Many Germans have still not forgotten the terrible images of the flood disaster in the Ahr Valley in the summer of 2021. The flooding of the Ahr River triggered by heavy rain killed more than 180 people and injured hundreds, some of them seriously. The extent of the damage ran into the billions. Even today, the entire valley suffers from the massive destruction caused by this extreme weather event.
In recent years, awareness has also grown significantly in Germany that heavy rainfall and flooding are not only weather extremes that affect other countries in Europe and other continents, but also pose an ever greater danger to life and limb as well as property in this country. No one can be sure anymore that it won’t hit him/her. Heavy rain can occur throughout the country and is nowadays a major hazard not only in mountainous regions.
The following article explores all the issues surrounding the relationship between heavy rainfall, flooding and climate change. He explains what is meant by heavy rain and what makes this weather phenomenon so dangerous. In addition, the paper provides an explanation of how human-induced climate change increases the frequency of weather extremes such as heavy rainfall. Last but not least, the questions of what damage from flooding German municipalities will have to prepare for in the future and what they can effectively do about it will be highlighted.
With the exception of meteorologists, few people have a sense of what heavy rain actually means. Rain is expressed in precipitation amounts per area and time unit. The German Weather Service (DWD) warns of heavy rain events in the following three levels:
When rainfall amounts of 15 to 25 liters per square meter within one hour or 20 to 35 liters per square meter in six hours occur, the DWD issues a prominent weather warning. The German Weather Service issues a severe weather warning for rainfall amounts between 25 and 40 liters per hour and square meter or 35 to 60 liters per square meter within six hours. If more than 40 liters of rain per square meter in one hour or more than 60 liters per square meter in six hours are expected, the DWD warns of extreme thunderstorms.
For a better classification of the flood disaster in the Ahr valley: In the summer of 2021, some areas received between 100 and 150 liters of rain per square meter within a short period of time. Such extreme heavy rains are therefore not unrealistic in Germany either.
Contrary to popular belief, heavy rain events can occur anywhere in Germany. These are not limited to mountains, where clouds usually accumulate. Heavy rain can also occur in flatter areas.
The great danger of heavy rain comes from a combination of man-made and self-reinforcing effects. Heavy rainfall events occur more frequently in hotter summers because the air can absorb more moisture at higher temperatures. What makes the situation worse is that during heat summers, precipitation hits parched soils that are unable to absorb the torrential rainfall. In addition, there is the weather phenomenon caused by the change in climate that heat phases last longer and longer. As a result, after several days and weeks, the air binds enormous amounts of moisture, which is then abruptly discharged when the weather changes.
In addition to parched soils that cannot bind the water, in many places in densely built-up Germany the water encounters sealed surfaces where it cannot drain away. In addition, the heavy regulation of many riverbanks causes the water masses to gain more and more volume and speed, as in a canal, until they finally burst their banks. The result of this combination of prolonged heat waves, parched soil, sealed surfaces and regulated rivers: Floods that cause massive damage to buildings and infrastructure, and increasingly cause deaths and injuries.
Is there a direct link between climate change and heavy rainfall?
It is now scientific consensus that the entire planet can expect more extreme weather events in the future, such as heat waves, storms and heavy rain. However, the climate is a very complex overall system that is influenced by a large number of factors that are far from being fully understood in their entirety and in their interaction. However, the observed accumulation of extreme weather events can be taken as an indication that the weather will become more extreme in the future.
Statistical evidence that climate change is leading to more extreme weather is not easy. This is especially true with regard to heavy rain and flooding. The reason for this is that, on the one hand, the observation periods for statistically viable statements are quite short. Second, recording a small-scale weather event such as heavy rain is relatively difficult. However, more accurate measurement methods, refined climate models and greater computing capacities have made it possible for some years now to link individual weather events with the overall climate with ever greater precision.
The link between climate change and rainfall is a relatively simple physical relationship on the one hand, but a complex phenomenon on the other. Since systematic weather records began in Germany in 1881, the average temperature in this country has increased by about 1.6 degrees Celsius.
A higher temperature of the air has a direct influence on the weather. When air warms by one degree Celsius, it can absorb seven percent more water. Thus, the relationship between average temperature and potential rainfall is not linear. Instead, the increase in temperature leads to a significant increase in moisture absorption in the air, which in turn increases the likelihood that heavy rainfall will occur in a short period of time.
