How can urban green spaces be planned to mitigate urban heat island effect under different climatic backgrounds? A threshold-based perspective

Abstract

Urban green space (UGS) was widely regarded as an effective nature-based solution to mitigate the urban heat island (UHI) effect, therefore, developing landscape strategies to enhance its cooling intensity (CI) is crucial. However, two main problems prevent the application of results to practical actions: one is the inconsistency of relationships between influencing factors of landscape and the thermal environment; another is the unfeasibility of some common conclusions such as simply increasing the amount of vegetation cover in highly-urbanized areas. This study compared the CIs of UGSs, investigated the influencing factors of CI and identified the absolute threshold of cooling (ToC_abs) of the influencing factors in four Chinese cities with very different climatic backgrounds (Hohhot, Beijing, Shanghai and Haikou). Results demonstrate that local climate condition affects the cooling effect of UGS. The CI of UGS is weaker in cities with humid and hot summer than in cities with dry and hot summer. Patch characteristics (area and shape), the percentage of water bodies within the UGS (Pland_w) and neighboring greenspace (NGP), vegetation abundance (NDVI) and planting structure together can explain a significant proportion (R² = 0.403-0.672, p < 0.001) of the CI variations of UGS. The inclusion of water bodies can ensure effective cooling of UGS, except in the tropical city. Besides, ToC_abs of area (Hohhot, 2.6 ha; Beijing, 5.9 ha; Shanghai, 4.0 and Haikou, 5.3 ha), and NGP (Hohhot, 8.5 %; Beijing, 21.6 %; and Shanghai, 23.5 %), NDVI (Hohhot, 0.31; Beijing, 0.33; and Shanghai, 0.39) were identified and related landscape strategies of cooling were proposed. The identification of ToC_abs values can provide easy-to-use landscape recommendations to UHI mitigation.

Keywords: Climate adaption and mitigation; Cooling effect; Local climate zone; The absolute threshold of cooling; Urban green space.