Climate change strongly linked to UK flooding

In normal weather the River Eden in Cumbria is a placid waterway, flowing at a rate of about 50 cubic metres per second.

After Storm Desmond arrived last month, it hurled 1,700 cu m, the contents of an Olympic swimming pool, into the Solway Firth every second — a historic record for any English river. A similar torrent was measured on the Tyne in Northumberland.

December’s record flow rates in northern England’s rivers — and similar records being broken in the east of Scotland this week — emphasise the exceptional nature of the winter rainfall and flooding, say scientists at the Centre for Ecology and Hydrology, the leading flood research body in the UK.

In December, Britain’s wettest month since Met Office records began in 1910, rain fell so heavily over such a wide area that serious flooding was inevitable, said Professor Alan Jenkins, CEH deputy director. In other words, any precautionary measures could not have stopped it — though mitigation schemes could help in some places.

Some have blamed upland landowners and farmers for exacerbating the impact of the rainfall through deforestation, as well as draining peat bogs and other natural reservoirs that could hold back or soak up floodwaters.

But Prof Jenkins does not accept that view. “There is very little scientific evidence that the recent extreme flooding is worse due to the impacts of the management of the uplands.”

As for the real cause, he said: “We are absolutely convinced that there is weighty scientific evidence that the recent extreme rainfall has been impacted by climate change.”

Climate scientists are moving away from cautious statements made in the past that, while man-made global warming makes certain types of severe weather more likely, it cannot be blamed for specific events.

All climate experts who have analysed this December’s record-breaking temperatures and rainfall in Britain see a significant role for global warming in addition to natural variability in the weather, according to Professor Adam Scaife, head of long-range prediction at the UK Met Office.

“When natural oscillations add to climate change, you end up breaking records,” he said.

Two natural oscillations are influencing UK weather this winter: El Niño, the heating of the tropical Pacific Ocean that occurs every few years, and an upper atmosphere phenomenon called the Quasi-Biennial Oscillation.

The more combative Professor Myles Allen, a climate scientist at Oxford university, said predictions of heavier winter rainfall in mid-latitudes as the world warms go back 25 years. “So none of this should really come as a surprise to anyone,” he added. “The hopefully diminishing band of diehards who continue to dismiss climate change concerns as ‘just green crap’ have mud on their hands.”

Climate change can increase rainfall in two ways. One is the undisputed physical fact that a warmer atmosphere holds more moisture, which can come down as rain.

A 1C rise in temperature increases the water vapour in saturated air by 7 per cent; the impact of this on rainfall depends on local circumstances but it typically translates into a 4-5 per cent increase in rainfall under UK conditions, said Prof Scaife.

The hopefully diminishing band of diehards who continue to dismiss climate change concerns as ‘just green crap’ have mud on their hands

The other impact of global warming — potentially more significant but harder to pin down — is to change circulation patterns in the atmosphere in a way that makes flooding more likely. In the British winter, that means a strong southwesterly airstream blowing moist, mild air from the Atlantic, with frequent depressions and weather fronts.

What comes next? The Met Office 30-day outlook promises a cold snap next week, followed by more unsettled weather with rainfall totals above average in the north and west. But Prof Scaife offered some reassurance: “There will be a much reduced risk of heavy rainfall and storminess for the second half of the winter.”

Taking a longer view, and bearing climate change in mind, Prof Jenkins of CEH warned that worse is to come. “As unprecedented as our last set of floods have been, there will be another bigger flood somewhere, sometime,” he said.

“We don’t know whether that flood is going to happen next month, next year, next millennium — but big events, further big events, are going to happen.”