German Environmental Aid: Offenburg is worst prepared for heat waves

Offenburg landed at the bottom of the ranking together with Lahr and Mannheim, three cities from Baden-Württemberg. Offenburg performed particularly poorly on tree canopy coverage: according to the DUH, it amounts to just under 13 percent there. Mannheim, which already ranked last in the previous year, continues to receive the worst grades in the Heat Vulnerability Index.

At the other end of the scale are Kiel and Wuppertal. They are the only two cities nationwide to which the DUH gave a green light in two out of three evaluation categories. According to the DUH, Kiel has a tree canopy coverage of around 32 percent, leading the overall ranking. Potsdam, Oldenburg, and Berlin also do not surpass Kiel's value for tree canopy coverage.

Losers and Winners in the Ranking

Despite Kiel's top position, the report shows significant losses: according to the DUH, around 7,500 trees disappeared in Kiel between 2018 and 2025. Extrapolated to all 195 cities examined, the loss amounts to more than 900,000 trees. In Kiel and Wuppertal, surface sealing also increased compared to 2018; none of the 195 cities received a green rating on the issue of soil sealing.

Demands on Federal Policy

The DUH's Federal Managing Director, Barbara Metz, warned of the consequences of this trend: "Wenn sich dieser Trend fortsetzt, leben wir in wenigen Jahren in menschenfeindlichen Betonwüsten." She also stressed that this development cannot be reversed quickly: "Diese Tatsache lässt sich nicht schnell wieder umkehren, denn Bäume brauchen Zeit, um zu wachsen."

Metz directed clear demands at the Federal Minister of Construction, Verena Hubertz (SPD): "Wir fordern von der Bauministerin verbindliche Vorgaben für die Entsiegelung, mehr Stadtgrün und damit mehr Schatten – bei jeder Sanierung, jedem Straßenumbau und jedem Bauprojekt."

Background: Heat and Health

The background to the assessment is the increasing heat stress in Germany. According to the German Weather Service (DWD), Germany has already warmed by 2.5 degrees Celsius compared to the pre-industrial period. According to experts, heat waves are occurring more frequently and more intensely with climate change. The Federal Ministry of Health points out that multi-day heat periods with temperatures above 30 degrees put a strain on the body and pose a risk particularly for elderly and pre-ill people.

According to the DUH, plants and green spaces cool their surroundings through evaporation, while sealed surfaces heat up and only release the stored heat back into the environment hours later. The environmental organization emphasizes, however, that the results are estimates that do not take all factors into account. Newly planted trees, for example, have so far barely been included in the assessment.

In light of the findings, the DUH considers Germany to be disproportionately affected by climate change. With the Heat Check, the organization aims to increase pressure on local authorities and federal policymakers to do more to combat soil sealing and the disappearance of urban trees.

The results of the Heat Check are based on aerial image analyses and statistical data on summer temperatures, population density, the share of concrete and asphalt, and the number of shade-providing trees and green spaces. The DUH publishes the report annually to draw attention to the health risks of rising temperatures in densely built cities.