Unprecedented warming over the past 30 years in Berlin, Germany, unaffected by urban effects

Climate change with increasing air temperatures results in amplified hazards for human health by excessive heat. Compared to their non-urban surroundings, cities typically show elevated air temperatures, causing urban dwellers to be even more threatened by heat in warmer conditions. Up until now, 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 measurements from urban climate observation networks are generally scarce. Here, we present robust air-temperature trends for the Climate Normal 1991-2020 using quality-controlled data from eleven urban and 14 non-urban weather stations in the Berlin region, Germany, covering a wide range of urban and non-urban settings. We analyse trends for four daily variables as annual and seasonal means, as well as during heatwaves. Our findings highlight that climate change and the city interact linearly on the analysed time scales. This results in similar air-temperature trends in urban and non-urban areas, yet at different absolute levels. An exception is the daily minimum air temperature in spring, which shows different trends for urban and non-urban stations. Investigation of the built-up area around the stations and in the study region shows no significant change during the study period. This highlights that the observed warming is due to regional climate change and not related to urbanisation processes. By comparing trends for the last 30 years (1991-2020) with observational data since the end of the 19th century, we show that the recent rise in air temperature is unprecedented in the study region, indicating accelerated regional climate change. Our study, a first presenting 30 years of data from an urban climate observation network, offers a blueprint for investigating climate change in other cities with sufficient data.