Added to this is the observation of changing air currents. In recent years, the German Weather Service has noted that classic weather influences are increasingly changing. Thus, the eastern German states and Bavaria are increasingly coming under the influence of the Mediterranean climate. This means that in spring and summer moist air from the Mediterranean region moves past the Alps to eastern Germany and Bavaria. Especially in spring, this weather phenomenon can bring severe storm potential, as the strong solar radiation has already warmed the land, but the humid air over the sea is still quite cold. These temperature contrasts cause the formation of powerful low-pressure areas and thunderstorm cells.
As shown in the last section, heavy rains are a classic weather phenomenon of a heating Earth’s atmosphere. In the course of the earth’s history, there have already been several warm phases with significantly higher average temperatures than today. Thus, for the study of heavy rainfall events and their consequential effects, the study of these geologic warm phases is of great importance.
In a study conducted by the renowned U.S. Harvard University, climate researchers recreated the environmental conditions of these geological heat phases in a model. The climatic conditions at that time are far from being comparable to the current ones. Nevertheless, the scientists’ climate-atmosphere model provides interesting insights into what our planet may face in the coming decades.
Because warm air stores more water vapor than cold air, torrential rains have always occurred in tropical areas. Air currents from the tropics are already carrying this moist air to northern latitudes. In their study, the scientists at Harvard University were able to demonstrate that climate change can amplify this effect through another phenomenon.
The strong heating of the surface of the sea and land masses favors the formation of a kind of barrier layer. This is a warm layer of air that prevents water vapor-rich air from rising into the troposphere. This means that normal cloud formation is disrupted and warm moist air accumulates under the barrier. Since it cannot rise to higher layers of air, the water vapor does not condense out and raindrops do not form. As a result, the atmosphere absorbs more and more water vapor, but precipitation fails to materialize. Climate scientists compare this phenomenon with the charging of a gigantic “water battery”.
The water vapor accumulates over days in layers near the ground until the “water battery” is no longer able to hold the moisture. A breach in the barrier layer causes air masses supersaturated with water vapor to rise from near the ground to the upper layers of the troposphere in a short time. This creates a self-reinforcing suction. These powerful vertical air movements cause the warm, moist air masses to cool abruptly, causing the bound water vapor to condense abruptly. The moisture discharges in a sudden and violent discharge in the form of heavy rain.
The findings of this study could mean that in the coming years, humanity will have to prepare for much more severe precipitation extremes than previously predicted. It is possible that the steady increase in average temperature is causing a significantly disproportionate increase in heavy rainfall, flooding, and high water events.
The change in global climate is not only causing an increase in average temperature, but also a change in weather patterns. In the meantime, it can be statistically proven that weather events not only become more extreme, but also last longer. Many people can say from their own experience that the number of consecutive heat days and weeks has increased in recent years. What was considered extreme in Germany decades ago, such as maximum daytime temperatures of over 30 degrees Celsius and tropical nights in which the thermometer does not drop below the 20-degree mark, has now become the norm in many parts of the country in the summer. Scientists from the USA concluded in model calculations that the number of heat days will more than double by 2050, even in more temperate latitudes.
This increase in heat waves is not only due to the increase in average temperature, but also to the change in air currents. Climate researchers can now prove that daily weather fluctuations are becoming smaller and weather patterns are therefore lasting longer.
These more stable weather patterns pose a great potential hazard. The heat, which lasts for several weeks, dries out the soils and massively increases the risk of forest fires. In addition, the hot air absorbs enormous amounts of moisture over days and weeks, which in turn can be released within a very short time in the form of heavy rain. After weeks of heat, this heavy rainfall hits parched soils that are unable to fully absorb the water. The result is flooding and mudslides.
Against this background, scientists assume that the number of heavy rainfall events will increase with global warming. Statistics over the past 20 years already indicate that there has been an increase in torrential rainfall in Germany as well. Although the number of heavy precipitation events varies greatly from year to year, the long-term trend indicates an increase in these extreme weather phenomena.
The evaluations of the German Weather Service show that in the period from 2001 to 2019 almost every place in Germany was affected by a heavy rain event. Even in hot and particularly dry years, there were numerous heavy rainfall events for the reasons outlined earlier.
