Hot days, tropical nights, water shortages: What will the towns and cities of Lower Franconia and their residents face if climate change continues unchecked as before? How many hot days of 30 degrees Celsius and more, how many tropical nights where it doesn’t drop below 20 degrees at night, are realistic? And where is it most unbearable in the cities?
Answers to the biggest questions – using the example of the city of Würzburg – climate researcher Prof. Heiko Paeth from the University of Würzburg and his colleague Christian Hartmann, who researches the topic “urban climate”. Both worked on the research project “Climate experience Würzburg“, in which scientists from the Technical University of Munich and the University of Würzburg studied the influence of urban trees on the microclimate in the city for three years.
Are the extraordinarily warm days, months and years really the result of man-made climate change?
Yes, proof of man-made climate change has been provided, says Professor Heiko Paeth. At least since the last status report of the United Nations Intergovernmental Panel on Climate Change (IPCC) in autumn 2021. “In principle, we are sure,” says Paeth.
How much warmer is it in Würzburg now compared to before?
A look at the temperatures measured in Würzburg from January 2010 to mid-2022 shows that the vast majority of months were too warm. And not too tight. In many months it was four to five degrees warmer in Würzburg than before. “It’s something extraordinary that we haven’t been able to associate with the natural change in the climate system for a long time,” says Heiko Paeth.
For explanation: In order to see how much today’s temperatures deviate from previous normal values - the so-called “temperature anomaly” – climate researchers compare the mean values of each month with the respective monthly mean values of the years 1961 to 1990. Because it’s about climate change, i.e. the long-term changes in the climate , to prove, current weather data is always compared with a 30-year reference period in the past.
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Sometimes a year is too wet, sometimes too dry: is the drought really getting more and more extreme?
In fact, the example of Würzburg shows that in 2010 very dry and very wet months alternated. But since the beginning of 2015 it has only been too dry.
For explanation: If it rains 20 liters per square meter more in Würzburg in one month than in the long-term reference period from 1961 to 1990, the blue bar in the graph shows 20 liters per square meter up. If it rains 80 liters per square meter less in the following month than in the reference period, the blue bar goes down into minus to -60 (i.e. 20 minus 80). The graphic shows the water deficit in the soil that has accumulated in Würzburg since 2010, the so-called “accumulated deviation”.
The summer of 2021 was completely rainy: have the groundwater and drinking water supplies been replenished?
The rainy summer of 2021 was actually able to compensate for the water deficit in the region a little (see graphic). But after the past months of 2022, which were again too dry, the water deficit that Würzburg had at the beginning of 2021 has almost been reached again. “Würzburg is now missing more than a whole year’s rainfall,” says Paeth. “It’s as if it hadn’t rained for a whole year in the past six years.”
Will it get even drier in Lower Franconia if climate change continues unabated?
“We already live in one of the driest regions of Germany,” says geographer Heiko Paeth. If climate change continues unabated, the dry periods will be even longer in the future. Although only by a few days, says the climate researcher. But every day that it doesn’t rain a drop increases the damage to agriculture and forestry: “If the weather continues to develop like this and we can afford the luxury of our own swimming pools in the garden at the same time, then we will soon have to ration our drinking water in summer .”
“Continue as before” scenario: what will happen to Lower Franconia if greenhouse gases continue to enter the atmosphere unchecked?
Heat days, i.e. days on which the temperatures reach at least 30 degrees or even climb above the 30 degree mark, will increase sixfold in the Würzburg, Kitzingen and Aschaffenburg area – to more than 30 a year. The number of tropical nights, in which it does not cool down below 20 degrees and which practically never existed in the past in the region, continues to increase. This has a team around Heiko Paeth in several different climate models up to the end of the 21st century for Lower Franconia. The result, the researchers warn: Extreme heat is affecting the health and quality of life of more and more people. It is often life-threatening for the elderly and infants.
Difference between town and country: is the heat in the city center of Würzburg becoming even more unbearable than, for example, in Gerbrunn in the district of Würzburg?
Yes, this is the result Research study “Climate Experience Würzburg“. One goal was to find out how pronounced the urban heat island effect is in Würzburg. In other words, the phenomenon that it is warmer in densely built-up cities than in the surrounding areas. For this purpose, the researchers have weather stations at seven different locations in the city and one was set up on the city limits in Gerbrunn.Weather data such as temperature, humidity, solar radiation, wind and precipitation have been recorded around the clock since January 1, 2018.
The result surprised even Christian Hartmann, who evaluates the data: “On hot days, differences of 8 degrees Celsius were measured!” says the urban climate researcher. So it could be that someone is sitting at 20 degrees in Gebrunn and at the same time someone else at 28 degrees in downtown Würzburg.
When and where is it warmest in the city center of Würzburg?
Urban heat islands are particularly pronounced on hot summer days, from May to September. “So you can’t be happy about lower heating costs in winter,” says Heiko Paeth. The city is particularly overheated in the afternoon and evening hours. The reason: the sun heats up the building fabric and then radiates this heat again, explains Christian Hartmann. This effect is strongest just before and just after sunset – especially on the Würzburg market square, because the air between the houses can hardly circulate there. Due to its basin location, the dense development, the high degree of sealing and the small amount of green space, Würzburg is predestined for this urban climate.
What helps against overheating in cities?
The heat island effect is defied at two weather stations in Würzburg: at the Ludwigkai and in the Ringpark. The Main acts like a natural air conditioning system, says Hartmann. In the evening hours, this could mean a temperature difference of up to 4 degrees. And the dense stock of trees in the Ringpark could cool the air down to 3.5 degrees on hot summer days, according to Hartmann. Provided the trees are doing well, have enough space and, above all, are well supplied with water.
The conclusion of the climate researchers: “We can reverse the warming trend and keep the city center cooler on hot days with green spaces, regardless of whether they are trees, grassy areas, roof or facade greening.” The scientists come to the conclusion that this requires urban architecture with 30 to 40 percent green spaces. In Würzburg, the proportion of greenery on the market square is less than ten percent. In the vicinity of the city center it is around 20 percent.
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