Parts of Gemany under water

Eberhard Faust: The thunderstorms, hail and torrential rain are being caused by a large-scale weather pattern that has dominated since mid-May. The prevailing trajectory of the jet stream has been in the form of a large northward arc around central Europe. This arc has stabilised weather systems and prevented them from moving onwards. A high-altitude, low-pressure system over western Europe, which has occasionally expanded as far as central Europe, has been continuously directing warm, humid subtropical air towards western and southern Germany. This resulted in heavy localised thunderstorms in those areas throughout May and the first few days of June, which, due to weak winds aloft, barely moved and simply discharged their rain where they were. This has been the case for several weeks already – and will persist for a few more days. At the same time, the jet stream arc brought a high-pressure system to northern Europe, which greatly affected northern and eastern Germany. Temperatures reached 34°C and higher in some places, breaking several local records. A number of “tropical nights” have even been recorded in Berlin and Frankfurt since the end of May, which means that temperatures did not drop below 20°C at night. It has been extremely dry in northern and eastern Germany due to the high pressure. Six of the 14 districts (Landkreise) in Brandenburg were put on the highest level of forest-fire alert during at least some of that period.

Germany’s National Meteorological Service, DWD, has announced that last month was the hottest May in Germany since records began in 1881. This is in line with the long-term trend of increasing monthly mean temperatures for May since 1881, and this April was of course already announced as the warmest April ever. Such observations clearly demonstrate the influence of man-made climate change. On the other hand, specific weather patterns – such as the heavy rain just discussed – are highly individual in themselves and cannot be traced to climate change per se. But the current weather is indeed very similar to what we already saw in May and June 2016, when Germany was also beset with recurring thunderstorms, with an omega block stabilising the weather front – so-called because it resembles the shape of the Greek letter omega (Ω). The villages of Simbach and Braunsbach were almost completely flooded that spring, and damage across Germany totalled €2.6bn. Studies have shown that the likelihood of heavy spring rainfall in the northern half of Europe has become significantly higher in the last two decades than it was in the decades before. And climate models are predicting that intense convective rainfall and periods of heat and drought are likely to occur more often in the future. Those two phenomena are related: the higher the ambient temperature, the more intense convective rainfall becomes. We can therefore consider the weather over the last few weeks to be a small foretaste of what climate change will be throwing at us a lot more frequently in the future.

Recently, we have been seeing repeated heavy rain and flooding pretty much everywhere south of a line between Emden and Chemnitz. Somewhere in Germany experienced thunderstorms with heavy rainfall every day between 22 and 27 May. In Vogtland, the DWD recorded more than 150 mm of rain in a matter of hours on 24 May. The Hunsrück and central and northern Hessen were hit on 27 May, there was localised flash flooding in parts of North Rhine-Westphalia on 29 May, and on the night of 31 May it was mostly the Saarland that was hit. Precipitation levels reached or exceeded 100-year return periods in just one or two hours in some places. Western and southern Germany in particular are likely to continue to be affected by the current weather pattern with thunderstorms in the coming days.

When a storm front does not move and a lot of rain falls in a limited area – as we have been seeing – the volume of water is often too much for drainage systems to cope with. If the land is flat, the water simply stays where it falls. But in hilly areas, water masses are swept along and gather earth and debris as they flow. In some cases, bridge passages then get blocked with debris, causing the water to rise. If there are steep slopes in the area, leading to heavy run-off, rubble and uprooted trees can seriously damage buildings. This has caused a number of landslides over the last few weeks, which blocked traffic routes.

Such thunderstorm cells can theoretically occur anywhere, and this is precisely why loss prevention is so challenging. But the local surroundings can also influence the risk of flash floods. Sparse vegetation, steep hills, and densely built-up spaces all contribute to accelerated surface water run-off. It takes a lot of planning and expense to build flood control infrastructure or to dig huge drainage culverts under roads. And given the fact that extreme weather events are typically very rare at any given location, such expense is often unwarranted. Municipal maps, on the other hand, showing the probabilities of heavy rainfall allow planners to identify where flooding, traffic disruption, blockages by debris and other threats are likely, and what drainage areas the local authorities need to protect. This information can help in drafting evacuation plans, and allows emergency personnel and residents to conduct suitable drills. The German Insurance Association (GDV) is currently working on a countrywide map of heavy rain zones, which they hope to have available by 2019. But households can also do their bit by ensuring that basement windows are well sealed, installing backwater valves, and refraining from storing valuable items in basements, for example. Flood insurance against conceivable losses should be a key component in any risk prevention plan, even far away from rivers, since heavy rain can happen in any region and affect everyone. As such events are localised and can happen anywhere, they are easily insurable. And premiums for this kind of enhanced natural hazard insurance tend not to be very high.