BioEd Online

On 15 April, the River Danube reached its highest level for some 111 years, forcing residents of Romania, Bulgaria, Hungary and Serbia to flee their homes. With weeks of snowmelt and rain swelling Europe's second-longest river, the waters haven't dipped below that level since. Meteorologists are warning that the flood represents yet more evidence that climate change is gearing up to play havoc with our weather. News@nature.com asks what caused the events of the past few days in Eastern Europe, and whether those living near other major river systems can expect the same this year.

What caused the Danube flood?

Some might say the flooding was down to bad luck more than anything else. Spring hit Europe quite quickly this year, after a relatively long period of winter cold. Temperatures rose by some 15°C over the space of a week in some parts of the Alps, leading to massive snowmelts. This led to two large surges along the Danube itself and in one of its main tributaries, the Tisza. In a freak coincidence, both of these flood waves arrived in the lower Danube together, at the same time as a long period of persistent rain.

This caused the river to burst its banks, inundating some 130,000 hectares of Romania, the hardest-hit country. The country's environment minister Sulfina Barbu has warned that flow along the river will be abnormally high for more than a month to come.

Did anyone spot it coming?

Staff in the flood-monitoring team at the European Union's Joint Research Centre in Ispra, Italy, spotted the danger in mid-March, when large amounts of snow began melting in the Czech Republic. But there was much more melting than they anticipated, says team member Ad de Roo, which meant that the computer models they used to calculate the rise in river levels downstream did not predict the extent of the inundation in April.

Is climate change to blame?

Single events are notoriously difficult to attribute to shifting climate rather than random chance. This is particularly the case for natural disasters involving precipitation — rain and snowfall — which tends to fluctuate more capriciously than temperature. Nevertheless, some experts say that this is a sign of what's in store. "While no single event can be attributed to climate change, the Danube scenario represents the kind of event that is likely to become more frequent according to climate-change predictions," says David Crichton, a climate expert based in Inchture, Scotland.

How will European flood risk change in the coming years?

"There will be an overall increase in precipitation, including wetter winters with more intense weather systems," says Simon Brown, a researcher at the Hadley Centre for Climate Prediction and Research in Exeter, UK.

Temperate areas of Europe and North America look set to encounter increases in rainfall all year round, in contrast to regions with a Mediterranean climate, which are expected to experience increasingly arid summers. "The Danube is right on the boundary," notes Brown. So other, less Mediterranean, rivers in Europe may be expected to experience greater increases in summer flood risk.

But with warming climate comes a reduction in the amount of snow on mountaintops, which might mean that spring floods — such as this Danube event — might actually become less commonplace.

Can we predict exactly which areas are becoming more flood-prone?

Not yet. In a recent review of the risk of flooding in Europe¹, Maarten van Aalst, a climate expert at the Red Cross/Red Crescent Centre on Climate Change and Disaster Preparedness in The Hague, the Netherlands, points out that climate models are currently not fine-tuned enough to pick out the individual flood risk for a given river system.

The climate models used to predict rainfall patterns typically use 100-kilometre square blocks of terrain. Evaluating the risk for individual rivers will require 10-kilometre resolution, says van Aalst. Climatologists are not currently convinced that their models are reliable at this scale, and even if they were, the computations would take 100 times longer.

So what can we do to prepare?

The European Flood Alert System is expanding, says de Roo. It currently includes several hundred precipitation stations run by European Union member states, and which feed data to a central database so that computer water-flow models can be run to predict the areas most at risk. But this system needs to be fully automated, de Roo says, ideally providing daily, or even hourly, updates.

Such a system would be adept at spotting floods caused purely by heavy rain, such as the one that struck the River Elbe in Germany in 2002. But the system will need more data from water-level sensors in rivers, notes de Roo, to determine flood risk in snowmelt-driven floods such as the Danube event.

Land use and the level of flood defences in a given area also strongly influence risk, van Aalst points out - a fact that Romania's residents know all too well since their waterlogged dykes burst this week. In a changing climate, anyone in a flood-prone area may be well advised to prepare for the worst.

Visit our newsblog to read and post comments about this story.