Urban Forests for a Moderate Climate
Urban Forests for a Moderate Climate
Heat waves sweeping across the globe have bigger effect in cities. Buildings of concrete and steel trap hot air and block circulation, while dark asphalt streets and stone pavements retain heat and cover natural soil, wiping trees and plant cover. Car engines also contribute to rising temperatures, alongside air conditioners, which cool on the inside while expelling hot air outside. All this causes hot air to be retained and "thermal islands" to develop in specific spots within cities. The temperature in these areas may exceed 10 degrees of what it is outside the city limits, or even in open and green spaces inside the city itself.
The "thermal islands" phenomenon in cities is not new. Rather, it has become a constant fact since cities began to expand, with taller buildings and crowded streets. What is new is that the increasing heat waves caused by climate change are making the situation worse.
High temperatures above normal human tolerance levels cause serious health problems, especially for patients with heart disease, diabetes and respiratory system problems. A third of the world's population is currently exposed to heat above normal levels on about 20 days per year. It is projected that half of the world's population will be exposed to risk due to heat waves by the end of the century.
Many cities, from Paris and London in Europe to Washington and Seattle in the United States, have opted to tackle heat waves by planting trees. It is traditional knowledge that trees serve as the lungs of the earth, as they absorb carbon dioxide and emit oxygen. They also act as a filter that extracts sulfur dioxide and nitrogen dioxide, and captures fine particulates with pollutants attached to them. But trees also have a direct effect on reducing heat in streets and buildings, as they emit moisture into the air and their shade reduces direct sunlight on walls and windows. This reduces the need for air conditioning, which in turn cuts harmful emissions.
This is where the idea of planting urban forests and increasing tree planting on road sides originated, not only for visual landscaping, but mainly for environmental goals. In addition to afforestation in public urban areas, some municipalities, as in the Netherlands, have launched programs to encourage residents to establish home forests, whose size and type of trees are determined according to the size and location of the garden. Local municipalities support these programs by offering free technical advice, seedlings and tax incentives.
Because of lack of space, several cities encourage the planting of trees on rooftops, as in Geneva, where some roofs look like hanging forests. There are also programs for growing certain types of vegetation on rooftops to create thermal insulation.
However, planners have to recognize that trees are living entities, not statues of steel, concrete and stone, which need constant care and adequate conditions to thrive and survive. Trees, like every living entity, are subject to disease and death. Also, trees often require many years to reach full growth before they can effectively tackle high temperatures, with the maturity period of some tree species exceeding 20-30 years. Therefore, cities that have initiated urban forestry programs often implement fast-track afforestation projects, in order to obtain benefits before it is too late.
One of the important factors for the success of urban forests is the selection of plant species appropriate to the natural conditions in a specified location, including soil, water and climate. During the past decades, many types of alien trees have been planted in the streets of Arab cities, which are not compatible with the local conditions, as they need continuous irrigation with large quantities of water, and threaten the local plant species.
However, this practice has been largely abandoned, and plants imported from regions with different climatic conditions have been replaced by more suitable ones. This does not mean restricting to local plants, as some imported tree species may be compatible with the local environment and more effective in tolerating urban conditions, with more capacity to absorb pollutants and provide shade to reduce heat. This depends on the shape and quality of the leaves. Researchers in China have found, for example, that conifers, such as pine trees, are better for absorbing pollutants in cities, if natural conditions allow them to grow, while trees with bigger leaves provide more shade.
Some Arab cities, where it is possible to grow trees with little care, due to their moderate climate and rainfall, such as Beirut, Damascus and Amman, have lost a large part of the green cover due to random urban expansion. In fact, green spaces have diminished in almost all cities of the Levant and North Africa, which have a traditionally moderate climate, due to urban expansion. This threatens the quality of life there, in addition to posing a major threat to human health, in the absence of the vegetation cover required to combat air pollution and the expansion of thermal islands.
In contrast, most cities in desert Arabian Gulf countries put a great effort into greening cities and afforestation. Perhaps Abu Dhabi is one of the most prominent examples of a successful urban afforestation program, which was launched by the founder of the state, Sheikh Zayed bin Sultan Al Nahyan, some decades ago, and continued after his death. Whereas in the past desalinated seawater was used to irrigate the trees, which is a very costly process, the irrigation of forests and road trees in Abu Dhabi now mostly depends on treated wastewater.
Sheikh Zayed had sharp vision, based on innate wisdom and a deep-rooted sense of belonging to nature and the need to respect and protect it. I remember asking him in 1997, while conducting a special interview for Environment and Development magazine, why he opted to grow forests in the desert and inside a barren city. He replied that he was doing this to curb the rise in temperatures, and for people and animals to enjoy the green cover. Sheikh Zayed himself was behind the cultivation of palm trees bearing dates in public areas of Abu Dhabi, not only to decorate and preserve nature, but also to make people benefit from the fruits.
Now, more than ever, all Arab cities are called upon to launch major programs to grow urban forests, and to use treated wastewater for irrigation. This is an indispensable effort to adapt to climate change, and to create green spaces where people can enjoy the green cover.