Tornadoes and man-made climate change - a perfect storm?

  • 09 Mar 2012, 13:45
  • Verity Payne

A spate of tornadoes hit the US Midwest on Monday, leaving dozens dead across several states. As tornado season gets going in the country with the fiercest debate around climate change, it must be about time for the media to ask the inevitable question - is there a causal link between a changing climate and tornados?

It's a perennial. Take last year, for example, which saw a record-breaking spate of tornadoes across the southern US during April, closely followed by the sixth most deadly US tornado on record hitting Joplin, Missouri, in May. The media and blogosphere were buzzing with discussion about potential links between tornadoes and man-made climate change.

At the time the Climate Science Rapid Response Team (CSRRT) released a useful summary of expert current understanding into the links between tornadoes and climate change. They explained that although US tornado activity has increased over recent decades, the increase in numbers seems to be confined to the least powerful tornadoes. And of course this trend could be a result of changes in the way tornadoes are measured or recorded in recent years.

It's not clear if a warmer climate means more or less tornado activity

Tornadoes need two things to form: warm moist air, and high 'wind shear' which causes the air rotate. A key 2007 study used climate models to project that whilst the number of warm, moist days is likely to increase with man-made climate change, wind shear will probably decrease. So at the moment it's not clear if a warmer climate means more or less tornado activity, as CSRRT concluded and we reported here.

This year one quite widely re-posted Reuters article - Scientists see rise in tornado-creating conditions - has caused a little confusion.

The article is itself a little unclear about where scientific opinion lies, and you could coming away with the impression that scientists agree man-made climate change will cause more, or bigger, tornadoes in the future.

Much of the article is fine - it starts by pointing out that the US experienced a warm winter which has led to the relatively early outbreak of tornado activity, and continues:

"Whether climate change will also affect the frequency or severity of tornadoes, however, remains very much an open question, and one that has received surprisingly little study."

The piece notes that the factors which cause tornadoes are likely to be affected in different ways, as we noted earlier:

"The scientific challenge is this: the two conditions necessary to spawn a twister are expected to be affected in opposite ways. A warmer climate will likely boost the intensity of thunderstorms but could dampen wind shear, the increase of wind speed at higher altitudes, researchers say."

So far, so good. But later on the article wants to set up some scientific disagreement, so it turns to the 'alternative view':

"Other scientists are not so sure, and they see a surge in tornadoes last year as ominous."

'Other scientists' here means Kevin Trenberth, head of the Climate Analysis Section at the US National Center for Atmospheric Research (NCAR), who has previously stated: "It is irresponsible not to mention climate change in stories that presume to say something about why all these storms and tornadoes are happening."

The rest of the article features Trenberth heavily. This seems fine as the views of a prominent NCAR scientist are worth featuring, in our view. But rather than rely on the views of just one scientist for the argument that tornadoes and climate change are linked, the piece intersperses carefully selected quotes from the 2007 paper which seem to back Trenbeth's views up.

But the 2007 paper was authored by, amongst others, tornado specialists Jeff Trapp of Purdue University and Harold Brooks of the US National Severe Storms Laboratory.

It seems strange that the article uses quotes from their paper to support Trenbeth's views rather than turn to these scientists themselves, who actually give a rather more cautious view elsewhere.

For example, Trapp says in an abc news report this week:

"Our biggest question is: how has, or will human activity affect tornadoes? [...] That's the thing that we haven't been able to unravel yet."

And co-author Brooks is very reticent to link tornadoes to man-made climate change yet, as he explained to Chris Mooney from Desmogblog last year:

"Brooks also thinks global warming is likely to impact many weather phenomena-increasing the risk of heat waves, for instance, and stronger precipitation events.
'But it doesn't necessarily mean that every bad weather event is going to get worse,' Brooks continues, and when it comes to tornadoes, 'I get really worried when people oversell the case.' After all, if we're wrong and we go through a series of quiet tornado years in the coming years, it will be just another weapon with which to attack those who want climate action."

The media's need to set up a clear disagreement often simply doesn't work for these kinds of science stories, where the reality tends not to be so clear cut. For the moment, on tornadoes at least, the general view from the scientific field seems to be: 'we don't know'.

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