Authors: Carl-Friedrich Schleussner (Climate Analytics and Humboldt University, Berlin); Stefan Rahmstorf (Potsdam Institute for Climate Impact Research, Potsdam); Özden Terli, meteorologist; Volker Wulfmeyer (Institute for Physics and Meteorology, University of Hohenheim, Stuttgart)
The devastating floods of July 2021 makes us concerned. More than 170 people have lost their lives and entire communities have been destroyed. The number of victims of this flood disaster exceeds that of all previous inland floods in Germany since 1900 combined.
The flood disaster was accompanied by a large number of record-setting precipitation events. Record rainfall and water levels were measured for many affected regions. Up to 182 mm of rain fell within 72 hours. Globally, we are seeing an increase in record rainfall in recent decades — a trend that is directly related to global warming.,
In view of approx. 1.2 ºC global warming, which corresponds to 2.0 ºC warming in Germany, the question in 2021 is no longer whether global warming — caused mainly by the use of fossil fuels — will influence the occurrence of extreme weather events, but in what way and to what extent. Temperature levels reached today are very likely to be the highest in 12,000 years. We have left the relatively stable climate of the Holocene, a key enabler for the development of agricultural societies and advanced civilisation. Now well within the Anthropocene, we are entering uncharted territory with an unchecked warming trend.
This also means that we still have no real idea of the actual consequences that 1.2 ºC of global warming will have for our societies. For the most part, we have yet to experience the really extreme weather — such as events that would be expected only once every 20 or 50 years- resulting from the global warming we have already caused. The flood event of 2021 is taking place on a planet that has already warmed by 0.4 degrees compared to 2002, the year of catastrophic flooding in eastern Germany. Our individual experience of the consequences of global warming is lagging behind the actual risks by years to decades.
Climate research has been warning for decades about the increasing risks posed by global warming, including the increase in heavy rainfall events. Scientific projections have proven to be accurate time and again. Due to the warming, we can expect an increase in extreme precipitation events and a decrease in days with only low precipitation (compare Fig. 1). This redistribution towards extreme events, such as heavy rainfall, is independent of changes in the annual amount of precipitation — even with overall decreasing precipitation, heavy rainfall events may increase. The relative increase is greater the less frequently the event occurs. In Europe, for example, every degree of warming doubles the amount of rain associated with particularly extreme rainy days that would statistically occur only once in ten years.
Human-made global warming leads to heavier precipitation almost everywhere in the world, and it increases in intensity by an average of about 7 % per degree Celsius of warming., This increase follows the thermodynamic principles described by the Clausius-Clapeyron equation, according to which a warming atmosphere can carry more moisture. For Central Europe, including Germany, the increase in heavy rainfall due to global warming has been clearly documented. For short-term precipitation extremes, the increase in Central Europe is in some cases even significantly higher than the 7 % mentioned.
This significantly increases the risk of flooding. The observed floods of the last decades in Europe are exceptional in this form in the last 500 years. Significant increases are also projected for future losses, unless additional adaptation measures are taken on a substantial scale. For Germany, the number of people exposed to flood risks could more than triple and damages more than quadruple by the end of the century.
Fig 1: Schematic representation of changes in the distribution of precipitation due to global warming. Due to a warmer atmosphere, there is a decrease in days with light precipitation and an increase in heavy rainfall events. The total amount of global precipitation is determined by the evaporation rate, which increases less than the heavy rainfall events. Therefore, less water remains for rain outside of heavy rainfall events. Figure adapted from reference 8.
There is also increasing evidence of rising risks from changes in circulation patterns. For example, a high-resolution modeling study of extreme precipitation events in Europe shows a slowing of weather systems and a sharp increase in the frequencies of quasi-stationary situations and the resulting precipitation extremes and flood risks. Such a stationary weather situation was a major contributor to the July 2021 flood disaster.
One mechanism for the increase in such stationary events discussed in the literature is related to the slowing of the predominant westerly wind circulation evident in observational data, due to a strong warming Arctic as a result of global warming. Such a slowdown has been linked to observed increases in the persistence of weather systems .
Model studies show that the number of weeks of prolonged intense precipitation in central Europe increases by 15 % already at 1.5°C warming. The mechanisms behind these circulation changes are complex and not yet fully represented in global — and thus lower-resolution — climate models. In the future, these relevant processes should therefore be simulated with high-resolution models not only on a global scale, but also specifically for certain regions., It is not yet possible to conclude whether there will be a further increase in weather system persistence with ongoing global warming, as climate models still provide contradictory results.,, Nevertheless, climate models that better represent observed atmospheric trends show a clear slowing of weather systems even in the future, and trends in the latest climate models are more pronounced than in previous generations of climate models.
However, the fact that different climate models do not yet provide robust conclusions about the future persistence of weather patterns does not call into question the increase in persistence in the observed data. Similarly, observed increases for summer persistence are independent of the scientific discussion on similar trends in winter. The extent to which the observed increase in persistence is a direct result of global warming remains the subject of current research. The aforementioned further increase in extreme precipitation according to the above Clausius-Clapeyron equation will continue regardless of the issue of persistence.
A detailed attribution study of the magnitude of the contribution of global warming to the flood disaster is still pending. Nevertheless, the best available science allows us to clearly state that human-made global warming undoubtedly contributed to the severity of the flood event. The catastrophe shows that no one, including people in a highly-developed country like Germany, is safe from the consequences of global warming. Even though increased adaptation measures to protect against climate impacts are urgently needed, the scale and severity of the event already show us the limits of our adaptive capacity.
This catastrophe underscores the urgency of stringent emission reductions. The promise made in the Paris Agreement to pursue efforts to limit the temperature increase to 1.5 ºC above pre-industrial levels needs to be the benchmark for climate action. This is the only way we will be able to contain and manage the enormous damage and costs of climate change. At the same time, climate protection creates enormous economic opportunities. We must act now if we are to achieve the Paris goals.
This text has been written by the authors as members of Scientists for Future and has been extensively reviewed by colleagues in the Scientists for Future community for scientific quality (in particular, for evidence of arguments). We thank the Scientists for Future for this collegial support.
- German: Die Flutkatastrophe im Juli 2021 in Deutschland und die Klimakrise — eine Stellungnahme von Wissenschaftler:innen der Scientists for Future
- Dutch: De overstromingsramp in Duitsland in juli 2021 en de klimaatcrisis – een verklaring van wetenschappers van Scientists for Future
 In the Em-DAT database on natural disasters, a total of 74 fatalities due to river flooding have been documented for Germany since 1900. Storm surges such as the North Sea storm surge of 1962 are not included. Data gaps especially before the 1980s cannot be excluded. Analysis here: https://twitter.com/mAndrijevic/status/1415701425930244101?s=20. The extreme weather events that cause the most fatalities in Central Europe are heat waves.
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