Global climate-driven trade-offs between the water retention and cooling benefits of urban greening

TL;DR

Urban greening helps reduce heat and flooding, but globally, most cities benefit more from one rather than both due to climate differences.

Contribution

A global analysis showing climate-driven trade-offs between water retention and cooling benefits of urban greening.

Findings

Water retention benefits increase in arid environments, while cooling benefits are stronger in energy-limited climates.

Common urban greening strategies rarely address both heat and flood risks effectively in most cities.

Irrigation in arid areas can enhance cooling without compromising water retention.

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.

Urban greening can help to mitigate both heat as well as flooding risks, but how these effects interact is not well known. Here, the authors provide a global analysis and show that most cities benefit from one of these two effects, but only few from both.

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