Heavy rain, long dry season
Studies show: After the flood is before the flood
January 4, 2024, 6:54 a.m
Rain, rain, rain: The situation in the flood areas is critical. And even if the floods have subsided, Germany must prepare for the next floods. Many studies show that these will become more common in the future. Just like another extreme weather event.
According to researchers, floods and droughts will continue to increase. “Many studies, including our own, show that as global temperatures rise, the number and intensity of extremes such as floods in Germany also increase,” says Fred Hattermann from the Potsdam Institute for Climate Impact Research (PIK), Hydroclimatic Risks Working Group.
One cause of the current flood is probably the seasonally high surface temperatures of the western Atlantic, which lead to high evaporation and therefore a lot of water vapor in the atmosphere, explains Hattermann. The westerly winds that prevail in our latitudes transported these moist air masses to Europe, where, as a result of a chain of low-pressure areas that moved quickly across Central Europe, there was heavy rainfall in autumn and winter and then floods. “The soils became saturated with water and then hardly absorb any water.”
The long-term average rainfall in Germany has hardly changed. “There are no strong trends for Germany, but the variability of precipitation is increasing,” says Hattermann. In short: There is heavier rain, but also longer dry periods.
According to Hattermann, there are two reasons for this. Firstly, climate change is causing the air to warm up, which can then absorb more water. “The amount of water per cubic meter of air has increased.” This initially means longer dry phases – and when it rains, more water falls on the earth.
Secondly: “We have more stable general weather conditions over Europe.” This is related to the influence of climate change on the jet stream, an air current over the northern polar region, although the individual connections have not yet been precisely clarified. Such a situation, for example, led to long-lasting high-pressure weather conditions such as those in 2018 and in spring 2023.
“A high-pressure area rotates clockwise and brings dry air from the Eurasian region to Germany,” says Hattermann. “In 2021 we had a low pressure area where the wind turned counterclockwise and brought moist air from the Mediterranean to Germany.” That led to the water masses in the Ahr Valley in 2021.