Cooling efficiency of trees and short vegetation in large cities across the globe

Abstract

Hot extremes are increasing with climate change. Urban green spaces can help to mitigate these extremes through evaporative cooling and shading. We assess the non-linear effects of tree and short vegetation on human thermal comfort, as quantified by the Universal Thermal Climate Index (UTCI), in 216 large cities worldwide. On average, the cooling efficiency of trees is 6 times larger than that of short vegetation. However, comparable cooling efficiencies of short vegetation are found in central North America, central and eastern Europe and southern Africa. These results highlight vegetation’s cooling potential in the hottest months and the usefulness of short vegetation where their cooling efficiencies are comparable to trees, while short vegetation is cheaper and easier to establish compared to trees. Notably, cooling efficiencies of both vegetation types can be negative in some cities, indicating that increasing vegetation cover does not always enhance, and may even reduce, human thermal comfort. In contrast, cooling efficiency derived from land surface temperature (LST) shows more consistent and stronger cooling. This divergence underscores that LST-based assessments can overestimate the comfort benefits of urban greening. Our findings emphasize the necessity to incorporate thermal comfort metrics like UTCI into urban heat mitigation assessments, and provide guidance for the planning and managing of urban green spaces to achieve efficient cooling sustainably with limited space and water resources.