Physiologically Active Vegetation Reverses Its Cooling Effect in Humid Urban Climates

TL;DR

This study investigates how physiologically active vegetation can switch from cooling to warming effects in humid urban climates, providing thresholds for effective greening strategies to enhance city heat resilience.

Contribution

It quantifies vegetation-climate interactions using machine learning, revealing the conditions under which vegetation's cooling effect reverses in humid urban environments.

Findings

Vegetation cooling strengthens at certain EVI and LAI thresholds.

High physiological activity can reverse vegetation cooling, increasing heat stress.

Quantitative thresholds guide climate-specific urban greening strategies.

Abstract

Efforts to green cities for cooling are succeeding unevenly because the same vegetation that cools surfaces can also intensify how hot the air feels. Previous studies have identified humid heat as a growing urban hazard, yet how physiologically active vegetation governs this trade-off between cooling and moisture accumulation remains poorly understood, leaving mitigation policy and design largely unguided. Here we quantify how vegetation structure and function influence the Heat Index (HI), a combined measure of temperature and humidity in 138 Indian cities spanning tropical savanna, semi-arid steppe, and humid subtropical climates, and across dense urban cores and semi-urban rings. Using an extreme-aware, one kilometre reconstruction of HI and an interpretable machine-learning framework that integrates SHapley Additive Explanations (SHAP) and Accumulated Local Effects (ALE), we isolate vegetation-climate interactions. Cooling generally strengthens for EVI >= 0.4 and LAI >= 0.05, but joint-high regimes begin to reverse toward warming when EVI >= 0.5, LAI >= 0.2, and fPAR >= 0.5,with an earlier onset for fPAR >= 0.25 in humid, dense cores. In such environments, highly physiologically active vegetation elevates near-surface humidity faster than it removes heat, reversing its cooling effect and amplifying perceived heat stress. These findings establish the climatic limits of vegetation-driven cooling and provide quantitative thresholds for climate-specific greening strategies that promote equitable and heat-resilient cities.