Urban heat islands (UHI) are areas with significantly higher temperatures than the regions surrounding them. The retention and generation of heat in the urban core contributes to climate change and lower air quality for people living in and near the heat islands. They also have a negative effect on rainfall downwind of the UHI. UHIs are also a leading cause of weather-related deaths in the United States. Cities which have insufficient cooling at night during periods of hot weather have shown increases in heat related deaths, much of which has been attributed to UHIs.
There causes are many, but fortunately solutions exist to counter their negative effects on the population and on the environment. Several of these solutions are doubly attractive as they simultaneously address other urban issues, such as quality of life and eliminating food deserts. They help make cities more attractive and livable for residents and visitors alike. Here are ten ways to mitigate the detrimental effects of UHI and create better cities for the present and for future generations.
10. Creating more pocket parks
Converting disused or abandoned urban properties into small parks, called pocket parks by urban planners and architects, has proven effective in combating the heat island problem. Post World War II Athens, Greece, underwent a housing boom, during which row upon row of apartment blocks replaced detached homes with surrounding greenery. Beginning in the late 1990s, Athens undertook projects to create pocket parks from small abandoned lots and in alleyways and mews. Roads designed to accommodate automobiles have also been converted to green spaces lined with paths for pedestrians and automobiles. The expanded greenery helps cool the city, as well as improves the air quality.
Other cities have seen the value of pocket parks, including their contribution to an enhanced quality of life by offering convenient opportunities to ease out of urban stress. Chicago has dozens of such small parks, with more planned. Mexico City has built about 3.5 acres of small pocket parks scattered across the city, including in disused spaces under bridges and overpasses. Paris is known for its wide boulevards and open spaces, but it too has adopted pocket parks as part of long term urban planning.
Pocket parks alone do not present the solution to UHIs, but they can and do contribute to the reduction of the urban core’s heat retention. At the same time, they offer little oases of peace and relaxation without leaving the urban area. Their use is influencing urban planners, both for quality of life and attempts at creating less heat-retentive urban cores. Whether in a large, international city or a mid-sized town, they are an effective tool against UHIs.
9. Create heat reflecting, rather than absorbing roofs
Dark colors absorb heat, while light colors reflect it. Thus, it makes sense that dark colored roofs absorb heat in daylight and release it during the relatively cooler hours of night. The slowly released heat remains, for the most part, in a bubble over the area, the very image of the UHI. On a day when the ambient air temperature is at 90 degrees, the temperature of a dark, heat absorbing roof can exceed 190 degrees. A dense concentration of such roofs traps a lot of heat, which is a significant contribution to the increased temperature of the UHI in comparison to the rural areas which surround it.
Heat reflective roofs are an obvious solution to the heat retention problem. And, as with pocket parks, they offer other advantages as well. They can reduce internal temperatures of the building, reducing the need for air conditioning. Absorbed heat is damaging to roofs; reflected heat reduces wear on roofs and contributes to a longer service life. Cool roofs thus benefit the community as well as the owner and occupants of buildings on which they are installed.
Beginning in 2012 in New York, volunteers began painting dark roofs white, in an attempt to reduce the city’s heat retention. As of 2018 they had painted almost 7 million square feet of formerly dark tar roofs, a lot of paint and a lot of time. Even with that effort, 99% of the city’s roofs remained untreated, an indication of the severity of the problem.
8. Creating rooftop gardens and farms
Painting roofs white, or treating them with a reflective coating, isn’t the only way to achieve a cooler roof. That’s a lot of wasted space up there. Putting it to use as areas where greenery is grown reduces heat absorption, improves air quality, and offers another benefit as well. Greenery in the form of fruits and vegetables can offer locally grown fresh produce within the urban core. It is neither far-fetched nor a vision of the distant future. In a growing number of cities urban rooftop gardening and commercial farming is a reality.
In Montreal, a grocer converted his 25,000 square foot roof into a produce farm, growing tomatoes, herbs, beets, lettuce, and other vegetables. Grown in soil, and watered by the store’s dehumidification plant, the rooftop farm’s produce is packaged and sold in the store below. In Tel Aviv, Israel’s Dizengoff Center, a large shopping mall, the flat roof supports two huge greenhouses which grow vegetables hydroponically. During the growing season it produces about 10,000 heads of lettuce per month, for sale to customers outside the mall. It grows a slew of other vegetables as well, and as in Montreal, provides a cooler roof for the buildings below.
Eagle Street Rooftop Farm sits above a three-story warehouse, across the East River from Manhattan in Brooklyn. Its 6,000 square feet of growing space supplies produce for sale at its own farmer’s market, as well as to restaurants in the area. Like the facilities in Montreal and Tel Aviv, it also maintains beehives for the harvesting of fresh honey. Cooling UHIs by growing fresh food on the roofs is a simple and easily achievable means of reducing urban heat, improving the air, and giving access to locally produced fresh fruits and vegetables.
