As the city of Joplin deals with the devastation from Sunday's tornado, some people might wonder whether these extreme weather events are getting more common because of climate change. The answer is that no one really knows.
A tornado is a rotating column of air that stretches from the bottom of clouds to the Earth's surface. They can occur in a wide range of shapes and sizes, typically manifesting as a funnel of condensation surrounded by a cloud of dust and debris. Wind speeds in an average tornado reach more than 100mph (160km/h) and the system itself is less than 100 metres across, but extreme events can be several miles across, with wind speeds of more than 300mph.
It is difficult to relate any individual weather event to climate change and, unlike with hurricanes, there is little robust research on whether the warming planet is causing any noticeable effects. Grady Dixon, assistant professor of meteorology and climatology at Mississippi State University, told AFP: "If you look at the past 60 years of data, the number of tornadoes is increasing significantly, but it's agreed upon by the tornado community that it's not a real increase. It's having to do with better (weather tracking) technology, more population, the fact that the population is better educated and more aware. So we're seeing them more often."
Writing on the Climate Central website, the policy analyst Andrew Freedman said climate change was already changing the environment in which severe thunderstorms and their associated tornadoes form, and that it was bound to have some influence on tornado frequency or strength. "But as of now, no discernible trend has been detected in the observational data, and studies of how tornadoes will fare in a warmer world show somewhat conflicting results."
He added: "Since more moisture gets added to the atmosphere as the climate warms, additional water vapour may help severe thunderstorms and tornadoes to form. On the other hand, wind shear is expected to decline due to climate change, which would argue against an increase in tornado numbers."
Around a thousand tornadoes hit the US every year, according to the National Oceanic and Atmospheric Administration. Though every state experiences them, tornadoes have tended to be most common around an area between the Rocky mountains and Appalachian mountains, nicknamed "tornado alley". For the most part, they form during the spring and summer, with the season coming earlier in the south and later in the north.
