How strategic green spaces can cool cities

When it comes to climate adaptation options, one of the key messages coming from the Intergovernmental Panel on Climate Change is for cities to integrate their grey and green spaces.

While modern cities have largely moved on from traditional practices, our forebears may have understood more than we realise about living with nature. Increasingly, researchers specialising in how cities can cope with climate change are encouraging us to regain local knowledge of nature.

City developments often don’t consider orientation, water, wind flows, and light. In face of urban warming, air conditioning remains almost the standard response in more developed nations, whatever the weather.

Air-conditioners can significantly reduce heat-health risk, but the cost of electricity prices many poor people out. Air-conditioners that emit heat to outdoor environments also increase heat stress for people outdoors. All while climate change is increasing the magnitude, frequency, and duration of extreme high temperatures.

Highly populated cities in warm and humid climates are particularly at risk. Urban areas are generally several degrees warmer than rural surroundings due to the replacement of natural surfaces with hard pavements and buildings, a phenomenon well known as the urban heat island effect. The loss of natural areas in the process of urbanisation reduces the mechanism of evaporative cooling and shading from tree canopy. Development that blocks natural channels of cities, such as valleys, riversides, and coastal fronts, also reduces natural ventilation.

Local-level governments that determine land use and development can play a crucial role in heat-risk reduction in several ways. For example, modifying planning regulations, land zoning and building codes for improving ventilation and green spaces can lower the urban heat island effect. This adjustment of land use can have huge impacts on the health and well-being of citizens, as well as improving social equality in heat adaptation. Enhancing ventilation of cities and increasing tree coverage in hotter and less affluent areas might help reduce the need of air conditioners, and help mitigate heat-related health risks for the most vulnerable.

The city of Stuttgart, Germany was famously a pioneer of ‘climatological planning’. A valley city, having suffered from poor access to light and fresh air for many years, Stuttgart was astonishingly forward-thinking in appointing climatologists to participate in urban planning in 1938. Building upon years of research, the city determined that cleansing winds were blocked from sweeping down forested hills by inappropriate building development. They adjusted the planning controls to specify a greater open-space to development ratio on hillside parcels of land. Critical geographical features that influence air flow, 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 a sea-land circulation of air through the city. During summer days, the breeze from the sea and forested mountains follows 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 transportation corridors and intensive development occupies the waterfront areas. This likely blocks the breezes from penetrating the city. Indeed, with more intense urbanisation compared with other cities in Taiwan, Taipei has shown a faster and more obvious warming trend in the past few decades.

Yet, when the time comes for urban regeneration, there is still a chance for planners to integrate climatological knowledge. They could identify ventilation zones with critical natural geographical features, and systematically allocate open spaces to allow prevailing winds to harness cooling services.

Green spaces are well known to regulate the micro-climate of a city, lowering the temperature of their surroundings. However, this cooling effect has its limitations; beyond 100m they can barely be perceived.

Development geometry around green spaces determines how far cool air can penetrate. If there are no adjacent heat sources, no physical barriers blocking air flow, and good winds, cool air might flow further. Hence, simply increasing ‘green coverage’ without considering relative location of green spaces cannot effectively reduce the problem of urban warming. Understanding this cooling mechanism is particularly critical for cities like Taipei, which has been densely built. In such contexts, green spaces need to be preserved and/or created more strategically.

Large green spaces possessing greater distance from their centre to the surrounding built environments, which are the source of heat, tend to form more stable cool islands of a city. Of course, not every city can simply designate large areas of land for green space, particularly for cities that are already crowded. Yet, small patches of vegetation can reduce temperature.

Clustering small green patches close to each other or distributing them around larger cool islands, such as rivers, parks and woodlands, can extend cooling. This understanding helps to provide alternatives to creating a large green space in a dense and hot city centre. Encouraging small areas of greenery adjacent to each other in school grounds, business precincts, private-owned lands, and under-utilised lands, can lower temperatures in the most crowded areas.

Not every kind of vegetation has 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 leaves. This process, called evapotranspiration, removes heat from surrounding areas. Trees, with larger a leaf surface area, generally have higher evapotranspiration rates than that of shrubs and grasses, removing more heat from their surroundings. More importantly, trees provide shade.

Some studies have found that open grass areas have a similar surface temperature to pavement in summer due to direct exposure to sunlight. Unlike in temperate countries, shading from either buildings or trees is critical for outdoor activities in the tropics. In summer, people often take a route or find a spot with shade and avoid open grasslands during the daytime. 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 and dry climates, we can see narrow streets amongst buildings, which allow streets to be shaded from sunlight. In temperate climates, however, open grasslands and sunshine is often prefered by the public.

Street width and building height ratio affects microclimate in a complicated way. On the one hand, narrow streets and taller buildings for shade avoids direct sunlight heating up the pedestrian level. On the other hand, it can be bad for ventilation or can cause wind tunnels. Wider streets allow cooling breezes, but increase periods of direct sunlight and heat during the day. In this case, trees with broad canopies are helpful to shade wide streets.

Satellite temperature measurement of Taipei City shows the 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 from camphor and Taiwanese rain trees. Indeed, they provide attractive shade and cooling while driving and walking across the area in summer.

Green space has multiple functions, such as recreation, social interaction, wildlife habitat, noise reduction, pollution filtering, runoff mitigation, and temperature regulation. These are crucial for quality of life in urban environments and benefit health and well-being of urban dwellers. However, these functions do not necessarily co-exist. Green spaces designed to maximise cooling benefits might not contribute to other desirable functions. For example, a cooling initiative favouring a large canopy might create uniform green spaces and thus reduce the overall biodiversity of cities.

In the era of cities, a changing climate means we need adaptation strategies to a warming urban environment. Greenery is often low-cost to implement, and important for a city's wellbeing.

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