By Abbas Nazil
A new global study has found that urban vegetation, often promoted as a cooling solution, can worsen the urban heat island effect in very dry cities under certain climatic conditions.
The research highlights that while trees and green spaces generally help cool cities, their impact is not universally beneficial and depends strongly on local levels of aridity and extreme heat.
Urban heat islands occur when cities become significantly warmer than surrounding rural areas due to heat-absorbing building materials, reduced airflow and human activities.
As global temperatures rise, this phenomenon is making life increasingly difficult for millions of urban residents, particularly during prolonged heat waves.
In response, city planners around the world have turned to nature-based solutions such as planting trees, expanding grasslands and introducing urban agriculture to reduce surface temperatures.
However, the new study, published in the journal Science Advances, suggests that this strategy can backfire in cities with very low rainfall.
Researchers analysed high-resolution satellite imagery and climate data from 761 major cities across 105 countries to assess how different types of urban vegetation affect surface temperatures.
Using machine learning models, they compared areas fully covered by vegetation with areas dominated by built-up surfaces.
The analysis differentiated between grasslands, croplands and trees, while accounting for factors such as surface reflectivity, heat storage and evapotranspiration.
Globally, vegetation was found to provide cooling benefits in most cities, with urban trees showing the strongest and most consistent cooling effect.
Urban grasslands cooled cities in 78 per cent of cases, while trees did so in 98 per cent of cities examined.
However, the pattern changed sharply in arid regions with annual rainfall below about 1,000 millimetres.
In these dry environments, grasslands and croplands produced higher surface temperatures than built-up areas in up to 22 per cent of cities.
Even trees, which performed better overall, caused net warming in a small number of arid cities.
The researchers also found that during periods of extreme heat, vegetation became far less effective at cooling.
Trees failed to provide cooling in about a quarter of cities experiencing exceptionally hot summers, while grasslands and croplands failed in most cases.
The study explains that in arid climates, vegetation reflects less sunlight than built surfaces, leading to greater absorption of solar radiation.
At the same time, limited water availability reduces evapotranspiration, weakening one of vegetation’s key cooling mechanisms.
Lower heat storage in vegetated areas also allows more energy to be released back into the atmosphere, further increasing surface temperatures.
These effects can outweigh the benefits of shade and airflow in very dry environments.
Despite these findings, the researchers stress that vegetation remains an effective cooling strategy in many parts of the world.
They argue that urban greening should not be abandoned but carefully tailored to local climate conditions.
In arid cities, planners are advised to combine vegetation with other measures such as reflective materials, improved urban design and the use of water-efficient plant species.
The study remarks that poorly planned greening efforts in dry cities could unintentionally intensify urban heat, underscoring the need for climate-sensitive urban planning.
