Quantifying cumulative cooling threshold of greenspaces using a newly developed 3D model across global cities

Urban greenspaces are widely recognized as an effective nature-based strategy to mitigate urban heat. However, previous studies have often oversimplified the cooling process of greenspaces, leaving their cumulative cooling effect insufficiently quantified. To address this gap, we developed a novel three-dimensional (3D) quantitative model to evaluate spatially accumulated cooling benefits. A total of 668 greenspace patches across 37 cities globally, representing varying climatic conditions, were selected. We then tested the relative importance of intrinsic drivers (landscape composition and spatial configuration) versus built environment drivers (landscape patterns of surrounding areas) in explaining the cooling effect. The results indicate that greenspaces provide an average cumulative cooling effect of 4.92 °C on the surrounding land surface temperature (LST), with a maximum reduction of up to 25.3 °C, far exceeding expectations. The cooling effect is primarily and positively influenced by patch area, the intrinsic green-blue proportion, and surrounding socioeconomic characteristics. More importantly, we emphasize the necessity of an accumulated perspective in the threshold value of efficiency (TVoE) assessments, as existing quantitative frameworks tend to largely underestimate the cooling benefits of large greenspaces. Our analysis reveals a U-shaped relationship between the aridity index and TVoE. To maximize cumulative cooling efficiency, optimal greenspace areas were identified as 2.25 ha for arid cities, 1.48 ha for semi-arid cities, 1.64 ha for dry sub-humid cities, and 1.8 ha for humid cities. This work opens new avenues for quantifying cumulative cooling effect from a 3D perspective, which may also scientifically inform urban greenspace planning and climate adaptation strategies.