Phenological Metrics in the Mitigation of Urban Heat: Timing and Thresholds

Urban vegetation is critical for mitigating summer heat, but previous studies have largely relied on static greenness metrics, leaving a gap in understanding of how vegetation phenology (its seasonal life cycle) regulates urban temperatures on a global scale. Here, we utilize interpretable machine learning and satellite data to quantify the influence of key phenological metrics, encompassing growth intensity (e.g., peak greenness), timing (e.g., start of season), and duration, on summer Land Surface Temperature (LST) across 24 major global cities. We found that: 1) a significant temporal mismatch exists in over 80% of cities, with vegetation green-up lagging seasonal surface warming by 50–100 days, creating a window of thermal vulnerability; 2) seasonal accumulation of Enhanced Vegetation Index (EVI) provides stable, linear cooling, whereas Maximum EVI (MEV) and EVI amplitude (EA) exhibit a distinct threshold effect, with their cooling benefits diminishing or even reversing beyond a critical point; 3) vegetation's cooling effect changes with context, delivering roughly 25% greater cooling in the top 10% of temperature extremes compared to moderate conditions; and 4) in certain contexts, vegetation's cooling effect is observationally weakened or even offset when it is masked by the dominant influence of a positively correlated warming factor, such as high elevation. These findings provide mechanistic evidence that simply increasing green cover is insufficient; future urban heat mitigation must shift to "phenology-aware" designs that synchronize vegetation's life cycle with seasonal heat peaks to achieve maximum cooling benefits.