Cooling effect of urban green and blue infrastructure: a systematic review of empirical evidence

Urban green and blue infrastructure(GBI) plays an extremely important role in regulating urban climate and assisting cities to cope with future climate change and enhance urban resilience. This paper presents a comprehensive review of urban green and blue infrastructure researches on cooling effect trends and hotspots, and the distribution of cooling effect studies in different climatic zones. Most studies focused on surface types and configurations, urban parks and water bodies, roof greening(building energy conservation), ecological benefits, thermal comfort, plant communities, etc. through remote sensing imagery, ground measurement, and comprehensive simulation. Most studies were influenced by the research field(perspective) and the research scale dependence of urban green and blue infrastructure cooling effect was extremely obvious. Based on the K9 ppen climate classification, the research methods, indicators and key issues are different at scale level. Through the analysis of cooling effect of green and blue infrastructure and its influencing factors from the landscape, patch(corridor) and community scale, we found that the cooling effect was mainly affected by the proportion of ecological land, landscape pattern(landscape scale); patch area, underlying surface type(patch scale); and plant configuration, single plant(community scale), etc. Meta-analysis was used to synthesize data on the cooling effect of parks and the results showed that, on average, a park was 1.39 ℃ cooler in temperate climate zone and 3.12 ℃ cooler in continental climate zone. On this basis, this paper summarizes three challenges in the future research on the cooling effect of urban green and blue infrastructure based on the practical needs of urban planning and design:(1) The cooling effect of green and blue infrastructure on city and block scale should be connected to determine key heat mitigation parameters of GBI planning and design, and furthermore provide quantitative indicators for urban green space system planning;(2) The multi-dimensional of the horizontal distribution and vertical transmission spaces on the cooling effect of green and blue infrastructure studies should be integrated, and the characteristics of blocks should be combined to determine the cooling threshold for guiding and optimizing the distribution of GBI in regulatory detailed planning;(3) The pivotal indicators and thresholds of each plant or plant community configurations in heat mitigation should be de