Dense canopies reverse the cooling effect of urban greening in humid cities.

Urban greening is widely promoted as an adaptation strategy to cope with rising heat stress; however, its effectiveness remains uneven, especially across rapidly warming cities of the Global South. Vegetation can cool urban environments but can also intensify humidity and thermal discomfort, raising questions about when green adaptation succeeds or backfires. Here we assess how vegetation structure and function influence the Heat Index (HI), a humidity-adjusted measure of perceived heat, across 138 Indian cities spanning diverse climates. Using high-resolution HI estimates and an explainable modeling framework, we find climate-dependent associations in which lower model-predicted HI corresponds to denser vegetation structure (EVI ≥ 0.4, LAI ≥ 0.05), whereas higher canopy activity (fPAR ≥ 0.5) is associated with higher HI, with earlier onset in humid urban cores. These findings indicate that green adaptation can shift from a cooling asset to a humid-heat liability under some conditions, underscoring the need for climate-responsive and equitable design strategies in the Global South.