The latest research published in the journal Nature shows that trees in cities reduce the urban heat island effect by about half on average, lowering air temperature by as much as 0.5 to 1.5 degrees Celsius. Scientists warn, however, that even ambitious planting programmes will neutralise only about 10 to 20 percent of the additional temperature increase expected by mid‑century.
The urban heat island phenomenon, abbreviated UHI from the English Urban Heat Island, is one of the best documented consequences of transforming natural ecosystems into densely built urban areas, because concrete, asphalt, brick and other construction materials have high heat capacity and low albedo, meaning their ability to reflect solar radiation. In practice this means that they absorb solar energy throughout the day, store it and then release it gradually at night, with the result that temperatures in a city are on average 1 to 3 degrees Celsius higher than in the surrounding rural areas, and in extreme cases the difference can reach as much as 7 degrees Celsius.
This problem is not merely a matter of thermal comfort, because during heatwaves the elevated temperatures, particularly when they persist at night and do not allow the body to recover, lead to an increased risk of dehydration, heat exhaustion, heatstroke, exacerbation of cardiovascular and respiratory diseases, and consequently to a rise in premature deaths. According to data from the World Health Organisation, heatwaves are already among the deadliest consequences of climate change in temperate and Mediterranean regions, and demographic and climate projections further compound the severity of the problem.
The United Nations estimates that by 2050 nearly 70 percent of the world’s population will live in cities, and at the same time all climate change scenarios predict an increase in the frequency, intensity and duration of heatwaves in all inhabited regions of the world, including Central Europe and Poland. In this situation, the search for effective, cheap and natural methods of cooling cities has become a priority for local governments, urban planners and public decision‑makers around the world.
The groundbreaking nature of the research published in Nature stems from the methodology used, because most previous analyses of the cooling role of urban greenery were based on satellite data measuring land surface temperature, abbreviated as LST. This method has a fundamental disadvantage from a human perspective, because the surface temperature of asphalt, concrete or roofs can be much higher than the air temperature in which people live, and on a hot, sunny day black asphalt can heat up to over 60 degrees Celsius, while the air temperature at a height of 1.5 to 2 metres above the ground, that is in the breathing and feeling zone of a person, may be 35 to 40 degrees Celsius in the same place. Relying exclusively on satellite data therefore led to an overestimation of the actual temperature felt by residents, and thus to an overestimation of the cooling effect of trees in some analyses, which is why the scientists behind the Nature study decided to investigate air temperature rather than surface temperature.
They used global air temperature data from ground‑based measuring stations and satellite data on tree cover, and then created a counterfactual model, that is a world without trees, which they compared with real conditions. The analysis covered nearly 9,000 cities worldwide, home to a total of about 3.6 billion people, made possible by combining vast datasets of climate, remote sensing and demographic information that have only become available at the appropriate spatial and temporal resolution in recent years.
The physiological mechanisms of cooling by trees and their effectiveness in different climates
Trees affect ambient temperature in two fundamentally different yet complementary ways, the first of which is shading, because tree canopies block direct solar radiation, preventing surfaces such as pavements, roads, squares, building facades and the people themselves from heating up. The shading effect is immediate and noticeable, because moving from a sunlit square to under a tree canopy causes a drop in perceived temperature of as much as 5 to 10 degrees Celsius depending on the time of day and humidity.
The second, even more important mechanism in the long term is transpiration, the process of releasing water vapour through the stomata on leaves, because plants take up water from the soil through their root systems, transport it through the xylem, that is the wood, to the leaves, and then release it into the atmosphere as water vapour. This process requires a supply of heat energy, which is taken from the surroundings, resulting in a local lowering of air temperature, and the mechanism is analogous to the cooling of the human body through sweating.
At the scale of an entire city, transpiration from a large number of trees can reduce near‑surface air temperature by more than 1 degree Celsius, even in the absence of direct shading. The results of the analysis surprised even the researchers themselves, because it turned out that trees reduce the urban heat island effect by on average almost half, and in absolute numbers this means that in many cities they lower air temperature by about 0.5 to 1.5 degrees Celsius.
The magnitude of cooling, however, is strongly geographically diverse and depends on three main factors, namely climate, city structure and tree species. In hot and dry regions, such as the south‑western United States with the cities of Phoenix and Las Vegas, the Middle East with Dubai and Riyadh, or Australia with Perth and Adelaide, the temperature differences between districts with a lot of greenery and those almost devoid of trees are greatest and can reach as much as 3 to 4 degrees Celsius. This is because in a dry climate transpiration is particularly effective, as evaporation occurs faster, and the lack of natural greenery around the city intensifies the heat island.
Inequalities in access to greenery as a climate dimension of social justice
One of the key and often overlooked conclusions from the research is that the benefits of the cooling role of trees are not distributed evenly across urban space, because most greenery is usually found in wealthier districts, on the outskirts of cities and in villa areas with single‑family housing. Poorer areas, often more densely built, with a higher rate of impervious surface coverage, a greater share of multi‑family housing and a smaller area of green space, have significantly fewer trees. In the United States, which has the most detailed data on this issue, lower‑income neighbourhoods have on average about 15 percent fewer trees than wealthier parts of the same cities, and afternoon temperatures there are on average about 1.5 degrees Celsius higher.
This phenomenon, described in the English‑language literature as climate apartheid or heat inequality, creates a vicious circle, because lower‑income people often live in lower‑quality buildings, have less access to air conditioning or cannot afford its expensive operation, work outdoors more often during peak sunlight hours in occupations such as construction, municipal services or delivery, and at the same time have the least access to the natural cooling provided by trees.
