In places like California, researchers have been working to understand how climate change is affecting droughts. But in the UK, it’s unusually wet weather making headlines of late. Southern England and Wales got soaked over the winter that ran into January 2014, leading to near-historic flooding. This led to a natural question: did climate change have a hand in it?

Climate is basically the statistics of weather, so the way we answer this is to use climate models to look for a change in those statistics. We can’t necessarily convict climate change for any particular weather disaster, but we can learn whether we should expect to see that disaster more often than we would in the absence of climate change. A home-run hitter on steroids is a common analogy—they'd clearly hit some out of the park anyway, but not with the same frequency.

Good statistics require a lot of samples, so to look at the English flooding, a climate model was used to generate more than 130,000 simulations of weather in the region. To do the computational heavy lifting, the team (led by University of Oxford researcher Nathalie Schaller) relied on weather@home running on volunteers’ computers. Some of the simulations were run with greenhouse gas concentrations, Arctic sea ice extent, and sea surface temperatures to match the 2013/2014 winter. The other simulations were run under approximated pre-industrial conditions: lower greenhouse gas concentrations, cooler sea surface temperatures, and the largest sea ice extent available from the satellite era (1986/1987).

The heavy rain around January 2014 was the result of persistent and strong low pressure to the northwest of the UK, which brought a constant stream of moist air and storms across Southern England from the west. This pattern was present for a record portion of the winter.

The model simulations were analyzed to see how frequently that weather pattern occurred and how much rain fell over the UK. Compared to the baseline simulations, extremes of this pattern were more likely in the modern conditions. A persistent stretch of west-to-east flow like in January 2014 happened about once every 150 years (on average) in the pre-industrial simulations. In modern conditions, that return period dropped to about 110 years.

Add in the fact that warmer temperatures meant more moisture in the air, and the result is increased precipitation extremes. Simulated January rainfall totals that were seen about once per century became about 70-year events. In other words, global warming does appear to be a steroid for this type of weather.

The researchers even went to the next step, plugging these numbers into a surface water model for the Thames River. There wasn’t much impact on daily peak river levels, since this is about sustained rather than intense rainfall, but 100-year highs in monthly averaged river levels became about 80-year events. For what is currently a 100-year flood, that means something like a thousand more properties under water.

The UK saw severe flooding again in the last few months, resulting from a similar sort of weather pattern. But the low pressure was located farther to the west this time, and the storms crossed the UK farther north, encountering topography that made precipitation locally quite variable, Oxford’s Friederike Otto told Ars. The research group (and volunteers’ computers running weather@home) is already hard at work analyzing the contribution of climate change, with some early results on December’s temperatures and precipitation total and Storm Desmond—the first of several to roll through and leave swollen streams in its wake.

Nature Climate Change, 2016. DOI: 10.1038/nclimate2927 (About DOIs).