According to climate scientists at Oxford University, heavy storms, such as the one that caused devastating flooding in the South of England in the winter of 2013/2014, are heavily facilitated by climate change. Anthropogenic greenhouse gas emissions increase the risk of extreme floods by more than 40 per cent, the researchers found. As temperatures keep rising, traditionally exceptional weather phenomena will become more normal.
The study lead by Dr Nathalie Schaller at Oxford’s Department of Physics is the first of its kind, analysing the once-a-century flood from end to end, accounting for atmospheric circulation, rainfall, river flow, inundation, and properties at risk. Two aspects of global warming primarily contributed to the extreme peak in rainfalls in 2013/2014, the researchers found: an increased water-holding capacity of the atmosphere, a so-called thermodynamic factor, and enhanced westerly air flow in January 2014, a dynamic factor. In their computer simulations, thermodynamic factors account for about two thirds of the increase in risk, dynamic factors make up the remaining third.
While it is not possible to link single extreme weather incidents to global warming, scientists are able to run probabilistic simulations. These simulations allow conclusions about how much more likely the event has been made by climate change. In this case, man-made emissions increased the risk of extreme rainfalls in South England by an average of 40 per cent in the researcher’s 100,000 different simulations. Lead author Dr Shaller said: ‘We found that extreme rainfall, as seen in January 2014, is more likely to occur in a changing climate. This is because not only does the higher water-holding capacity lead to increased rainfall, but climate change makes the atmosphere more favourable to low-pressure systems bringing rain from the Atlantic across southern England.’
In late 2013 and early 2014 heavy rainfalls led to floods submerging 5,000 houses and totalling costs of more than £450 million. Among the areas worst affected were Somerset, Devon, Dorset, Cornwall and the Thames Valley. The researchers warn that rising temperatures will make such extreme and costly weather phenomena significantly more likely.
The study can also be used to explain the occurrence of Storm Desmond in late 2015, which devastated parts of Cumbria and other northern regions and left 60,000 households without power. However, “A positive attribution for an extreme rainfall event like Desmond is still rare”, said co-author Friederike Otto from the Environmental Change Institute at Oxford University.
For the simulations, the research team made use of the weather@home project which combines the computing power of thousands of personal computers in households around the country. The study has been published in the journal Nature Climate Change.