Strategic Green Spaces: How to Make the Most of Their Cooling Effects
Urban green spaces are one of the most appropriate and accessible ways to mitigate the effects of rising temperatures in the urban environment. As the global climate warms, cities around the world face more frequent and intense heat waves, putting their citizens at risk. Many cities are using techniques to reduce the impact of urban heat islands, which are produced when natural land cover is replaced with surfaces that absorb and retain heat, such as pavements and buildings. This raises the temperature by several degrees compared to the surrounding atmosphere. Cities have their own micro-climate, influenced by this phenomenon, combined with a series of often overlooked factors. For a climate strategy to be effective, all factors must be taken into consideration.
Heat risk levels are also strongly correlated with the social structure of a city. Neighborhoods that are less affluent and historically marginalized have less access to green spaces, putting them at greater risk. Through planning regulations and land zoning, impacts on disadvantaged citizens can be reduced, improving their health, well-being and living standards. Scientists such as Winifred Curran and Trina Hamilton are suggesting that improving vegetation can increase property values and lead to displacement of long-term residents. They suggest a “just green is enough” strategy of creating strategic interventions to support local communities.
Simply increasing ‘green coverage’ without considering the relative location of green spaces cannot effectively reduce the problem of urban warming. – Wan-Yu Shih, Assistant Professor in the Department of Urban Planning and Disaster Management at Ming-Chuan University, Taiwan
How cities adapt to heat waves in the wake of climate change
Green Corridors and Climate Planning
In 1938, the forward-thinking German city of Stuttgart decided to hire a meteorologist to study weather conditions and their relationship to urban development. The city is located in a valley basin with low wind speed. It is highly industrialized, dependent on automobile manufacturing infrastructure, and densely populated. The combined conditions resulted in poor air quality and high levels of pollution. This early study is considered by many to be the beginning of “urban climatology”. Just a year later, at the start of World War II, urban meteorologists were appointed to coordinate civil air defense measures, which included the use of artificial fog to hide from air raid squadrons, a relatively effective measure before radar technology. After releasing large amounts of artificial fog, it was discovered that the fog cloud was melting faster than intended in some areas. At the same time, it lasts longer in other districts. The voluntary large-scale study had a significant impact on urban planning, as it raised awareness of fresh air corridors and their impact on urban climate.
Through planning and regulations, Stuttgart encouraged the development of open space on hillside parcels, allowing wind currents to sweep the forested hills surrounding the city. Geographical features such as rivers, valleys and other green spaces are protected in urban planning as ventilation corridors, allowing prevailing winds to utilize cooling services. In managing rising temperatures, the first challenge is to understand the topography, work with it and protect the natural assets of the place.
Green spaces are known to reduce average land surface temperatures, but their cooling effects have limitations. Under most conditions, at distances greater than 100 meters, these effects are barely perceptible. Because heat sources and physical buildings form barriers to airflow, the topography and geometric shapes of their surroundings determine how far cold air can penetrate. Because of this, increasing the amount of green coverage without considering their local conditions will have limited efficiency in reducing urban temperatures. In dense urban areas with limited space availability, understanding these conditions is critical to creating an effective strategy.
Large green spaces are ideal for creating stable, cool islands, but not every city can implement them. Studies show that small clusters of green areas can help distribute cool air and significantly reduce urban heat. Distributing small green spaces around large cool islands, such as rivers or parks, can extend its beneficial effects to a larger area. The type of vegetation is also used. Plants take water from the ground and slowly evaporate it through their leaves, thus lowering the temperature of the air, a process called transpiration. Trees are more efficient than shrubs and grasses because of their large leaf surface and ability to provide shade.
Road geometry affects the microclimate of a city in complex ways. Their width and orientation determine the solar exposure of surrounding buildings. In hot and dry climates, narrow streets are recommended to ensure adequate shade and avoid overheating. On the other hand, narrow streets can limit air movement and obstruct natural ventilation channels, which is an especially important factor in humid climates. In contrast, wide streets allow for air circulation but also maximize direct sunlight at street level and surrounding buildings. Planting trees along streets can potentially offset some of the adverse effects of street geometry. A tree canopy covering at least 40 percent was found to counteract the warming effect of asphalt.
Green spaces in cities can fulfill multiple roles: they can be spaces for social interaction, recreation and play; They can provide habitat for wildlife and increase the biodiversity of cities; Vegetation helps reduce noise and acts as a pollution filter in addition to aiding in stormwater management and temperature regulation. Wan-Yu Shih, an assistant professor in the Department of Urban Planning and Disaster Management at Taiwan’s Ming-Chuan University, cautions, however, that not all of these roles will be able to co-exist. Strategic planning is needed to define the most effective functions that green spaces can play in efforts to adapt urban environments to the social and environmental challenges they face.