By Wan-Yu Shih, Ming-Chuan University, Taiwan Taipei, February 25 (360info) Green spaces are not in themselves the answer to overheated cities. Careful urban planning must take into account their strategic location.
When it comes to climate adaptation options, one of the key messages from the Intergovernmental Panel on Climate Change is that cities integrate their gray and green spaces.
While modern cities have largely abandoned traditional practices, our ancestors may have understood more than we realize about living with nature. Increasingly, researchers specializing in how cities can cope with climate change are encouraging us to rediscover a local knowledge of nature. Urban developments often do not consider orientation, water, wind flows and light. Faced with urban warming, air conditioning remains almost the standard response in the most developed countries, whatever the weather. Air conditioners can significantly reduce heat and health risks, but the cost of electricity costs many poor people. Air conditioners that radiate heat to outdoor environments also increase heat stress for people outdoors. Meanwhile, climate change is increasing the magnitude, frequency and duration of extreme high temperatures. Highly populated cities in hot, humid climates are particularly at risk. Urban areas are typically several degrees warmer than rural areas due to the replacement of natural surfaces with pavements and hardstanding buildings, a phenomenon well known as the urban heat island effect.
The loss of natural spaces in the process of urbanization reduces the mechanism of evaporative cooling and shading of the tree canopy. Development that blocks natural city channels, such as valleys, riverbanks and coastal fronts, also reduces natural ventilation. Local governments that determine land use and development can play a crucial role in reducing heat-related risks in several ways. For example, changing planning regulations, land zoning, and building codes to improve ventilation and green spaces can reduce the urban heat island effect.
This land use adjustment can have huge impacts on the health and well-being of citizens, as well as improving social equity in heat adaptation. Improving city ventilation and increasing tree cover in hotter, less affluent areas could help reduce the need for air conditioners and mitigate heat-related health risks for the most vulnerable. The city of Stuttgart, Germany, was known for being a pioneer in “climate planning”. A city in the valley, having suffered from a lack of access to light and fresh air for many years, Stuttgart was surprisingly forward-thinking in appointing climatologists to participate in city planning in 1938.
Based on years of research, the city determined that the purifying winds were being prevented from sweeping through the forested hills by improper building development. They adjusted planning controls to specify a greater open space to development ratio on hillside plots. Critical geographical features that influence airflow, such as mountain forests, rivers, valleys and other green spaces, are designated ventilation zones in the urban plan. Taipei is a subtropical city nestled in a basin. Some studies suggest sea-land air circulation through the city. During summer days, breezes from the sea and forested mountains follow the major river valleys of the Danshui, Keelung, and Dahan rivers through the city, cooling as they pass.
However, modern urban development has ignored this natural mechanism. Many mountain valleys are developed into transport corridors and intensive development occupies the riparian areas. This probably prevents breezes from entering the city. Indeed, with more intense urbanization compared to other cities in Taiwan, Taipei has shown a faster and more obvious warming trend in recent decades. However, when the time comes for urban regeneration, planners still have the possibility of integrating climatological knowledge. They could identify ventilation zones with critical natural geographic features and systematically allocate open spaces to allow prevailing winds to harness cooling services.
Green spaces are well known for regulating a city’s micro-climate, lowering the temperature of their surroundings. However, this cooling effect has its limits; beyond 100m you can barely see them. The geometry of the development around green spaces determines how far fresh air can penetrate. If there are no adjacent heat sources, no physical barriers blocking airflow, and good winds, cool air can flow farther. Therefore, simply increasing “green coverage” without considering the relative location of green spaces cannot effectively reduce the problem of urban warming. Understanding this cooling mechanism is especially critical for cities like Taipei, which has been densely built up. In such contexts, green spaces need to be preserved and/or created more strategically. Large green spaces with a greater distance from their center to surrounding built environments, which are the source of heat, tend to form more stable cool islands of a city. Of course, not all cities can simply designate large areas of land for green spaces, especially for already crowded cities. However, small patches of vegetation can reduce the temperature. Grouping small patches of green close together or spreading them around large cool islands, such as rivers, parks, and woods, can prolong the cooling. This understanding makes it possible to propose alternatives to the creation of a large green space in a dense and hot city centre. Encouraging small patches of greenery adjacent to each other in schoolyards, commercial areas, private lots, and underused lots can lower temperatures in more populated areas. Not all vegetation types have the same cooling effect. In general, areas planted with trees are cooler than areas covered with shrubs and grasses. Vegetation draws water from the ground and evaporates it from the leaves.
This process, called evapotranspiration, removes heat from surrounding areas. Trees, which have greater leaf area, generally have higher evapotranspiration rates than shrubs and grasses, thereby removing more heat from their environment. More importantly, trees provide shade. Some studies have shown that open grassy areas have a surface temperature similar to pavement in the summer due to direct exposure to sunlight. Unlike in temperate countries, shade from buildings or trees is essential for outdoor activities in the tropics. In the summer, people often take a route or find a shady spot and avoid open grasslands during the day. The decision to increase the number of green spaces for cooling should therefore consider both vegetation types and human activity in the open spaces. Understanding the local climate is very important for heat adaptation. In hot, dry climates, we may see narrow streets among buildings, allowing streets to be shaded from the sun. In temperate climates, however, open grasslands and sun are often preferred by the public. The ratio of street width to building height affects the microclimate in complicated ways. On the one hand, narrow streets and taller buildings for shade prevent direct sunlight from heating up the pedestrian level. On the other hand, it can be bad for ventilation or cause wind tunnels. Wider streets allow for cooling breezes, but increase periods of direct sunlight and heat during the day. In this case, trees with wide canopies are useful for shading wide streets. Satellite temperature measurement of Taipei City shows particularly obvious and stable cooling effect of street trees on two major boulevards in summer. These are roads created with linear greenery and large canopies of camphor and Taiwanese rain trees. Indeed, they provide attractive shade and cooling when driving and walking in the area in the summer. Green spaces have multiple functions, such as recreation, social interaction, wildlife habitat, noise reduction, pollution filtering, runoff mitigation, and temperature regulation. These are crucial for the quality of life in urban areas and benefit the health and well-being of city dwellers.
However, these functions do not necessarily coexist. Green spaces designed to maximize cooling benefits may not contribute to other desirable functions. For example, a cooling initiative promoting a large canopy could create uniform green spaces and thus reduce the overall biodiversity of cities. In the age of cities, climate change means we need strategies to adapt to a warming urban environment. Greenery is often inexpensive to implement and important to a city’s well-being. (360info.org) AMS AMS
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