Should we be concerned about the global cool down

A hot future for cities - is it possible to cool down?

Vineyards in Copenhagen and desert heat in Rome: this could already be a reality in 2050. Scientists from ETH Zurich simulated the heat development in 520 cities around the world. They used a scenario with a moderate increase in global temperatures from the World Climate Report.

"We found that 77% of cities in the future will very likely have a climate that is more similar to a city in a different climate zone than its current climate," the scientists write.

That means: in 2050 it will be as warm in the rather cool Seattle in the northwest of the USA as it is currently in San Francisco, which is over a thousand kilometers further south. In Germany's capital Berlin, temperatures in the warm summer months could climb as high as they are currently in Canberra, Australia: that would be an increase of more than 6 degrees Celsius.

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In Karlsruhe, this trend is viewed with concern. The city in southwest Germany is already one of the warmest in the country. The city administration assumes that the number of extremely hot days with over 30 degrees Celsius will double to at least 60 days per year by 2050.

In Singapore, the pioneer in facade greening, house walls of 100 hectares are already overgrown

Death rates will rise

"There will be more tropical nights when the temperature is around 25 degrees," says Norbert Hacker from the Office for Environment and Occupational Safety in Karlsruhe. His office deals with adaptation and the consequences of climate change in the city.

“Hot nights are actually almost worse than the heat during the day because the body can no longer recover. (...) there are also clear connections between heat development and death rates. "

If climate change continues unchecked, the World Health Organization (WHO) estimates that the heat-related death rate among older people worldwide will be ten times higher by 2050 than in 1990.

In the extremely hot summer of 2018, 490 people died from the effects of heat in Berlin alone, exact figures for all of Europe are not yet known.

City infrastructure has to change

On warm days, cities heat up a few degrees more than the surrounding area. This is due to the urban heat island effect. It arises from the fact that building facades and asphalt store the solar energy and thus the heat, and air conditioning systems direct warm air onto the streets, where the traffic generates additional heat.

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Half of the people on earth already live in cities that have so far hardly been prepared for rising temperatures. “Cities were usually built for very specific climatic conditions. The smallest changes such as precipitation or temperature can have a major impact on people, "writes Emily Clark, co-author of the ETH study, by email to DW.

In Karlsruhe, the administration is already working with experts to think about how to cool the city down and prepare it for the heat.

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Water and green walls

More fountains and water dispensers should be built for the heat in everyday life. There are already 200 wells in Karlsruhe, many of which are fed with drinking water. Every flat roof in the city has to be greened with plants today. In the Südstadt-Ost residential area, "the roofs of 5,000 citizens have now been greened," says Klaus Weindl from the Horticultural Office. This is an important step "because on the one hand it retains precipitation, it causes evaporation and is beneficial for the urban climate". In addition, the greening of house walls is to be expanded. The plants provide fresh air, shade and cooling because the buildings heat up less. However, you will not be able to keep up with Singapore, the pioneer in facade greening. The current 100 hectares of overgrown house walls are to be doubled in the East Asian metropolis by 2030.

"(It is our) long-term goal by planting trees - they have the greatest climatic balancing effect - to bring more coolness into the city," says Norbert Hacker from the city of Karlsruhe.

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"Future trees" against tree death on the roadside

Around a third of the 1,000 new trees planted each year in Karlsruhe are trees of the future

But that is precisely what could be difficult. Due to the higher temperatures and the prolonged drought in the city, trees are more likely to suffer from water shortages and are more susceptible to disease and fungus. "The number of felled trees in Karlsruhe is increasing," says Mario Köhler from the Green Lungs research project, "an estimated 20-30% have to be felled because of the consequences of the high temperatures."

The city is therefore planting more and more so-called "trees of the future" that can cope better with the heat. These include the hackberry tree and the American ash. "Trees that are planted on the side of the road are up to permanent due to the radiation from the asphalt on hot days Exposed to 50 degrees Celsius. Some of them get tired ”, says Andreas Ehmer from the city's own tree nursery. Around a third of the approximately 1,000 new trees planted each year in Karlsruhe are trees of the future.

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More green could also help elsewhere. In particular, the many old buildings from the Wilhelminian era with their sealed inner courtyards heat up a lot on hot days. "We want to use information campaigns and advice to encourage homeowners to unseal and green these areas," says Hacker from the city of Karlsruhe. The city cannot force people to adapt their properties to the climate.

"Overall, it is also about changes in behavior in everyday life," says Petra Mahrenholz from the "Competence Center for Climate Impacts and Adaptation" at the German Federal Environment Agency. This could include: closing the windows during the day and lowering the blinds to keep the heat out - common practice in Spain. It is only aired at night.

Less heat due to white walls

In Karlsruhe, public spaces and buildings should only be designed in light colors and house walls - if possible - plastered in white. White heats up less than dark colors. The temperature difference on a white roof compared to a black one can be up to 23 degrees Celsius, writes NASA in a report. In New York, the Cool Roofs initiative has already painted several hundred thousand square meters of roofs white - this saves 2,282 tons of CO2 every year because the buildings need less cooling.

Although climate change will also be noticeable in more and more German cities, so far only "under five percent" of the municipalities in Germany have actually taken concrete measures, estimates Petra Mahrenholz from the Federal Environment Agency.

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Will that be enough? In Karlsruhe, at any rate, there has not yet been "a special budget for climate adaptation," says Hacker, and he sees no approach at all in the city's plan. And major projects, such as new fresh air corridors or the conversion of streets and parking lots into shady green spaces, that is not yet planned in Karlsruhe.

But Hacker and his colleagues are already working on an action plan for the heat emergency. Hackers can well imagine building urban cold rooms to protect people from the heat. “But that's not the way things are at the moment. That may still come if the temperatures continue to rise. "According to the Zurich scientists, they will.

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    Author: Ruby Russell