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Tornadoes, extreme weather, and climate change

The return of tornado season with a vengeance has people asking again about a possible link to climate change. At the same time, tantalizing new preliminary research finds “some evidence to suggest that tornadoes are, in fact, getting stronger.” I talked to the lead scientist behind that research.

Tom Karl, the director of the National Climatic Data Center, explained in a 2011 email:

What we can say with confidence is that heavy and extreme precipitation events often associated with thunderstorms and convection are increasing and have been linked to human-induced changes in atmospheric composition.

2013 thunderstorm 640

Insured losses due to thunderstorms and tornadoes in the U.S. in 2013 dollars. Data and image from Property Claims Service, Munich Re.

And a September 2013 study from Stanford, “Robust increases in severe thunderstorm environments in response to greenhouse forcing,” points to “a possible increase in the number of days supportive of tornadic storms.” In particular, the study found that sustained global warming will boost the number of days experiencing conditions that produce severe events during spring, representing “an increase of about 40 percent over the eastern U.S. by the late 21st century.”

Tornadoes “come from certain thunderstorms, usually super-cell thunderstorms,” explained climatologist Dr. Kevin Trenberth in an email last year, but you need “a wind shear environment that promotes rotation.” Global warming, it was thought, may decrease the wind shear and that may counterbalance the impact on tornado generation from the increase in thunderstorm intensity. But the Stanford study found that most of the decline in wind shear occurs on days that weren’t suitable for tornado formation anyway.

Trenberth, the former head of the Climate Analysis Section of the National Center for Atmospheric Research, notes:

The main climate change connection is via the basic instability of the low level air that creates the convection and thunderstorms in the first place. Warmer and moister conditions are the key for unstable air.

The climate change effect is probably only a 5 to 10% effect in terms of the instability and subsequent rainfall, but it translates into up to a 32% effect in terms of damage. (It is highly nonlinear). So there is a chain of events and climate change mainly affects the first link: the basic buoyancy of the air is increased. Whether that translates into a super-cell storm and one with a tornado is largely chance weather.

Many scientists would agree with the December assertion of Penn State meteorology professor Paul Markowski and National Severe Storms Laboratory senior research scientist Harold Brooks that, “Because of the inconsistency in [historical tornado] records, it is not known what effect global warming is having on tornado intensity.”

But Florida State University researchers have worked hard to reanalyze the historical tornado data to make them more consistent. Lead researcher, Prof. James Elsner, said in September, “The risk of violent tornadoes appears to be increasing.” In particular, the trail of distruction from tornados appears to getting longer and wider. As the FSU news release noted, “The Oklahoma City tornado on May 31, 2013, was the largest tornado ever recorded, with a path of destruction measuring 2.6 miles in width.”

Elsner presented his findings at the annual meeting of the American Geophysical Union in December along with these charts:

Elsner Tornado

As LiveScience reported at the time:

Beginning in 2000, tornado intensity — as measured by a twister’s damage path — started rising sharply, said Elsner, of Florida State University. “I’m not saying this is climate change, but I do think there is a climate effect,” he said. “I do think you can connect the dots.”

Elsner told me that he has not changed his views at all since then, but he made clear the preliminary nature of this work: “Much more research is needed before definitive conclusions can made.” He said, “We do know that the measured path length is increasing. To what extent the trend is due to changes in recording is an open question.”

After April 2011 saw records set for most tornadoes in a month and in 24 hours — “The Katrina of tornado outbreaks“ — I examined the climate/tornado link in great detail here, looking at the data, the literature, and expert analysis. That piece concluded:

  1. When discussing extreme weather and climate, tornadoes should not be conflated with the other extreme weather events for which the connection is considerably more straightforward and better documented, including deluges, droughts, and heat waves.
  2. Just because the tornado-warming link is more tenuous doesn’t mean that the subject of global warming should be avoided entirely when talking about tornadoes.

Early March 2012 saw what was likely “the most prolific five-day period of tornado activity on record for so early in the year,” as meteorologist Dr. Jeff Masters put it.

Then we had an unusually long “tornado drought” from May 2012 to April 2013, which came to a stunning end that spring 2013, punctuated by a string of devastating Oklahoma tornadoes in May 2013. Now, after a very slow start, the 2014 tornado season has kicked into overdrive, as “Tornadoes Kill 16 in Arkansas, Oklahoma, and Iowa.”

For an extended review of the scientific literature with analyses from leading experts, read the second half of this post.

In general I do think it’s best to avoid statements like “global warming is to blame for” or “global warming caused” or “this is evidence of global warming,” in regards tornadoes.

Finally, while tornadoes will continue to grab the headlines wherever they flatten cities and take lives, it is virtually certain that other extreme events — and ultimately the permanently changed climate — will cause the greatest harm attributable to human emissions of greeenhouse gases.

The population hasn’t even acclimatized to the climate change we’ve had already — in part because the GOP and the fossil-fuel-funded disinformation campaign have obfuscated efforts to inform the public.

We’ve only warmed about a degree Fahrenheit in the past half-century. If we keep listening to the disinformers, we are on track to warm nearly 10 times that this century (see literature review here). In short, we ain’t seen nothing yet. Or, as one CP commenter put it:

“Mother nature is only warming up.”

Finally, yes, we need to improve housing for those in tornado alley. That’s a great thing for blogs that don’t focus on climate to write about. Just as obviously we need an aggressive strategy for reducing carbon pollution that also supports real adaptation.

You can donate to help the victims of the current wave of tornadoes through American Red Cross disaster relief here.

What do you think? Leave a comment below.

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