Thinking
On a hot day, the value of a tree can be immediate.
Its canopy interrupts direct sun. The ground beneath it is cooler than exposed paving. A footpath becomes walkable; a courtyard becomes a place to pause. The benefit is physical, spatial and easy to understand.
So a claim that greenery can sometimes coincide with greater heat stress sounds wrong—or worse, like an argument against trees.
It is neither.
New research is showing that urban vegetation affects more than temperature. Plants provide shade and cool through transpiration, but transpiration also adds moisture to the air. In a humid place with limited air movement, lower air or surface temperature does not always translate into lower physiological heat stress.
The design question is not whether cities need trees. They do. It is whether we are measuring and arranging greenery in ways that reflect how people actually experience heat.
Cooling has more than one mechanism
Trees influence outdoor conditions in several ways.
Canopies block solar radiation before it reaches people and hard surfaces. This reduces radiant exposure and the heat stored by paving and façades. Leaves also release water vapour through transpiration. The phase change consumes energy and can cool leaves and surrounding air.
But the human body also relies on evaporation. Sweat must evaporate from the skin to remove heat. When the air is already humid, that process becomes less effective.
This creates a trade-off that temperature alone cannot describe. Vegetation may reduce surface or air temperature while adding enough moisture to change a humidity-sensitive heat index. Wind and radiant temperature complicate the picture further.
That is why the metric matters. Land-surface temperature, air temperature, Heat Index, wet-bulb temperature, Physiological Equivalent Temperature and Universal Thermal Climate Index are related, but not interchangeable. Some include humidity; some include wind and radiation more fully; satellite measures may not describe conditions at pedestrian level.
“Cooler” needs a named measure.
What researchers found across 138 Indian cities
In May 2026, researchers published an analysis of 138 Indian cities across tropical, semi-arid and humid climates. The study reconstructed Heat Index at one-kilometre resolution between 2003 and 2020 and used an explainable modelling framework to examine vegetation and urban form.
It separated vegetation structure from vegetation function.
Structure included indicators such as greenness and leaf area. Function included the fraction of photosynthetically active radiation absorbed by vegetation, associated with biological activity including transpiration.
The relationships varied by climate and urban condition. Denser vegetation structure was associated with lower model-predicted Heat Index after particular thresholds. Higher canopy activity, however, was associated with higher Heat Index in some conditions, with the reversal appearing earlier in humid, dense urban cores.
This is a statistical and model-based result at city scale. It does not show that planting a particular tree causes a hotter experience at a particular corner. Heat Index also does not directly account for local wind or solar radiation—the very conditions that make a shaded position beneath a canopy feel different from open paving.
The study’s value lies in the distinction: more vegetation activity and more vegetation structure are not thermally identical, and their effects change with background climate and urban form.
The shape of the city changes the result
A second study, published in August 2026, examined 251 rapidly developing Chinese cities. The researchers classified background climates, constructed a three-dimensional urban-form index and assessed how vegetation transpiration affected wet-bulb temperature.
As conditions became hotter and more humid, transpiration was increasingly likely to have a warming effect on wet-bulb temperature. In humid-heat cities, the authors attributed 63.8% of the upward trend in the number of cities showing that effect to urbanisation.
Under compact high-rise forms, transpiration raised wet-bulb temperature mainly through increased humidity; the cooling pathway was not statistically significant. For the same increase in transpiration, the study reported population-weighted wet-bulb exposure intensity 23 times that of low-rise dispersed forms.
That 23-times result is not a multiplier for a Brisbane courtyard. It belongs to the study’s cities, model and exposure definition. It nevertheless makes the interaction between planting and three-dimensional form difficult to ignore.
Tall buildings are not inherently incompatible with trees. Compactness can provide shade and support walkability. But closely spaced forms can also restrict air exchange. The same moisture source will behave differently in an open, breezy setting and a sheltered canyon.
This is not a case against trees
The wider evidence remains clear: urban trees can deliver substantial cooling, particularly through shade.
A 2024 meta-analysis synthesised 182 studies across 110 cities or regions and 17 climate types. It found broad pedestrian-level cooling benefits, with effectiveness shaped by background climate, urban morphology and tree traits. The authors also noted cases of reduced cooling or warming associated with stagnant air, long-wave radiation trapping and physiological changes such as stomatal closure.
The conclusion is not “less green”. It is “more specific”.
Trees also provide benefits outside thermal comfort: habitat, stormwater interception, air-quality effects, amenity, identity and psychological value. Removing them because one heat metric worsened could create a poorer place and discard the radiant-shade benefit that the metric did not fully capture.
Equally, meeting a canopy target does not prove that a space is thermally safe. A percentage of cover is an input. Comfort and exposure are outcomes.
Shade and ventilation are not opponents
The architectural challenge is to create shade without sealing the space from useful air movement.
That may mean coordinating canopy height and density with street orientation, building spacing and prevailing breezes. It may mean preserving low-level openings in a courtyard rather than filling every edge. It may mean locating dense planting where it intercepts afternoon sun while keeping routes for air exchange. Species, soil volume, irrigation and canopy development all affect the result over time.
Hardscape still matters. A tree surrounded by dark, heat-storing surfaces is working in a different energy environment from one combined with lighter, permeable and lower-heat-capacity materials. Deep shade from a structure can complement living canopy where soil or water limits tree growth.
Water must be considered too. Irrigation can sustain transpiration and plant health during heat, but in a humid microclimate it also participates in the moisture balance. The right response is not to deprive trees of water. It is to understand where the water, vapour and heat move.
Design for the person at 1.5 metres
Urban heat is often mapped from above. People experience it close to the ground.
A public-realm assessment should therefore look at the hours and routes that matter: the school gate in the afternoon, the hospital entrance used by people with limited mobility, the queue outside a sports venue, the civic square during an event, the worker walking from public transport and the shaded seat used at midday.
Useful questions include:
Where does direct sun reach bodies and surfaces through the day?
What is the mean radiant temperature at the places people wait?
Can air move through the occupied zone, not only above the canopy?
How does humidity change during the hottest periods?
Are drinking water, rest and cooler refuges available?
Who must use the space, and who can choose to leave?
Measurements and models should include the relevant combination of air temperature, humidity, wind and radiation. Post-occupancy monitoring can then test whether the designed microclimate exists after the planting matures and the buildings are operating.
Beyond the public park
The issue reaches into many project types.
Commercial and workplace projects increasingly use planted atria, terraces and courtyards as amenity. If these spaces are sheltered and humid, planting alone may not make them usable at lunch. Healthcare landscapes need shade and rest opportunities for people with different thermal sensitivity, while routes must remain direct and accessible. Sports precincts need to consider crowds, radiant loads from large paved areas and the timing of events. Schools and civic projects must account for people who cannot simply change their schedule.
In every case, the question is performance rather than appearance. A space can photograph as green while remaining uncomfortable at the moment it is most needed.
Greener, cooler and more breathable
The choice is not between trees and comfort.
It is between treating greenery as a countable object and designing it as part of a microclimate system.
Trees should still shade streets, frame public life and support urban ecologies. But their thermal performance depends on the atmosphere around them: the humidity already present, the surfaces beneath them, the buildings beside them and the air that can move through.
More greenery is a valuable ambition. Less heat stress is a separate outcome.
Good design refuses to assume they are automatically the same—and works to achieve both.