last. Yet, how do violent tornado outbreaks really trend over time? Data posted at the National Climatic Data Center website gives us clues:
Although F1+ tornadoes do not show much trend over time,
F3+ tornadoes actually show a pronounced and statistically significant downward trend (trend line not shown). Keep in mind that the way we observe tornadoes has changed over time. We are likely to identify more tornadoes given today's technology than we did in the past. The most intense tornadoes, however, are the most likely to have been observed and recorded, and they seem to be declining with time.
Study of the effects of climate change on severe weather is a young field. One major problem is that climate models do not simulate severe weather, so it is difficult to perform experiments to diagnose the causes of changes over time. Regional models embedded in climate models might be helpful. In any case, we can suggest hypotheses that might explain these changes. One possibility, for example is that climate change might reduce the size of temperature differences between cold Arctic and warm tropical air masses that meet over the plains of the United States that favor violent tornadoes. Tornadoes rely on the wind blowing from different directions at different heights, a condition that can be reduced if these temperature differences weaken over time.
My bottom line is that there are plenty of good reasons to take action on climate change. Yet, we should be honest and not claim that one of those reasons is to reduce violent tornado activity. Given the data, it would be astonishing if climate change were actually increasing activity in these severe storms.
****After posting this commentary, after some communication with experts in tornado data, I now understand that prior to 1977, intensity of tornadoes was biased to higher values relative to estimates of intensity of more recent tornadoes. This adjustment eliminates the downward long-term trend. At the same time, this change does not seem to suggest a trend upward, suggesting that it remains inappropriate to claim that violent tornadoes become more active with climate change.