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Tornadoes and climate change - what does the science say?

  • 22 May 2013, 16:15
  • Freya Roberts

Sourced under creative commons

Monday's tornado in Oklahoma highlights the threat extreme weather poses to human life, and has prompted some to ask if human-caused climate change is partly to blame. Unfortunately, that's a question scientists still can't answer.

The overwhelming response in the media and online in the wake of the tornado has done a pretty good job of accurately reflecting the science, which is so far unclear over whether theres a link between climate change and tornadoes.

This statement from scientist Michael Wehner sums up the situation pretty well:

"With tornadoes, what we don't know is as much as what we do know."

Tornadoes 101

Before we dig into the science in more detail, here's a quick introduction to what tornadoes are and how they form.

Tornadoes are narrow, spinning columns of air reaching from a the base of a thunderstorm down to the ground. They actually only account for a fraction of the energy released in a thunderstorm, but that energy is concentrated on a small area.

As the video below shows, tornadoes need two critical conditions to form: warm moist air and high 'wind shear'. Wind shear is the spinning motion caused when winds at different heights blow at different speeds. It's the moisture and high winds which cause most of the destruction when a tornado hits.

What does the science say?

Trying to establish whether tornadoes activity will change as the climate warms is tricky for a number of reasons, as a recent report from the Intergovernmental Panel on Climate Change (IPCC) highlights.

First, scientists don't have a complete, good quality data set on tornadoes that have already occurred. Without a long reliable record, scientists can't easily look to see how tornadoes have changed since temperatures started rising.

Second, computer models used to simulate the climate can't tell scientists much about tornadoes either. That's because these models work on large scales, simulating changes in the ocean and the atmosphere on a global scale. In comparison, tornadoes are small weather events. As Dr Suzanne Gray, a meteorologist from Reading University explains:

"Tornadoes are too small-scale for current climate models to simulate, so it is not possible to say very much about how strength and occurrence might alter under climate change."

Then, there's one final problem. Climate change is likely to affect the two critical conditions for tornado formation - atmospheric moisture and wind shear - in opposite ways. The atmosphere is expected to hold more moisture as temperatures rise, making tornadoes more likely. But wind shear will probably decrease, having the opposite effect. Scientists can't say whether one force will override the other.

Climate change and the Oklahoma tornado

Given how hard it is to find any link between climate change and tornadoes, it's no surprise scientists say individual events, like the tornado which struck Oklahoma, cannot be pinned to climate change. As IPCC chair Rajendra Pachauri reiterated yesterday:

"One really cannot relate an event of this nature to human-induced climate change. It's just not possible. Scientifically, that's not valid."

Scientists worldwide are continually researching tornadoes to find out how their frequency and intensity might change in the future. As time goes on, the record of past tornadoes will grow too - providing a bigger set of data to spot trends in. It seems logical that climate change will have some effect on tornadoes, but for now it's very hard to say what that effect will be.

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