Guest article by City of Aachen

Experts take soil samples to determine their cooling capacity.
Stadt Aachen/Sarah Koll

Keeping the City Cool

Aachen's Soils Are Like Air Conditioners

Next summer will be here before we know it. And it will be hot again, with more and more scorchers likely; worldwide, nationwide, in Aachen. According to the German Weather Service’s forecast, we will see an average of 48 summer days and 10 to 20 heatwave days annually from 2031 to 2060. Experts talk of “summer days” when the maximum high temperature exceeds 25 degrees Celsius and of “heatwave days” when the maximum high temperature rises to more than 30 degrees.

Soils can provide relief on such days thanks to their natural cooling capacity. Whenever water evaporates from the soil, it acts like a natural air conditioner and cools the surrounding air. Urban greenery and urban soils are thus highly significant for evaporation and urban cooling.

A map is now being drawn up for the entire urban area of Aachen that will show potential soil cooling capacity and evaporation. The purpose of this map is to identify and present information on soils with a high cooling capacity and thus areas that are particularly worthy of protection. Soil mapping is used to validate soil conditions in selected areas. In Aachen, the focus of those responsible is on, for example, Elisengarten, Ferberpark, the spa gardens in Burtscheid and at Eurogress, Westpark, the parks around Frankenberg Castle and at Moltkebahnhof, Von-Halfern-Park, the park at Hangeweiher, Kennedypark, and many more.

Plants, trees, and meadows, like those here at Hangeweiher Park, help to provide a cool urban climate.
Stadt Aachen/Andreas Herrmann

Soils Are Also a Valuable Resource in Economic Terms

Dr. Michael Kastler, an agricultural engineer at ahu GmbH, is confident that “the realization of how important the cooling capacity of soils is for a harmonious urban climate is slowly gaining ground.

Aachen is at the forefront of this innovation with its modern planning tool for urban development.”

In addition to ahu GmbH, the Soil Protection Subdivision at the Climate and Environment Department of the City of Aachen and the Bönsch und Schomers Office are entrusted with this task.

The cooling capacity of soil is enormous and can be calculated very precisely: To match the work a one-hectare overgrown park area can do would require – assuming an electricity price of 0.43 euros per kilowatt hour – about 1.6 million euros per hectare per year with air conditioning. Soils are therefore also a valuable resource from an economic perspective. Only through careful and sustainable use of soil can we succeed in preserving and securing them for future generations. Sustainable land development is only possible if industrial brownfield lands are regreened and particularly valuable soils are better protected.

All findings about soils with a high cooling capacity, the known cold air generation areas, and the movement of colder air over the known cold air corridors form an important data basis for soil as a protected good, for the local urban climate, and urban planning processes. Where are the heat islands in the city? Where can soils be improved so that they can be used to store water?

“Sustainable urban and settlement development also has to address the foreseeable impacts of climate change on settlement areas. Soils play a central role in climate mitigation,” says Heiko Thomas, Head of the Environment Department of the City of Aachen.

For him, sustainable urban development and climate protection can only go hand in hand: Heiko Thomas, Head of the Environment Department of the City of Aachen.
Stadt Aachen

Green Spaces As “Refrigerators of the City”

Such green spaces, coupled with soil improvement measures and sustainable water storage solutions, can become very effective “refrigerators of the city.” But unfortunately, it also works the other way around: Without an adequate water supply, a green space in a city park heats up almost as much as tarmac in the summer. Therefore, the soil cooling capacity of meadows and forests, of parks and gardens must be protected and promoted just as much as cold air currents over the many stream valleys and so-called “green finger” areas of the city of Aachen.

Nature provides Aachen residents with many valuable gifts; we just have to make use of them. The map illustrating the potential cooling capacity of soils in the Aachen urban area is expected to be available for all to see in spring 2024.

– Author: Elena Reinders