Tree shade is one of the fastest ways to make heat more bearable. It cuts direct sunlight, protects people walking or working outdoors, and remains essential for Heat Action Plans. A new study by researchers from the Indian Institute of Technology Gandhinagar (IITGN) adds a sharper planning question: if greening is so important, why does the same strategy cool some urban areas more reliably than others?
The study, published in Nature Communications , analysed 138 Indian cities from 2003 to 2020 across tropical savanna, semi-arid steppe, and humid subtropical climates. Instead of relying only on land surface temperature, the team reconstructed the Heat Index, which combines temperature and humidity and is closer to how heat is felt by the human body. The researchers used extreme-aware downscaling of temperature and humidity to build one-kilometre Heat Index maps that better capture dangerous high-heat conditions.
The analysis brought together multiple satellite and urban datasets. Vegetation was represented through Enhanced Vegetation Index (EVI), Leaf Area Index (LAI), and Fraction of Absorbed Photosynthetically Active Radiation (fPAR), which respectively capture greenness, leaf-area density, and physiological activity. Night-time lights were used as a proxy for built intensity, while Local Climate Zones helped describe urban form. The team then used explainable AI methods to identify which features pushed the Heat Index up or down and where critical thresholds appeared.
"Greening is essential for climate adaptation, and shade gives people immediate relief," said Professor Udit Bhatia, corresponding author of the study and Associate Professor at IITGN. "Our results show that one-size-fits-all plantation targets miss part of the problem. Cities need greening strategies that are designed for shade, moisture, and ventilation together."
The key finding is that green cover is not a single variable. Canopy structure and canopy activity can affect humid heat differently. Across the 138 cities, the researchers found that vegetation cover and canopy structure were associated with lower model-predicted Heat Index once EVI reached about 0.4 and LAI about 0.05. But high canopy activity, especially fPAR around 0.5 and above, was associated with higher Heat Index in some settings, with the warming signal appearing earlier in humid, dense urban cores.
The reason is physical and intuitive. Trees cool through shade and evapotranspiration. In dry air, evapotranspiration can be strongly beneficial because the released moisture evaporates into air that can still absorb it. In humid and compact neighbourhoods, however, extra moisture can remain trapped near the ground. The result can be a higher Heat Index even though shade continues to provide immediate relief at the street level.
Lead author, Dr Angana Borah, Research Graduate, at IITGN's Department of Civil Engineering, said the findings point to a more practical approach to urban greening. "The question is not whether cities should green. They should. The question is what kind of green, where, and how much," she said. "In dry cities, vegetation can provide strong cooling benefits. In humid and compact neighbourhoods, planners also need to think about airflow and moisture build-up."
For Indian cities, the takeaway is not to slow greening, but to make it climate-responsive. Shade trees, parks, roadside planting, open spaces, and ventilation corridors need to be planned as one system. In humid and dense neighbourhoods, species choice, canopy spacing, pruning, irrigation, and street geometry may all matter because they influence how much shade is created and how quickly moisture can disperse.
This has strong equity implications. The people most exposed to dangerous heat often live or work in dense, poorly ventilated neighbourhoods and have limited access to cooling. Better planned greening can help these communities by improving shade today while reducing long-term humid-heat stress more effectively.
The authors caution that the study works at a one-kilometre city-scale resolution and does not produce a tree-by-tree or species-by-species rulebook yet. Street design, building geometry, irrigation, pruning, and local species traits all matter. "By making model outputs interpretable, explainable AI tools like those used in this study are making complex climate-urban interactions easier for planners and policymakers to understand and act on," added Professor Bhatia. The approach echoes India's commitments at the recently concluded India AI Impact Summit 2026 to deploy AI for the Welfare for All, including AI for Social Good. The next step is to connect these city-scale patterns with finer street-level and plant-level data so planners can make more precise greening decisions.
The central message is constructive: trees remain a must-have for heat adaptation, but cities need more than green-cover targets. Under a hotter and more humid future, greening will work best when it provides shade, manages moisture, and leaves space for air to move.
Research team:
Dr Angana Borah, Research Graduate, Department of Civil Engineering, Indian Institute of Technology Gandhinagar
Ms Adrija Datta, PhD Scholar, Department of Earth Sciences, Indian Institute of Technology Gandhinagar
Mr Ashish S Kumar, Department of Civil Engineering, Indian Institute of Technology Gandhinagar
Dr Raviraj Dave, Department of Civil Engineering, Indian Institute of Technology Gandhinagar, and Sustainability and Data Sciences Laboratory, Northeastern University, United States of America
Professor Udit Bhatia, Department of Civil Engineering, Department of Earth Sciences, and Department of Computer Science and Engineering, Indian Institute of Technology Gandhinagar
Paper: Dense canopies reverse the cooling effect of urban greening in humid cities .
