Floods are hitting Britain two weeks later than they did 20 years ago, and it’s because of climate change, a new study has suggested.
The large-scale study, conducted by TU Wien, shows that the timing of floods has shifted considerably over the last 50 years across Europe. The study authors have been able to link this to our warming climate.
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Jamie Hannaford, co-author and leader of the Hydrological Status and Reporting Group in the UK, says that floods come later to north western Europe, including Scotland, western Ireland, coastal Scandinavia and northern Germany, because of ‘north Atlantic oscillation’; fluctuations in the difference of atmospheric pressure at sea level between the equator and north pole.
However, there’s a debate over whether north Atlantic oscillation is actually caused by climate change.
Hannaford says flooding trends in the north west are consistent with anthropogenic warming (pollution), but changing Atlantic pressure is also caused by a natural variability in the Atlantic itself. This makes the relationship between flooding and climate change harder to pin down in these areas.
Interestingly, there’s even variation within the UK, even though rainfall is consistent.
In southern England and the north of France, the study found a shift toward earlier timings. Hannaford hypothesises that this is because we have wetter autumns with earlier, persistent rain.
This increases soil moisture and groundwater stores, which fills river catchments earlier on in the year compared to decades past. Then, when peak rainfall comes later in the year, rivers overflow.
Jim Dale, senior risk meteorologist at British Weather Services, says that it may only be a matter of weeks but, in climate terms, that’s quite huge.
“Climate change, specifically rainfall and rising temperatures, is slightly changing the seasons, which is why we see variation in flood timings,” he says.
Hannaford says the team chose to study flood timings because it’s a more sensitive indicator of climate influences on flooding, compared to looking at magnitude or frequency of floods. They can use the timings to unpack the processes that are driving flood variability on a large scale.
These processes are more obvious in colder countries in the north-east of Europe such as Sweden, Finland and the Baltic States, where floods now tend to occur one month earlier than in the 1960s and 1970s.
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“Rising temperatures result in snow melting earlier,” he says. “That’s almost certainly due to human induced climate warming because it’s unequivocal that the climate has warmed and this is melting ice.”
On parts of the Mediterranean coast, flood events occurring later in the season are aligned with the warming of the Mediterranean.
For the study, which has been published in the journal Science, Hannaford and Günter Blöschl, the paper’s main author, collected and analysed 50 years of data of the timings of floods from more than 4,000 hydrometric stations located in 38 European countries, making this the biggest study of its kind.
Hannaford says it’s important to monitor flood changes so we can better coordinate our flood risk management in a warming world.
“If we do see these shifts continue at the rate they have been changing, it might impact agriculture, hydropower and water resources,” he says.
“This study means we should be less surprised by ad-hoc events that seem slightly out of season,” Dale adds.
This isn’t the first time that floods have been blamed on our changing climate, but before this study we were unsure of the exact reason why.
Previous flood research was often concerned with the size of floods, but this can be affected by urbanisation, intensifying agriculture and deforestations – more than just climate warming.
Therefore, looking at the timing of flooding can tell you more about what causes them.
Hannaford says this study should hammer home the importance of long term monitoring so we can see the emerging climate variability, whether or not we can directly link this to human-driven climate change, and prepare for future flooding.