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Climate change has shifted the timing of European floods

From BBC - August 10, 2017

Climate change has had a significant impact on the timing of river floods across Europe over the past 50 years, according to a new study.

In some regions, such as southern England, floods are now occurring 15 days earlier than they did half a century ago.

But the changes are not uniform, with rivers around the North Sea seeing floods delayed by around eight days.

The study has been published in the journal Science.

Floods caused by rivers impact more people than any other natural hazard, and the estimated global damages run to over a $100bn a year.

Researchers have long predicted that a warming world would have direct impacts on these events but until now the evidence has been hard to establish.

Floods are affected by many different factors in addition to rainfall, such as the amount of moisture already in the soil and other questions such as changes in land-use that can speed up water run-off from hillsides.

This new study looks at this issue in some depth, by creating a Europe-wide database of observations from 4,262 hydrometric stations in 38 countries, dating back to 1960.

The analysis finds a clear but complex impact of climate change on river flooding.

The most consistent changes are in north-eastern Europe around Scandinavia where earlier snow melt due to warmer temperatures is leading to earlier spring floods. Around 50% of monitoring stations are seeing floods eight days earlier than they did 50 years ago.

The biggest changes are seen along the western edge of Europe, from Portugal up to Southern England. Half the stations recorded floods at least 15 days earlier than previously. A quarter of the stations saw flooding more than 36 days earlier than in 1960.

In these regions, the issue is not snow melt - it's more about saturated soils.Maximum rainfall tends to occur in the autumn and gets stored in the soils. Heavier and earlier rain means that the groundwater reaches capacity earlier.

"It's the interplay between extreme rainfall and the abundance of rainfall," lead author Prof Gnter Blschl, from the Technical University of Vienna, told BBC News.

"In southern England, it has been raining more, longer and more intensely than in the past. This has created a rising groundwater table and higher soil moisture than usual and combined with intense rainfall this produces earlier river floods."

However, around the North Sea, in the Netherlands, Denmark and Scotland, the trend is towards later floods.

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