Decline in Violent Tornadoes: Blame Global Warming?

“Now, we know that no single weather event is caused solely by climate change.  Droughts and fires and floods, they go back to ancient times.  But we also know that in a world that’s warmer than it used to be, all weather events are affected by a warming planet.” — President Obama, Georgetown University, June 25, 2013

Well, here’s something else that goes back to ancient times: sophistry — sham wisdom raised to an art and marketed through deceptive rhetoric.

When someone tells you that “no single weather event is caused soley by climate change” but also that “all weather events are affected by a warming planet,” what is he really saying? He implies that you should blame global warming for particular weather disasters — but that you should not ask him (or anyone else) to provide specific evidence. How convenient!

What reminded me of the President’s climate rhetoric is the lead post on today’s WattsUpWithThat. Guest blogger Paul Hornwood presents graphs based on NOAA data showing that “Violent Tornadoes Are on the Decline in the U.S.” So if it’s true that “all weather events are affected by a warming planet,” should we blame global warming for suppressing violent tornadoes? If we’re swayed by the President’s rhetoric, yes, especially since Hornwood is not reporting a “single weather event” but a multi-decadal trend.

Quick background. As Hornwood explains, reliable analysis of tornado frequency and strength starts in the 1970s. National observation practices only began to develop in the ’50s and ’60s, and many higher-rated tornados back then were ‘measured’ after the fact. In 2007, NOAA upgraded its rating system to the Enhanced Fujita (EF) scale.

Here’s the big picture:

The data show significant year-to-year variability but also a decline in the number of tornadoes since 1970. But wait, there’s more. . .

Even as the number of tornadoes declined, the percentage of very strong tornadoes (EF-2 to EF-5) also declined.

To be sure, the percentage of EF-1 tornadoes increased since 1970 — but that is a mathematical necessity, given that percentages of stronger (EF-2 to EF-5) tornadoes decreased.

Finally, Hornwood presents a NOAA graph of very strong (EF-3 to EF-5 ) tornadoes — storms big enough to be reported in decades before widespread use of doppler radar.

Hornwood concludes:

Analysis of long term climatic trends can often be fraught with these sort of data issues, whether tornadoes, hurricane or temperatures. Nevertheless, the official data is the best we have got. And its message is clear.
 
Tornadoes since 1970 have been declining both in number and intensity.