Recent unprecedented warming in Berlin, Germany, not influenced by urban effects

Climate change is accompanied by increasing air temperatures, resulting in amplified hazards for human health by excessive heat. Cities typically show elevated air temperatures as compared to their non-urban surroundings such that urban dwellers are even more threatened by heat. So far, studies could not conclusively clarify how climate change and urban effects on air temperature interact with each other over time scales covering decades since multi-decadal atmospheric data from urban climate observation networks are generally scarce. Here, we present robust air-temperature trends for the climate normal period 1991-2020 using quality-controlled data from eleven urban and 14 non-urban weather stations in Berlin, Germany, and the surrounding region, covering a wide range of urban and non-urban settings. We analysed trends for four daily variables as annual and seasonal mean values, as well as during heatwaves. The results show that climate change and the city interact linearly on the analysed time scales, also during heatwaves. This results in similar air-temperature trends in urban and non-urban areas but at different absolute levels. Investigation of the built-up area around the stations and in the study region shows no significant change in the study period, highlighting that the observed warming is due to regional climate change and not related to urbanisation processes.

By comparing trends for the last 30 years with those at two stations with observational data for longer time periods, we show that the recent rise in air temperature is unprecedented in the study region, indicating accelerated regional climate change. Our study, the first one presenting 30 years of data from an urban climate observation network, offers a blueprint for investigating climate change in other cities with sufficient data and supports the design of solutions for adapting cities to climate change.