Met Office: Climate change made UK's wet winter in 2013/4 seven times more likely

05.11.15

Extreme weather

By:

Roz Pidcock

The winter of 2013/4 saw a succession of storms batter the UK, leading to heavy rain and severe flooding across large parts of the country. The odds of those storms bringing such extreme wet weather were seven times higher than on a planet that wasn’t warming, according to a new analysis by Met Office scientists. Today’s study is part of a special issue published in the Bulletin of the American Meteorological Society that looks in depth at extreme weather events across the world in 2014. Unusual streak The winter of 2013/14 was one of the most exceptional periods of rainfall in England and Wales in at least 248 years. It marked the wettest December and January in the UK as a whole since 1910.

The simplest answer to what caused the floods was what today’s Met Office analysis calls an “exceptional clustering of vigorous storms” sweeping the UK one after the other. Each storm dumped a huge amount of rain, while winds whipped up storm surges that overwhelmed coastal flood defences. With the storms so quick in succession, rivers burst their banks and the ground quickly became saturated, leaving nowhere for the excess water to go. The unusual run of storms was caused by the North Atlantic jet stream – a river of fast flowing air high up in the atmosphere – being further south than usual, the paper explains. That meant more storms were directed towards the UK than might otherwise be. There are a number of possible reasons for why this happened, all of which are linked to natural climate cycles, the authors say. So, what role, if any, did climate change play? The answer lies not in the unusual run of storms itself, but in the amount of rainfall the storms delivered, and how that’s changed over time. The UK’s wet winters First, the authors separated out the last 20 years of winters to represent the “current climate”. They then grouped together all those in which the jet stream and atmospheric circulation patterns developed in a similar way to 2013/4. They found that the chances of extreme rainfall was eight times higher than in winters without those characteristic weather patterns. Dr Nikos Cristidis, a senior scientist working on extreme weather attribution at the Met Office and lead author on the new study, tells Carbon Brief:

[The circulation] translates into more rainfall over the UK. South westerly winds drive moisture from warmer parts of the Atlantic over the UK, hence, we tend to get more rain.

While other winters saw conditions develop in a similar way, they didn’t all end up like 2013/4 because other random factors affect rainfall from one year to the next, Christidis adds:

You may have favourable conditions for extremes, but this doesn’t necessarily mean extremes will actually occur.

So far, the results are enough to tell us what kind of conditions are likely to bring heavy rain to the UK. But they don’t tell us whether such events are different today compared to pre-industrial times. To work out the role of climate change, the scientists had a different test. Seven times more likely Taking only winters with similarly stormy conditions to 2013/4, the scientists ran 76 simulations using seven different climate models to examine what happened once they removed human influences, leaving just those from natural climate cycles. This is known as an attribution study. The researchers found that in “the real world” where greenhouse gases warm the planet, extreme heavy rain over 10 consecutive days – like we saw in 2013/4 – is seven times more likely than on a planet without human influence. Put another way, if the same sequence of storms that we saw in 2013/4 happened on a planet that wasn’t warming, the chance of seeing rain as heavy as we did would be a lot lower. Of course, that assumes all other things are equal, such as the height of coastal defences, the amount of paved ground, the extent of building on floodplains, soil saturation, and so on. You can see the results from the study in the graphs below. Taking the most recent 20 years of model data (between the dashed lines), comparing the red line in the bottom left panel with the blue line in the bottom right panel shows a higher modelled average rainfall for events lasting 10 days when human activity is included (left), compared to only natural factors (right). It’s worth noting that the increase in average rainfall holds for events of 10-days duration (bottom panels), but not for the winter season as a whole (top panels).

The new study’s results are more of a hint at a role for climate change in the UK’s 2013/4 winter floods than conclusive evidence. That’s because the shift towards heavier rainfall in today’s climate compares to a non-warming one is not statistically significant, the paper explains. This means that it doesn’t pass the strict statistical tests scientists carry out to eliminate the possibility that it happened by chance. Christidis tells Carbon Brief: “The climate change signal may still be weak for winter precipitation in the UK, but here we provide some evidence that it begins to emerge and may already be manifest in 10-day rainfall events. This, I think, is an important result, even though it is not yet conclusive.” While it may not be hard evidence, the result fits in with what scientists expect from a warming climate. A warmer atmosphere holds more moisture so when a storm hits, there is more water available to fall as rain. And when several hit in quick succession, the effects are amplified.

Attribution With the UK’s weather patterns being notoriously complicated, disentangling how much of the unusually wet winter in 2013/4 was down to natural factors and how much was attributable to human influence is complicated, to say the least. So, was Met Office chief scientist, Dame Julia Slingo, right to tell the media in the wake of the flooding that “all the available evidence suggests there is a link to climate change?” Yes, Christidis tells Carbon Brief:

We do expect to see a move towards more warm wet winters in the UK under the effect of climate change. Julia’s assessment of the available observational evidence is consistent with this expectation. Our work provides additional evidence based on a new evidence from state of the art climate models.

Evidence is stacking up that climate change played a role in bringing more rain than we would have seen otherwise in the winter of 2013/4. This, in turn, contributed to the flooding we saw, but was by no means the only factor. What scientists aren’t doing is linking the existence of the storms themselves to climate change. While this might all seem like a frustratingly nuanced conclusion, the distinction between what drives heavy rainfall, storminess and flooding is important when it comes to understanding climate change. They are linked, but not the same.

Main image: Flooding in the centre of the village of Datchet in Berkshire became more and more extensive today as the level of the River Thames continued to rise. © Mark Kerrison/Demotix/Corbis