The urban heat island under extreme heat conditions: a case study of Hannover, Germany

Abstract

Global warming has resulted in higher frequencies of climate extremes, such as drought periods or heat waves. Heat waves are intensified in urban areas due to the urban heat island effect. Studies are inconclusive as to whether the urban heat island effect is intensified during heat waves. Using the city of Hannover, Germany, as a case study, we analysed the intensity of the urban heat island under unprecedented summer heat conditions in the years 2018, 2019 and 2020, which were among the hottest in Germany since weather records began. We compared the intensity of the urban heat island across these years with the non-heat year of 2017. Differences were analysed for various inner-city urban locations and an urban park, while accounting for their distinct land use and land cover characteristics. We identified the urban heat island effect across all years investigated in the study and also found a significant intensified urban heat island effect during the years of unprecedented heat, when night-time temperature minima are considered. The urban heat island was identified on a lower level, however, with maximum daily temperatures when compared to the non-heat year. The lowest intensity of the urban heat island was found for the urban park site, highlighting the need for more city-wide greening strategies, including tree-covered and open green spaces, to provide all residents with the cooling services of green spaces.

Spatial Gradient Differences in the Cooling Island Effect and Influencing Factors of Urban Park Green Spaces in Beijing

“…Existing studies have confirmed these differences. For example, the central heat island intensity of dense urban areas in cities is the strongest [26], the new urban areas in metropolitan areas are more vulnerable to the heat island effect [27], and highly continuous dense and sprawling urban expansion will enhance the heat island effect [28]. As an integral component of urban green space systems, urban parks are directly influenced by factors such as the positioning of urban functional zones [29], socioeconomic factors [30], and differences in heat island patterns [31], which affect their intrinsic characteristics and functions.…”