These are therefore the same people who are most exposed to the health effects of heatwaves and at the same time least protected by green infrastructure, which is why the fight against the urban heat island cannot be reduced to simply increasing the number of trees across the whole city. It is necessary to take into account the issue of spatial justice and direct planting primarily to the most deprived and most heat‑exposed neighbourhoods, and the researchers estimated that for more than 200 million people worldwide urban trees lower the local temperature by at least 0.5 degrees Celsius.
Limits of a tree‑only strategy in the perspective of mid‑century
Despite the impressive results, the Nature study also brings an important warning for policy‑makers and urban planners who in recent years have often presented tree planting as a universal, cheap and uncontroversial solution to the problem of urban heat, because the scientists estimated that current urban trees, meaning those already existing, are capable of neutralising only about 10 percent of the additional temperature increase expected by mid‑century. This applies to moderate climate change scenarios, that is assuming that greenhouse gas emissions will be significantly reduced, and even with ambitious tree planting programmes assuming an increase in tree cover of 50 to 100 percent at the city scale, this rate would rise to at most about 20 percent.
This means that over a 25‑year horizon, regardless of how many trees we plant, the temperature increase caused by climate change will vastly exceed the ability of trees to compensate for it, in other words trees are essential but absolutely insufficient. Continuing to rely solely on greenery as the main heat adaptation strategy is not only naive, but can be downright dangerous, because it creates the illusion that the problem has been solved while in reality it is growing. Experts in urban planning and climate adaptation agree that trees should be treated as one element in a broader portfolio of measures, not as the only solution, because the design of public space and buildings with a view to limiting heat accumulation is of key importance.
The materials used to build ground surfaces matter greatly, because light, highly reflective surfaces with high albedo, such as light concrete, light paving stones or solar‑reflective paints on roofs, the so‑called cool roofs, heat up much less than dark asphalt. It is estimated that replacing dark asphalt with light concrete on car parks, pavements and local roads can lower air temperature at the district scale by 0.5 to 1 degree Celsius, and programmes of such cool roofs and cool pavements are already being implemented in many US cities, such as New York, Los Angeles and Phoenix, and in Europe, in Milan, Athens and Seville.
Complementary city cooling strategies from green roofs to ventilation corridors
Green roofs and green walls, that is vertical gardens, complement trees in places where ground‑level planting is impossible, for example in dense, compact city centres, because green roofs not only thermally insulate buildings, reducing the demand for air conditioning, but also lower ambient temperature through plant transpiration and retain rainwater.
At the scale of an entire district, green roofs can lower air temperature by 0.3 to 0.5 degrees Celsius, which combined with other solutions gives a tangible effect. Appropriate shaping of street space by designing ventilation corridors that allow cool air to flow from the suburbs and green areas into the city centre is crucial, because narrow, deep, east‑west oriented street canyons are most prone to overheating, while wider streets oriented in line with prevailing winds can effectively remove hot air. In some cities, such as Stuttgart or Vienna, spatial plans already incorporate such ventilation corridor systems as an integral part of adaptation policy.
Reducing the sealing of ground surfaces by creating pocket parks, that is small squares with vegetation, unsealing surfaces by replacing concrete and asphalt with permeable gravel or grass surfaces, and protecting existing green spaces from development are actions of proven effectiveness. From the research published in Nature and the wider literature on climate change adaptation in cities, concrete recommendations can be drawn for local and regional authorities, because tree planting should be carried out in a targeted and equitable manner, not randomly.
Priority should be given to districts with the highest population density, the highest share of sealed surfaces, that is asphalt and concrete, and the lowest income of residents, that is where temperatures are highest and residents are most exposed to the effects of heatwaves. Thermal maps of cities, created on the basis of satellite data and ground‑based measuring stations, should form the basis for designating priority intervention areas, and planting programmes should go hand in hand with programmes to replace surfaces with light ones, with green roof programmes on public and commercial buildings, with the creation of ventilation corridors and with the protection of existing greenery from felling for new developments.
Monitoring effectiveness and the need for systemic action beyond urban greenery
Success measures should take into account the cooling effect, not just the number of trees planted, because many cities boast about the number of new plantings while not monitoring how many trees die from drought, disease, pests or felling for developments, nor what the actual cooling effect of those trees is after 10 to 20 years of growth. Systems are needed to monitor tree cover and air temperature at neighbourhood resolution to evaluate the effectiveness of actions taken, and it is also necessary to embed urban greenery policy in the broader context of climate change mitigation.
Tree planting, although insufficient to completely offset temperature increases, brings many additional benefits, such as carbon sequestration, air pollution reduction, reduced stormwater runoff, increased biodiversity and improved mental health of residents. Even if it does not stop temperature rise, it is still an extremely valuable action for many other reasons, but it cannot be a pretext for abandoning the deep, systemic changes in the economy and transport that are necessary to stop the very phenomenon of global warming.
The latest research unequivocally confirms that trees are an extremely effective and cheap tool for cooling cities, capable of lowering local air temperature by more than 1 degree Celsius and bringing relief to hundreds of millions of residents during heatwaves, and their role is especially valuable in the context of growing climate inequalities. The poorest people, living in the most densely built and least ventilated districts, are most exposed to the effects of heatwaves and at the same time have the least access to greenery, so directing new plantings precisely to these districts is not only a matter of efficiency but also of basic social justice.
At the same time, however, continuing to rely solely on trees as the main adaptation strategy is doomed to fail by the 2050 horizon, because with the projected temperature increase even moderate climate scenarios vastly exceed the physical ability of trees to compensate for that increase. It is therefore necessary to implement other solutions in parallel, such as light surfaces and cool roofs, green roofs and walls, ventilation corridors, surface unsealing and protection of existing greenery from development, because the cities of the future will have to be designed as systems in which trees interact with other technical and urban planning solutions to create a coherent, heat‑resilient urban fabric.
