The value of small urban green spaces in mitigating urban heat: a fine-grained nationwide analysis

Abstract

Urban green spaces (UGS) are increasingly recognized as vital nature-based solutions for mitigating urban heat, particularly against a background of accelerating urbanization, anthropogenic warming, and intensifying heatwaves. However, small UGS (<1 ha) have long been undervalued in cooling strategies due to their fragmented distribution and presumed limited cooling capacity. Here, we conducted a comprehensive assessment of the multi-dimensional cooling effects of 60,064 UGS—ranging from 0.1 ha to 200 ha— across 36 major Chinese cities during the summer of 2020. The key findings are as follows: (1) Although small UGS comprise only 11.31 % ± 4.78 % of the total UGS area, they dominate in patch number (71.97 % ± 8.29 %) and contribute disproportionally to multi cooling effects, including cooling area (50.62 % ± 10.18 %), cooling intensity (55.36 % ± 9.8 %), cumulative cooling (64.96 % ± 8.85 %), cooling gradient (60.96 % ± 9.52 %), cooling area efficiency (94.42 % ± 1.99 %), and cooling intensity efficiency (94.91 % ± 1.95 %). (2) The cooling performance of small UGS is predominantly influenced by anthropogenic factors, with higher built-up and road areas enhancing their localized “urban oasis” effect. (3) The threshold values of cooling intensity efficiency (0.49–0.63 ha) and cooling area efficiency (12.38–19.90 ha) increase progressively from medium cities to megacities, correlating significantly correlations with urbanization factors. Meanwhile, these thresholds are a 6–24 % lower than those estimated without considering small UGS, suggesting the existence of diseconomy of greening for heat mitigation by ignoring small UGS. (4) Finally, six distinct clusters of cooling service bundles were identified, each characterized by distinct landscape characteristics, providing a framework for more tailored UGS planning and management strategies in diverse urban settings. These findings highlight the crucial role of small UGS in urban thermal mitigation and provide actionable ecological knowledge to enhance urban cooling efficiency towards sustainable cities and society.