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Shifting flood patterns across Europe linked to climate change

06 September 2019 | Adaptation

Climate change has increased the severity of flooding across north-west Europe over the last five decades, a continental-scale study by researchers in 24 European countries has found. The largest increases in flooding were seen northern parts of the UK.

Led by scientists at the Vienna University of Technology in Austria, the multinational team looked at river flow data from almost 4000 monitoring stations across Europe between 1960 and 2010. They identified clear regional patterns of both increases and decreases in floods across Europe over the 50 year period. These ranged from increases in flood levels of 11% per decade in northern England and southern Scotland to decadal decreases of 23% in parts of Russia.

According to the researchers, the changes are broadly consistent with climate model predictions for the next century and provide evidence that climate driven changes in flooding are already occurring. They add that their results support the idea that climate impacts now need to be considered when designing flood management strategies.

“This [study] is the first time that we see evidence of climate change in flood observations,” Günter Blöschl, a hydrologist at the Vienna University of Technology, told Physics World. “People have done simulations in the past of climate impact on floods and people have looked at individual catchments and looked at trends in flood observations, but this is the first time we actually see climate impact on flood observations at the continental scale.”

Key drivers are changing

As well as flow data, the scientists assessed changes in key drivers of flooding, including precipitation, soil moisture and air temperature. This enabled them to identify three regional trends. Increases in autumn and winter precipitation and wetter soils are increasing floods in north-west Europe, while in southern Europe floods are decreasing due to a reduction in precipitation and increased evaporation of soil moisture. Floods are also decreasing in eastern Europe, but this is driven by reduced snow cover and snowmelt due to warmer spring temperatures – the team found that in some areas spring air temperature has increased by as much as 1 °C per decade.

In north-west Europe around 69% of flow-gauging stations showed an increasing flood trend, with an average increase of 2.3% per decade. In southern Europe three quarters of stations showed a decrease in floods, with an average drop of 5% per decade. In eastern Europe about 78% of stations show a decreasing flood trend, with an average decrease of 6% per decade.

According to the researchers these changes are linked to changes in atmospheric circulation patterns, such as storm tracks and jet streams. The poleward shift of the subpolar jet stream – which sits between the cooler mid-latitude air and the warmer equatorial air – and changes in its associated storm tracks, and the slowing of the northern jet stream, have led to increases in precipitation in north-east Europe and increased the chances of stalled weather patterns.

Pressure difference

Blöschl says that we now understand why flooding has increased in north-west Europe. “The pole is warming more strongly than the equator, so the pressure difference between the pole and the equator is changing, which shifts [weather systems] further to the north,” he explains. Also, the speed at which these systems move across Europe is a little slower than in the past, so the duration of extreme rainfall events is longer than before.

The northward shift of the subpolar jet has also allowed the expansion of the Hadley cell, a tropical atmospheric circulation. This, according to the researchers, has led to a decrease in precipitation and an increase in the evaporation of soil moisture in southern Europe. Over the period studied, they found that soil moisture reduced by around 5% per decade in this region.

Jamie Hannaford, a hydrologist at the UK‘s Centre for Ecology & Hydrology, who was involved in the research, says that the study “adds to a growing body of evidence that shows that flood magnitude has increased in the UK over the last five decades, particularly in parts of northern and western Britain”.

“We show this is part of a continent-wide pattern of changes in flooding which is in line with what we may expect in a warming world,” Hannaford adds. “This highlights the importance of long-term hydrological monitoring and the benefits of data sharing and collaboration at a European scale in order to better understand the mechanisms behind observed changes in flooding.”

 

Source: Physics World