The heat island phenomenon rises the temperature of cities, increases the energy demand for cooling and deteriorates comfort conditions in the urban environment. To counterbalance the impact of the phenomenon, important mitigation techniques have been proposed and developed.
It is widely accepted that an increase of the urban green infrastructure, and particularly of the tree cover, improves urban resilience [1-2]. Trees provide urban overheating reduction, pollutant removal, carbon sequestration, retention and detention of storm water runoff, while improving residents’ health [3-8]. The American Forestry Association in 1989 estimated that the value of an urban tree is close to $57,000 for a 50 years mature specimen. This value includes the benefits for air conditioning, soil protection, air pollution and wildlife habitats (9).
The present fact sheet aims to provide information and recommendation on the impact of urban greenery on the urban climate and in particular on the potential decrease of the urban overheating. It does not discuss the impact of urban greenery on the specific energy demand of buildings. The fact sheet considers four types of urban greenery: Urban parks, street parks, green roofs and vertical green infrastructure attached to buildings.
Greenery contributes to decreasing the ambient temperature through evapotranspiration and shading. In parallel, when impervious surfaces like asphalt and concrete are replaced by vegetation, the stored heat during the day and the emitted sensible heat during the day and night are significantly reduced. The potential temperature decrease because of the increased green infrastructure fraction depends on the difference of the thermal balance between the non-vegetated control/reference scenario and the vegetated one. It is mainly affected by the specific climatic conditions, the availability of soil moisture, the type of vegetation and the way it is distributed in a city. As a result, increased vegetation fractions may contribute to reducing the ambient temperature during the night or the day or both, or even cause some warming effects under specific conditions.
To increase the urban green infrastructure, additional trees placed in urban parks or streets as well as building integrated greenery like planted roofs and vertical greenery systems may be used.
Figure 2 : Bergpark Wilhelmsshohe Germany. Picture from : Bergpark Wilhelmshöhe - Urban Park in Germany - Thousand Wonders.
Green roofs are partially or completely covered by plants over an engineered planting substrate on specialized build up of polymer materials. They are differentiated by the type of plants they may support. Extensive type green roofs are covered by low vegetation while intensive green roofs may support growing of shrubs and small trees. Important beneﬁts are associated with the use of green roofs. Because of the solar and heat protection they provide, they contribute to lowering the energy consumption of buildings, while through latent heat processes, they decrease the surface temperature of the roofs and reduce the release of sensible heat to the atmosphere. In parallel, green roofs help with the storm water runoff management, provide better air quality, reduce noise, prevent erosion and increase the durability of the roof materials (10).
Figure 3 : Green Roof in Athens, (9)
There is an acceptable correlation between additional tree cover and the ambient temperature drop during the peak day period and at night. The average maximum drop of the peak daily temperature may not exceed 1.8 ◦C, when the tree fraction reaches its maximum, while the average night-time maximum mitigation potential is much higher and close to 2.3 ◦C. Given the serious spatial limitations in cities, a potential increase of the tree cover by 20% may initiate a temperature decrease close to 0.3 and 0.5 ◦C during the daytime and night-time, respectively.
The local landscape and climate conditions highly affect the potential temperature drop in a city; however, the order of magnitude of the temperature drop remains close to the previously mentioned levels. Given that most of the cooling demand is during the peak daytime period, the shift of the maximum mitigation potential of trees during the night reduces the expected cooling contribution. Potential problems of night warming and serious increase of humidity levels reported by numerous studies should not be neglected, especially when additional trees are deployed in urban canyons, reducing the sky view factor, and when additional green infrastructure is planned in humid climates.
There is a statistically significant correlation between the peak daily temperature decrease caused by higher green infrastructure fractions and heat-related mortality. When the peak daily temperature drops by 0.1 ◦C, then the percentage of heat-related mortality decreases on average by 3.0%
Urban greenery and in particular trees contribute seriously to urban cooling 10 to 15 years after plantation. However, urban trees and the rest of the urban greenery systems present a very long-life span and may contribute to urban cooling for more than 30-50 years.
Figure 4: Street Trees. Picture is taken from : 20 Incredible Benefits of Urban Street Trees (arbor1.com).
Figure 5 : Vertical Greening in Volksbank Austria. Picture is taken from: Green Buildings - Volksbank Blog.
Several European cities increase the percentage of urban green infrastructure to improve the urban microclimate. The greenness of European cities has increased by 38% over the last 25 years, with 44% of Europe’s urban population currently living within 300 metres of a public park, (11). On average, some 40% of the surface area of European cities is made up of urban green infrastructure, with around 18.2 m2 of publicly accessible green space per inhabitant, (11). Figure 1, gives the available public green space per inhabitant in Europe, (12).
Figure 1. The available public green in European cities per inhabitant, (12)
Urban Greenery is the most mature mitigation measure that supports the improvement of the urban microclimate and counterbalances the impact of urban overheating.
Especially green roofs are extremely well developed, matured, commercial greenery system, available to the market. Green roofs are tested under all climatic conditions, and they provide a considerable contribution to decrease urban overheating.