Creating a thermally comfortable city through urban green infrastructure: An international review of greening policies

Urban green infrastructure has been recognized as a pertinent Nature-based Solution in urban heat mitigation and climate change adaptation. Greening efforts, especially in compact cities, face multiple constraints and challenges that require innovative justifications. To effectively improve urban greening, government agencies and local communities worldwide have devised diverse greening policies, which often include optimizing the cooling potential. This study comprehensively reviewed greening policies in 21 cities on five continents, analyzing how urban greening works as a strategy for moderating urban thermal environments. Information was gathered from portal websites and city authority documents. Targeted searches covered greening motivation and policy formulation, implementation, monitoring and evaluation. The cities' concerns and approaches to developing greening policies were evaluated through in-depth investigation and comparison. Four key themes were distilled from the review. (1) Political and economic motivation: raising awareness regarding the economic value of the thermal moderating effect of urban greening; (2) Policy refinement and formulation: considerations over the quantity and quality of tree canopies; (3) Policy feasibility and implementation: incorporating considerations of local climate features and climate change into practices; and (4) Policy monitoring and evaluation: promoting urban microclimate monitoring and greening assessment through the collaboration of governments, universities, businesses, the public and the environment. By offering a comprehensive understanding of greening policies by cities with high-quality governance, this review provides a snapshot of collective best practice intending to foster the scientific promotion of urban greening to fulfill the coupled objective of urban greening and urban cooling.