Urban Spatial Patterns and the urban heat island effect - Evidence from German regions

Abstract

The urban heat island (UHI) effect, known as the temperature gap between urban and rural areas, has been widely observed in human settlements of all sizes. The adverse influences of the UHI effect have been widely documented, as it raises energy consumption, worsens air quality, and increases risks of heat-related mortality. For a long time, city planners and policymakers have debated whether well-designed urban forms can potentially reduce the UHI effect. A wide-ranging study has suggested that high-density and compact urban development can increase urban heat intensity. However, others have concluded that low-density and dispersed development may also exacerbate the urban heat intensity due to higher energy consumption per capita and expanded urban impervious surface. This inconclusive debate between urban density and the UHI effect has led to a dilemma in optimal urban management, calling for more efficient spatial strategies in urban thermal planning.

Several studies have proposed that polycentric spatial development may offer a promising alternative that reconciles this dichotomy, as it assumes to integrate the benefits of both compact and dispersed development (Han et al., 2022). A polycentric urban system encompasses multiple centers, characterized by a more evenly distributed employment and a network of interconnections among these centers. This concept embodies two aspects of urban activities: decentralization and compactness. “Decentralization” suggests that households and firms are less concentrated around a single center but dispersed across extensive suburban areas. “Compactness” implies that these decentralized activities re-agglomerate outside the traditional urban core to form urban centers. Consequently, polycentric regions distribute urban activities across multiple centers rather than a single center, resulting in lower densities in urban cores and reduced spatial aggregation in urban footprints. These considerations suggest that polycentric spatial development offers a more sustainable and efficient approach to urban thermal planning.

As far as we know, Germany-based investigations on the relationship between urban spatial patterns and the UHI effect mostly focused on case studies, thus lacking universal conclusions that can be generalized to a broader range of metropolitan regions. Furthermore, existing studies have primarily examined mitigating urban heat intensity through the lens of “compactness versus dispersion” utilizing “traditional” landscape metrics. The effectiveness of polycentric urban configuration has received limited empirical testing.

This study aims to achieve two objectives. First, we assess whether the more compact or fragmented urban forms can contribute to the UHI effect, employing five “traditional” landscape metrics. Second, we investigate the capacity of polycentric spatial patterns to mitigate urban heat. We utilize city regions (Großstadtregionen) as the spatial unit and the MODIS land surface temperature (LST) dataset to obtain the UHI measures. We quantify five traditional landscape metrics based on McGarigal & Marks (1995) and the degree of polycentricity following the works of Green (2007). For empirical testing, we employ both naïve and multivariate OLS regressions to analyze the influences of urban spatial configuration on the UHI effect. We further utilize nonlinear local weighted regression and kernel density estimates to compare temperature variations between polycentric and monocentric regions.

The results indicate that certain features of urban fabric, including fragmentation, mixed land use, and regular-shaped urban patches, have the potential to mitigate the UHI effect. Moreover, dispersing multiple smaller greenspaces throughout the urban area demonstrates a greater cooling effect compared to having a single large and more aggregated park. This study confirms that polycentric development yields greater benefits in reducing urban heat for large-sized city regions compared to medium- and small-sized ones; and its effectiveness is mostly pronounced near urban center(s).