Satellite data: DLR detects high heat stress in major German cities

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The first major heatwave in August may have passed in Germany, but urban planning measures to adapt to climate change are likely to remain in demand. In an analysis, the German Aerospace Center (DLR) systematically examined the distribution of surface temperatures in all major German cities with over 100,000 inhabitants for the first time. The results confirm that heavily sealed and densely built-up urban areas suffer particularly from heat.

The study makes the geographical location and structure of heat stress in more than 70 cities such as Berlin, Munich, Frankfurt, Stuttgart, Cologne and Hamburg spatially comprehensible. This is based on thermal images from the US Landsat satellites, which are intended to create a sound, data-based foundation for targeted measures to combat heat in conurbations.

The DLR scientists examined the average surface temperatures for the summer months of June, July and August from 2013 to 2024. Although surface and air temperatures are not identical, they are directly related, which makes it possible to identify heat islands. The experts have correlated this data with various variables such as city size, location and structure types – such as single-family home areas or block development –

Precisely identifying heat hotspots

In order to simulate the temperature distribution even more accurately, the researchers combined earth observation data with urban climate models. This enabled them to record the heat distribution down to a meter and precisely simulate important factors such as the shading of individual areas. This enabled them to find out where heat stress is at its highest.

"We now have extensive data from remote sensing to identify, quantify and monitor spatial heat hotspots", explains Thilo Erbertseder, DLR researcher for urban climate and health. "This knowledge can be used to reduce the urban population's exposure to heat." It should be noted that the perceived heat stress for people also depends on factors such as humidity, wind speed and radiation.

Cooling green and water areas

The analysis confirms that structures with low building density and a high proportion of greenery have significantly lower temperature differences to the surrounding area than densely built-up areas. Green and blue infrastructure (vegetation and water areas) have a significant cooling effect and are therefore already being used as an adaptation measure to reduce urban temperatures.

Hannes Taubenböck from DLR's Earth Observation Center (EOC) emphasizes that there are conflicts of use in every city between initiatives to adapt to climate change and the need for housing and transport infrastructure. The researchers therefore wanted to provide knowledge "so that social and political decisions can be made more consciously". Possible initiatives that could be implemented based on this data included targeted information for the population during heatwaves, the provision of cool rooms, heat protection-oriented renovations and long-term urban planning interventions. With the help of Landsat data, the EOC has also already visualized large-scale losses of tree cover in Germany: from January 2018 to April 2021, over 500,000 hectares of green vegetation were affected. The main triggers are considered periods of severe heat and drought, which in turn boosted insect pests.

Outlook and methodology

In future work, DLR intends to investigate the effectiveness of various mitigation instruments and link urban climate models with remote sensing data to evaluate possible scenarios. The Landsat earth satellites used this time fly over Germany in the late morning or around midday and record the temperatures of the surfaces, for example on tree tops in forests or over water areas.

The team then used this data to calculate the average temperatures for the summer months. According to the researchers, the comprehensive measurement by remote sensing solves the problem that air temperature measurements in cities are often not sufficiently available to identify local hotspots and enable targeted planning.

"Information from space is fundamental to an even deeper understanding of processes such as climatic developments in cities and conurbations," says DLR Executive Board Chairwoman Anke Kaysser-Pyzalla. Thanks to the procedures and methods developed at DLR for processing satellite data, the institute can provide reliable information and applications for a wide range of recommendations for action.

(nie)