The effects of climate change

It is indisputable that a warmer climate will lead to the atmosphere having a higher water vapour content. The upshot will not only be larger amounts of precipitation generally but also extreme rain intensities in regional or local severe weather situations especially during the summer, as observations in many places have confirmed over the past few years in particular.

In no way should this be considered inconsistent with the general tendency towards drier summers in certain regions (e.g. in Europe); it must rather be seen as an indication of greater variability in precipitation and hence more frequent extreme events at both upper and lower ends of the intensity distribution scale.

Although there will be less rain in the summer, it will be more concentrated in time, so that more flash floods will occur. The fact that losses occur is attributable to these very extremes and not to a change in the mean values.

The costs that arise from flood events and in connection with them must therefore be expected to increase dramatically. Particularly over dense urban areas — i.e. areas with high concentrations of values — the more intense convection may lead to local severe weather events that induce extreme precipitation intensities.

These often involve a high density of lightning strokes, hailstorms, and gale-force gusts, sometimes even tornadoes. On account of the large proportion of impervious surfaces in urban areas, the torrential rain runs straight into the drainage systems, which are not designed to cope with such volumes, with the result that underpasses, cellars, and sometimes subway tunnels are flooded with water.

Much milder and wetter winters expected

At the same time, the trends observed in recent decades and the climate models lead us to expect much milder and wetter winters in many regions. This will have a substantial impact on the flood risk because precipitation will come in the form of rain rather than snow.

Without the buffer that snow provides, the precipitation runs off directly into rivers and streams. This effect is intensified by the fact that in winter — when the level of evaporation is low — the soil is almost completely saturated so that the surface acts as a natural impervious cover.

Furthermore, for about 30 years now, Europe has seen a distinct increase in westerly weather patterns during the winter; these are very rainy low-pressure systems that often trigger floods.

Isolated extreme events are nothing new, a fact to which the numerous high-water marks on historical buildings bear witness. Consequently, even such exceptional floods as those that swamped central Europe in the summer of 2002 cannot be cited as proof of global warming.

On the other hand, the indications that climate conditions have already changed significantly are so strong and unmistakable that no unbiased observer can deny them. Extremes have increased in frequency and intensity or they now occur more often in seasons that are untypical for them.

This development is due at least in part to anthropogenic global warming and is likely to continue and even accelerate in the future. As it will be impossible to reverse for decades to come, design assumptions must take into account that a 100-year discharge will be higher in the future.

The state, the emergency services, the population, and the insurance industry must come to terms with the fact that there will be more frequent and more catastrophic events with generally greater losses.