Tree shade remains one of the quickest and most practical ways cities can blunt extreme heat. It is relatively inexpensive, it takes effect straight away, and it can make streets noticeably more tolerable for people who have to spend time outdoors.
However, new research suggests that blanket tree-planting goals - the kind that simply aim to boost overall green cover - can end up overlooking what matters most.
In some situations, urban greening schemes may even make humid heat feel more oppressive.
The study was carried out by researchers at the Indian Institute of Technology Gandhinagar (IITGN).
To do this, the team examined 138 Indian cities over the period from 2003 to 2020, covering tropical savanna, semi-arid steppe, and humid subtropical climate zones.
How the body feels heat
A great deal of urban-heat research concentrates on land surface temperature - essentially, how hot the ground becomes.
Here, the researchers took another route by reconstructing the Heat Index, which combines air temperature and humidity in order to reflect how heat is actually felt by the human body.
That distinction matters: in high humidity, 35°C can feel like 45°C, and land surface temperature on its own does not capture that.
Factors that influence heat
Across all 138 cities, the researchers produced Heat Index maps at a one-kilometre resolution.
They drew on satellite observations to quantify vegetation in three separate ways: overall greenness, leaf density, and the level of physiological activity in the canopy.
They also used night-time light data as a stand-in for urban density, then applied explainable AI techniques to determine which variables drove the Heat Index up or down - and to pinpoint where critical thresholds appeared.
City trees lower heat through two main pathways: shade and evapotranspiration. Shade is the simpler one to understand - foliage blocks direct sunlight so the ground, and the people beneath the canopy, stay cooler.
Evapotranspiration, by contrast, is the release of moisture from trees into the air; in dry conditions that moisture evaporates readily, carrying heat away.
Moisture that gets trapped
The catch is that the value of evapotranspiration is highly dependent on local conditions. When the air is dry, the added moisture evaporates efficiently and can deliver substantial cooling.
But in humid, tightly built neighbourhoods, the same added moisture may linger close to street level instead of dispersing.
In those settings, air can remain warm and heavy. As a result, the Heat Index can rise even when shade is offering immediate, surface-level relief.
The team observed this pattern across the dataset. Once greenness and leaf density passed certain thresholds, vegetation cover and canopy structure tended to align with a lower Heat Index.
Yet high canopy physiological activity - trees working intensely and releasing larger amounts of moisture - was linked to a higher Heat Index in some contexts, especially in humid and compact urban areas.
“The question is not whether cities should green. They should. The question is what kind of green, where, and how much,” said study lead author Angara Borah from IITGN.
“In dry cities, vegetation can provide strong cooling benefits. In humid and compact neighborhoods, planners also need to think about airflow and moisture build-up.”
One-size-fits-all targets don’t work
A central message of the paper is that treating green cover as a single figure to maximise is too blunt an approach.
A city might technically meet a tree-planting target by adding dense, high-moisture canopy in already humid neighbourhoods - only to find that residents do not feel cooler, or are, in terms of perceived heat, worse off than they were previously.
“Greening is essential for climate adaptation, and shade gives people immediate relief,” said senior author Udit Bhatia.
“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.”
In practice, this means treating shade trees, parks, roadside planting, open spaces, and ventilation corridors as parts of one connected system, rather than as separate interventions.
In dense and humid neighbourhoods, the specifics become important: the species selected, the spacing of the canopy, pruning practices, and the way streets are laid out to enable air movement.
All of these factors affect how much moisture builds up and how quickly it can disperse.
Strategies to help vulnerable people
There is also a clear equity dimension that needs to be stated directly.
In Indian cities - as in cities worldwide - those at greatest risk from dangerous heat are often people who live and work in densely packed areas with poor ventilation and limited access to air conditioning or other cooling options.
These are precisely the places where poorly designed greening may provide less benefit than expected, or could even increase heat stress.
Getting greening right in these neighbourhoods is not a minor detail.
It is the difference between a measure that genuinely supports the most vulnerable people and one that looks impressive on a policy dashboard while failing to reach them.
A framework for city greening
The researchers emphasise that their analysis operates at a one-kilometre, city-scale resolution.
At this stage, the work does not yet offer guidance at the scale of individual streets, specific species choices, or detailed planting arrangements.
Reaching that level requires linking city-scale patterns to finer, street-level and plant-level datasets - which the team highlights as the next logical step.
What the study does offer, though, is a way of thinking about urban greening that is far more useful than relying on a single coverage target.
Cities in a hotter future
In a future that is both hotter and more humid, the cities that reduce felt heat most effectively are likely to be those that design green infrastructure with care.
Shade, moisture management, and airflow need to be planned to work in tandem rather than treated as isolated fixes.
And those choices have to be tailored to each neighbourhood’s climate and built form.
Cities do need more trees - but they also need the right trees in the right places, designed to function together instead of being added piecemeal.
That goal is harder to define than a percentage of green cover, but for people trying to get through summer in dense urban neighbourhoods, it is far more consequential.
The study was published in the journal Nature Communications.
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