The connection between climate change and the current floods
Days of rain have led to severe flooding in parts of Bavaria and Baden-Württemberg. Entire areas are under water, dams have burst and people have had to leave their homes. The water levels of the Danube and Inn in Austria have also risen dangerously. This is due to the large-scale weather situation, also known as the V-b weather situation: a low is slowly moving from northern Italy across the eastern Alps to Poland. What does this have to do with the climate crisis? Due to global warming of almost 1.5 degrees Celsius, extreme weather conditions are on the increase. On the one hand, this has to do with the fact that the air contains around seven percent more water vapor per degree Celsius and, on the other hand, the slower jet stream leads to longer-lasting weather conditions - be it heat or rain.
The combination of an above-average rainfall in May and the low pressure system with plenty of water vapor from the Mediterranean led to the tense situation with flood levels in Bavaria and Baden-Württemberg. The soil is already saturated with rain and many rivers are barely able to absorb it, as there was around 30 percent more rain than normal this May and in some places - such as Sigmarszell on Lake Constance with 160 liters per square meter in 48 hours - as much precipitation as once every 50 to 100 years. This led to water levels that are statistically only reached once every hundred years. But this could be the new normal.
Climate experts have been using climate models to warn for years that extreme weather events will increase due to the climate crisis. Also in Bavaria and Austria. "We also know that this V-b weather situation has become much more frequent in recent decades. In this respect, we are therefore well advised to be even better prepared for this type of extreme event in Bavaria," says climate researcher Harald Kunstmann from the Campus Alpin of the Karlsruhe Institute of Technology.
Heat leads to more water vapor in the atmosphere
2023 was the warmest year ever measured since records began. The warmer the air above the Earth's surface, the more water vapor it can absorb. If the atmosphere warms by one degree, it stores around seven percent more water vapor, experts estimate. This increase leads to more precipitation. Warmer therefore also means wetter, says Stefan Rösner, who is responsible for climate monitoring in Europe at the German Weather Service: "This means that the air can also absorb seven percent more water vapor and precipitation can be correspondingly higher." This makes extreme heavy rainfall, such as that seen in recent days, increasingly likely.
The influence of the jet stream
While high and low pressure areas used to alternate quite quickly, we have been seeing weather patterns lasting longer for some time now. This May, for example, there was a prolonged period of rain in Bavaria and Austria. Last summer, we had a very dry heat phase lasting several weeks. The so-called jet stream is the driving force behind passing areas of low and high pressure in our latitudes.
At an altitude of 9,000 to 14,000 meters, a wind band with wind speeds of up to 400 kilometers per hour stretches from west to east around the entire globe. This jet stream is caused by the large temperature differences between the North Pole and the equator. When the cold air masses of the north meet the warm air masses of the south, they pick up speed - the earth's rotation determines the west-east direction of the strong wind band.
The massive warming of the polar ice caps and the melting of the poles is having a variety of small-scale effects on climate and weather patterns worldwide, including on the jet stream in our latitudes - it is weakening and also moving more strongly to the north and south.
The slowdown in the jet stream means that weather patterns last longer in our region. The weather virtually stays in place, with serious local consequences such as long periods of heat and drought or prolonged torrential rain.
Feedback mechanisms
"And if you think about it further, if rain and heat events persist over a region for longer, then the likelihood of these events becoming more extreme also increases - due to feedback mechanisms." This is according to Kai Kornhuber, who researches weather extremes and climate impacts at the Columbia Climate School in New York and the International Institute for Applied Systems Analysis (IISA) in Laxenburg near Vienna. In the example of heat, feedback means that soil dryness intensifies the heat. "This is because the evaporation of moisture in the soil leads to cooling. And if this mechanism is no longer guaranteed, hot areas become even hotter." The same applies to wet conditions: At some point, the soil is just full, if you want to call it that, and can no longer absorb moisture - and the level rises.
"This is not a good sign for the time being, because it also indicates that we are increasingly moving into regions that are more difficult to assess," says Kornhuber.
Riparian area
