Did Man-Made Global Warming Cause the Joplin Tornado?

Environmentalist Bill McKibben all but says yes in a clever op/ed today in the Washington Post: 

Caution: It is vitally important not to make connections. When you see pictures of rubble like this week's shots from Joplin, Mo., you should not wonder: Is this somehow related to the tornado outbreak three weeks ago in Tuscaloosa, Ala., or the enormous outbreak a couple of weeks before that (which, together, comprised the most active April for tornadoes in U.S. history). No, that doesn't mean a thing.

It is far better to think of these as isolated, unpredictable, discrete events.

McKibben goes on to cite other recent weather disasters as evidence of the dangers that a warming world poses. An interesting feature about disasters is that they get reported, but when things go pretty much as normal, they don't. However, this understandable focus on disaster can mislead us. We have to be careful not to see what we want to see rather than what is actually there. So what about the tornadoes? The AFP reports:

"This year is an extraordinary outlier," said Harold Brooks, research meteorologist at the National Oceanic and Atmospheric Administration's (NOAA) National Severe Storms Laboratory in Norman, Oklahoma. …

Brooks said the contemporary US tornado record, which dates back to 1950, "can be a difficult thing to work with."

But when scientists examine the most complete records available and adjust for changes in how tornadoes were reported over time, "we see no correlation between global or US national temperature and tornado occurrence," Brooks said.

Nor are the storms themselves getting larger than they used to be, even though it may seem so after learning of massive twisters like the one in Missouri that tore apart a six-mile (10 kilometer) long, half-mile deep stretch of land.

"Tornado deaths require two things. You have to have the tornado and you have to have people in the right or the wrong place," Brooks said. …

The tornado record does not show a steadily increasing trend toward bigger deadlier storms, he said. For instance, "2009 was a really low year for tornadoes. Some recent years have been big, some recent years have been small," he said.

Since modern records on tornadoes began, the deadliest outbreak was on April 3, 1974. The "Super Outbreak" claimed 310 lives when 148 tornadoes over a 24-hour period swept across 13 states.

The single deadliest tornado in US history, described in early accounts, killed 695 people when a massive twister tore up parts of Missouri, southern Illinois and southwestern Indiana in 1925.

Humans are pattern-seeking animals and we often tend to see what we want to see. That's where data—as opposed to speculation—comes in.