Abstract
Urban greening can potentially help mitigate heat-related mortality and flooding facing the >4 billion urban population worldwide. However, the geographical variation of the relative combined hydrological and thermal performance benefits of such interventions are unknown. Here we quantify globally, using a hydrological model, how climate-driven trade-offs exist between hydrological retention and cooling potential of urban greening such as green roofs and parks. Using a Budyko framework, we show that water retention generally increases with aridity in water-limited environments, while cooling potential favors energy-limited climates. Our models suggest that common urban greening strategies cannot yield high performance simultaneously for addressing both urban heat-island and urban flooding problems in most cities globally. Irrigation, if sustainable, may enhance cooling while maintaining retention performance in more arid locations. Increased precipitation variability with climate change may reduce performance of thinner green-infrastructure more quickly compared to greened areas with thicker soils and root systems. Our results provide a conceptual framework and first-order quantitative guide for urban development, renewal and policymaking.
Highlights
Urban greening can potentially help mitigate heat-related mortality and flooding facing the >4 billion urban population worldwide
When the impact of seasonality in calculated actual evapotranspiration (AET) is considered in regard to the cooling potential, we find that locations with high seasonal variability in cooling potential tend to be those with higher mean cooling potential
With respect to hydrological efficacy, previous work indicates that the KöppenGeiger climate classification is a poor diagnostic indicator, at least for retention of green roofs[23] within the main climate classes
Summary
Urban greening can potentially help mitigate heat-related mortality and flooding facing the >4 billion urban population worldwide. The urban stream syndrome (USS) reflects the widely reported degraded physical, chemical, and biological conditions caused by urbanisation of watersheds[6,7] and the consequent increase in flood risk[8] In combination, these present pressing health and water management challenges associated with substantial financial (e.g., infrastructure) and social (e.g., human wellbeing and mortality) risks, and increasingly so under climate change[7,9] due to the impact on heat and moisture fluxes. These present pressing health and water management challenges associated with substantial financial (e.g., infrastructure) and social (e.g., human wellbeing and mortality) risks, and increasingly so under climate change[7,9] due to the impact on heat and moisture fluxes To combat these problems, a commonly proposed strategy for urban development and renewal[10] is to increase the proportion of ‘urban greening’ in the form of green roofs, green walls, or vegetated urban spaces[2,11]. This is of critical importance since, owing to the various potential interactions between different urban greening interventions, to consider any one in isolation risks unknowingly providing a potential disservice from another
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