Severe floods in central Europe are not becoming more common, say scientists in Germany who have compiled a historical record that stretches back almost 1000 years.
Even the devastating floods that left cities from Prague to Dresden awash with water in 2002 do not suggest an upward trend, Manfred Mudelsee at the University of Leipzig and his colleagues found.
Many reports at the time suggested that the floods were the kind of extreme weather event expected to become more frequent as a result of global warming. But “if they can’t find trends, then there is no reason to attribute the flooding to climate change” says Mike Blackburn, a climate scientist at the University of Reading, UK.
However, while the frequency of major floods may not be increasing, the economic damage and the number of people at risk of flooding may well be going up. This is due to, for example changes in land-use and in the possessions damaged by flooding.
The damage caused by the 2002 floods cost the insurance industry $4.1 billion, says Jens Mehlhorn, head of the flood group at Swiss Re in Zurich. The average loss over the previous 15 years was $1 billion a year, and there is an “upward trend in losses”.
“People now live in more risky areas and the vulnerability of the insured objects has increased – for example if a PC or television gets wet, you have to throw it away,” he told New Scientist.
Elbe and Oder
The University of Leipzig study focused on two rivers – the Elbe and the Oder. It was the Elbe, which flows from the Czech Republic, through Germany and into the North Sea, which was responsible for the flooding in the summer of 2002. The Oder also emerges in the Czech republic and runs northwards. Together they drain rainfall from 150,000 square kilometres of central Europe.
Mudelsee found evidence for 328 major floods on the Elbe, with the earliest in 1021, and 218 floods on the Oder, beginning in 1269. “We then applied a statistical test to see if we could find a significant trend over the last few decades” he told New Scientist.
There was no trend at all in summer floods, while those in winter have become less frequent. Global warming could explain this, says Mudelsee. Warmer winters would mean that fewer rivers become blocked with ice and burst their banks.
A separate study mapping the flood threat to home-owners in the UK found a rising trend, but blamed people, not climate change.
The team, led by Edmund Penning-Rowsell, at Middlesex University’s Flood Hazard Research Centre, mapped how factors such as climate change, the density of buildings and the quality of flood defences would affect flooding over the next century.
The researchers predict that 3.5 million people in the UK will be at high risk of floods by 2080, compared to 2.6 million now. Particularly dramatic was a map of the Thames Gateway, where the government plans to build 120,000 houses – the zone lies entirely on fluvial land and a tidal flood plain.
Ilan Kelman, from the Centre for Risk in the Built Environment at Cambridge University says: “If we build poor houses in dumb places, the flood risk increases, even if the hazard becomes less extreme.”
Journal reference: Nature (vol 425, p 166)