For decades, we have seen urban greenery as a universal remedy for the urban heat island phenomenon and global warming. A new large-scale study published in Science Advances, which analyzed 761 megacities in 105 countries, however, this assumption fundamentally questions. Researchers have uncovered a surprising paradox: under certain conditions, vegetation can actually increase temperatures in cities instead of reducing them.

Where does the cooling effect end?

A key factor that determines whether trees and grass cool us down is the level of humidity in the environment. The study identified the empirical limit of 1000 millimeters of annual precipitation. In cities located below this threshold – that is, in arid (dry) and semi-arid areas – urban vegetation exhibits a significantly weaker cooling effect or even net warming.

According to scientists' findings, vegetation, especially grasslands and farmland, net warming in up to 22 % locations studied. Even urban trees, which are generally more effective at regulating temperature, lose their cooling capacity in 2 % cities located in dry zones.

Biophysical mechanism of warming

Why do plants fail in drought? The answer lies in the balance between three factors: albedo (reflectivity), evapotranspiration (water evaporation) and heat storage.

1. Lower albedo: Vegetation tends to absorb more sunlight than lighter built-up areas because it has a lower albedo.
2. Limited evaporation: In arid regions where water is scarce, plants limit evapotranspiration to survive.
3. Radiation dominance: When cooling by evaporation (evapotranspiration) is not enough to compensate for the absorbed solar energy, the result is an increase in surface temperature.

Additionally, vegetation has been shown to store less heat than buildings, which in dry conditions means that more energy is immediately released back to the surface, contributing to warming.

Trees as the last "warriors" in the heat

During extreme summer heat, the differences between types of greenery become even more pronounced. Urban trees can mitigate temperature rise in about 75 % of the world's cities. They owe this to their deep roots, which allow them to access groundwater even in times of drought, and to the roughness of their crowns, which facilitates heat exchange with the atmosphere.

On the contrary, grass and urban fields fail in 71 % to 82 % locations During extreme heat, their shallow roots dry out quickly, leading to a cessation of cooling and subsequent warming of the surrounding environment.

The Future of Urbanism: Intelligent and Adaptive Solutions

These findings suggest that planting greenery is not a one-size-fits-all solution for every city. In dry, water-scarce areas, massive grass planting may even be counterproductive.

The study authors therefore propose climate adaptive strategies:

• In dry cities, they can be materials with high albedo (e.g. "cool" light roofs) a more effective tool for combating heat than watering-intensive greenery.
• Humid regions should continue to maximize the planting of trees that provide the greatest cooling and ecological benefits there.
• Economic maturity also plays an important role: richer cities (e.g. Phoenix in the USA) are able to investments in irrigation maintain the cooling effect of greenery even in desert conditions, which is often unattainable for developing countries.

Green cities of the future will need to combine natural elements with technologies to respect local climate limits and responsibly manage scarce water resources.

An analogy for better understanding: Imagine a plant on a hot day like a person wearing a dark T-shirt. If a person can sweat (evapotranspiration), the evaporation of sweat cools them down even in dark clothing. However, if they have nothing to drink (drought in arid regions), they stop sweating. The dark T-shirt (low albedo plant) then simply absorbs the sun's heat, causing the person to overheat much faster than if they were standing on concrete in a white T-shirt. JRi

The study was published in the journal science.org