This Is What Scientists Know About What Caused The UK Floods

Partly climate change, partly sheep, partly just lots and lots of rain.

1. It's not as simple as more rain = more flooding.

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Obviously that's true to an extent. But Nick Reynard, an expert in the science of flood hazards at the Centre for Ecology and Hydrology (CEH), says the translation of rainfall into floods is "non-linear". That is, a small amount of extra rainfall in some places can lead to large floods, while in others, where the capacity to absorb the rain is greater, even a large increase in rainfall might not lead to floods at all.

"In the northwest, where the recent floods have been, most of the catchments [areas] are neutral – they respond relatively linearly to changes in rainfall," he says. "But saying rainfall is going to increase therefore flooding is going to increase, broadly that's true, but there's a lot of detail within that."

2. No two floods are alike.

Prof Alan Jenkins, the deputy director of the CEH, says that "as unprecedented as these floods have been", there will always be a bigger one along eventually, whether next month or next millennium. "And the next flood will not have the same characteristics as these ones, which themselves are very different from the ones in the summer of 2007 and winter of 2013–14."

The type of land, the weather, how saturated the ground is, the peak flow of the rainfall, the duration of the rainfall, and the impact of snowmelt – as well as human interference like flood defences and dredging – are just a few of the factors that make each flood hard to predict.

3. These floods are, in some ways, unprecedented.

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Jamie Hannaford, a hydrologist at the CEH, says the recent floods have set lots of records. The River Eden in Cumbria and the River Tyne have both seen their highest flows in 50 years of records – about 1,700 cubic metres per second, up from an average of around 50m³/s for the Eden and about 45³/s. They've also affected a much larger area than most floods.

4. Climate change is making heavy rain more likely.

Jenkins says "there is weighty scientific evidence that recent rainfall has been impacted by climate change".

His colleague Dr Chris Huntingford, a climate modeller with the CEH, says research from about 20 different groups agrees that 1°C of climate warming translates to about a 7% increase in rainfall on average. "We've always had very heavy rainfall incidents," he says, but climate change is making them more common.

The 7% figure is an average, rather than universal – some places will have less rain, others more, as the patterns of rainfall change – but, says Huntingford, for the UK and northwest Europe, 7% is about what is expected.

5. And the way we farm our hills is probably making floods worse.

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Jenkins says that it is "probably" true that the deforestation of hills, often for sheep-grazing, means that water runs off them more quickly, joining waterways and rushing down to populated flood plains. But, he warns, there is "very little published scientific evidence" to support that, so it's impossible to know.

6. There are things we can do, but no one of them can fix the problem. "Rewilding" the uplands might be a good idea.

That is, planting more trees, removing sheep, stopping the draining peat bogs for grouse shooting – and possibly reintroducing beavers, who will build dams and slow the rush of water from highlands to plains. Jenkins says there would probably be a reduction of the runoff and that rewilding has several other benefits, so it's worth a try in some areas.

But, he adds, "what I'm not saying is that it makes sense to say that everybody everywhere should go out and plant lots of trees. We don't have the evidence."

7. We also need more flood defences.

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Reynard warns that rewilding can't be the whole story. "You can't solve the problem by reforesting the Cotswolds to protect London." So traditional, man-made flood defences – built-up levees, runoff channels, flood walls – will be needed on flood plains and coasts.

8. And we need to take steps to limit climate change.

Huntingford and Reynard both say that since a single degree of warming leads to 7% more rain, the recent Paris agreement to reduce emissions and limit warming to less than 2°C – or 1.5°C if possible – is important.

9. Dredging can make things worse in some situations.

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Dredging a river so the water runs faster through it can reduce flooding right there. But it can also move the water faster downstream and make flooding worse for people further down the river.

Also, in some situations, despite calls for dredging, it wouldn't help. "Remember the flooding on the Somerset Levels?" says Jenkins. "A lot of local people were saying that the overtopping was caused by the lack of dredging, but it's a lot more complicated than that. The River Parrett has no slope on the Levels, it's flat. To get the water off the levels to the Bristol Channel, one would have had to dredge so deep, it wasn't practical."

10. And it may reach a point that we need to stop building on some flood plains – or even move people out of them altogether.

"Developers need to understand the risk they're exposing people to," says Reynard: You need to know the flood risks of an area. Huntingford says: "It's a question of money. Should we build more defences, or should we move people?

"That's obviously a pretty drastic option. But maybe, ultimately, people may have to move."

11. But there just isn't enough research to really know the best thing to do.

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"Hard evidence is hard to come by," says Jenkins. Field experiments – trying a small-scale project, such as introducing beavers to an area or allowing trees to regrow – take years to get results, and it's not always possible to extrapolate from them to a national or regional scale. Historical data and hydrological models exist and can be run right now but they're far from perfect, especially as the climate is changing so rapidly.

"Our climate models are getting better," Huntingford says, "but everything is so interconnected: oceans, air currents, sea ice.

"People's lives are being wrecked and we need to give them better information. It pains us that we have to say there's so much uncertainty."