Basically, extreme storms, torrential rain and flooding can occur anywhere in Germany. In mountainous and hilly areas, however, the heavy rain can be particularly heavy. There, rain and thunder clouds are more likely to accumulate and precipitation to fall faster and more intensely than in other areas. In Germany, therefore, the large mountain regions such as the edge of the Alps, the Ore Mountains and the Black Forest are particularly affected by heavy rain and consequently by flooding. However, the disaster in the Ahr Valley in 2021 made it clear that the risk has increased even in valleys where heavy rainfall events were previously less frequent.
That global warming is causing an increase in extreme weather events is largely undisputed among experts. However, it is much more difficult to forecast when and where extreme weather may occur. The disaster in the Ahr Valley has once again made it clear that every minute counts when it comes to flooding caused by heavy rain.
Current weather models are already quite good at more accurately predicting the severe weather potential of certain weather patterns. But the question of where exactly dozens and hundreds of liters of rain per square meter fall on the earth can usually only be answered when the heavy rainfall occurs. Meteorologists and climate researchers are therefore working to improve the accuracy of their forecasts.
Gaining a better understanding of the extreme weather phenomena triggered by climate change is now not limited to climate scientists. Due to the billions of euros in damage caused by floods every year, the insurance industry now also has a vested interest in better providing for the occurrence of extreme weather. In 2021, the German insurance industry recorded record losses from natural events of 12.6 billion euros. The flood disaster in the Ahr valley alone caused damage worth more than eight billion euros.
In a four-year project, the German Insurance Association (GDV), together with the German Meteorological Service (DWD), has therefore investigated the frequency and distribution of heavy rainfall and the damage caused by it throughout Germany since 2001. The focus of the investigation was on damage to buildings.
Conclusion of the joint studies by GDV and DWD: Nationwide, there have been more than 11,000 heavy rain events since 2001. Thus, this form of extreme weather is anything but rare in Germany. In addition, any place in the entire federal territory can be affected by heavy rain and flooding. There is no region where the population can consider itself safe from flooding. Short and heavy rain showers occur with a similar high probability throughout the country. Long-lasting heavy rain, on the other hand, is more common in mountainous regions.
In addition, the study demonstrated that most damage from precipitation does not result from continuous rainfall, but rather from shorter precipitation events lasting up to nine hours. This result is consistent with the German Weather Service’s finding that with each degree of global warming, rainfall during heavy rain events increases by up to 14 percent. The reason for this disproportionate increase in rainfall as average temperatures rise is that in enlarged thunderstorm cells, disproportionate amounts of moisture are transported to higher layers of air, where they enhance cloud dynamics.
In view of the fact that the future increase in global warming is very likely to be at least two degrees, Germany must also prepare for a significant increase in heavy rainfall and consequently flooding. The clustered occurrence of heavy rain is a major problem in both urban and rural areas. In rural areas, the heavy regulation of rivers and the lack of natural flood zones mean that water masses can no longer be removed from streams or absorbed by nature. In hot years, this effect is further intensified by desiccated soils. The Ahr Valley disaster showed that a river like the Ahr can burst its banks within minutes and become a deadly hazard for local residents.
In cities, flood hazards are primarily due to heavy land sealing. The concreting over of huge areas with a simultaneous lack of green spaces means that when it rains too much, streets and cellars in almost all cities in Germany are under water.
Climate change is already a major challenge for Germany, and not just for farmers and foresters. Flooding caused by heavy rains makes global warming a real threat to everyone in the country.
As a consequence of the latest scientific and statistical findings, the German insurance industry is already calling for building regulations to be adapted to the new flood hazard situation. Flood protection must be a new priority in the future, according to insurance companies. In future, it will no longer be permitted to build in flood plains.
In addition, cities and communities in Germany need to think about how they can make their settlement areas “climate-proof” for the future. To prevent large-scale flooding in populated areas in the coming years and decades, it will not be enough to plant a few more trees and put up sandbags for flood protection. Every German municipality will have to develop an overall strategy for dealing with heavy rainfall and flooding. Time is pressing – the Ahr Valley disaster could happen again anywhere in Germany at any time.