7. Lighter pavement in streets and parking lots
As anyone walking across an asphalt parking lot on a hot sunny day can attest, dark pavements absorb a lot of heat. And pavement is an essential part of any city, for vehicles and pedestrians to move about. An American Environmental Protection Agency report suggests that anywhere from 35-40% of an average city’s urban area is covered in pavement, making it a major contributor to UHIs. And cooler pavements are more difficult to achieve, since they cannot be covered with greenery. Other factors also need to be considered, such as driver and pedestrian safety, visibility, and pavement durability.
Current types of pavements can be treated with materials, such as coatings, to increase the amount of light which is reflected, rather than absorbed. Other types have been studied for suitability for future repaving of streets, roads, and parking lots. One is permeable pavement, which is coarser in density, allowing for greater convection during the daylight hours, and reduces the amount of heat which is stored in the pavement for later release. It also allows for more absorption of water, reducing storm runoff during heavy rains. Lighter colored pavements also reduce the need for candlepower in street lighting.
The use of cool pavements can reduce heat retention, leading to a reduction of the temperatures of UHIs, as well as offer better visibility to drivers at night. There are cost concerns. Replacing or modifying existing pavements in cities is a seriously complicated proposal, and implementation affects traffic patterns, commuting times, and other complications of urban life. But the potential advantages to be gained may well outweigh the costs, when overall adverse effects of UHIs are considered.
6. Create better blue spaces in urban areas
Blue spaces are water within an urban area, whether running streams, lakes and ponds, or fountains and pools. Properly designed and maintained they mitigate heat islands. Improperly designed or maintained they may make matters worse. Water cools primarily through evaporation at the surface. But it also absorbs heat, with standing water temperature rising during the course of the day as sunlight warms the surface.
Man-made blue spaces such as fountains run by pumps can actually add to the heat retention effect, since pumps create friction while operating. They also contribute to power demand, another source of heat retention. The cooling effect of blue spaces in cities is enhanced when it is combined with the use of green spaces, which in effect creates an island within an island, or a cool space surrounded by a larger UHI. More such islands reduce the overall heat retained by the UHI.
More blue spaces in urban areas have been shown to produce other benefits beyond temperature reductions. Living near blue spaces has been shown to reduce stress, improve physical exercise, and lower obesity rates, all considerable advantages. Of course, to gain the full benefit of natural blue spaces, such as rivers, centuries of effluents into the water needs to be cleaned up in all too many urban areas.
5. Plant more shade trees in the urban area
The larger the shade canopy, the less sunlight reaches the heat retaining surfaces of a city. This includes not just pavements and rooftops, but the exterior walls of brick, stone, and concrete buildings, as well as the interiors of those sided with glass. Shade trees can also protect large areas of blue spaces, and the combination of increased blue and green spaces is one of the most effective ways of reducing heat retention during daylight hours.
Urban parks and forests offer other advantages as well. Both quality of life and the quality of the air are improved by large, shaded, forested areas in cities. A 2014 study by Georgia Tech revealed Louisville, Kentucky, as one of the fastest growing UHIs in the United States. It also revealed that a major contributor to the increased heat was the lack of shade trees in many areas of the city, including the downtown area. The relationship between shade trees and heat retention is clear. More trees = cooler city.
Cities typically lose trees to a variety of causes, including pests, blight, new construction, and natural activities such as thunderstorms and winter blizzards. The Georgia Tech study found that planting more trees and increasing the shade canopy is the single most effective method of reducing heat buildup and the resulting UHIs. Simply put, trees and shade offer natural protection from the potentially damaging rays of the sun and the heat they produce.
4. Vertical gardens improve air quality and lessen heat retention
Vertical gardens are becoming more and more popular in cities around the world, both for their esthetic value and for their cost-effective impact on heat retention. In Singapore, 18 towers built for the purpose are covered with growing plants, and backed with solar powered lights. The result is a dramatic display of vegetation which helps cool the city at the same time it provides a major tourist attraction. In Mexico City, officials took a more practical approach. Columns supporting bridges and elevated roadways are lined with felt, through which plants grow, watered by their own rainwater collection system.
In Paris, the façade surrounding the entrance to the Musee du Quai Branly is covered with more than 15,000 plants covering its vertical walls, framing the entrance in greenery. The Athenaeum Hotel in London has seven stories of plants climbing an exterior vertical corner of the building. Vertical gardens, besides offering dramatic displays of plants, cool the air, improve air quality, and also protect the structure on which they are grown from absorbing heat during sunlit hours.
Beirut, as well as other communities in Lebanon, has also turned to vertical gardens to address the issues of UHIs, air quality, and beautification. Vertical gardens are a part of the solution for all three, but only when used in conjunction with other attempts at mitigation. They also help reduce city noise for residents, and plants act as a natural filter for rainwater. They can also be used to produce herbs and other consumable plants, though as of this writing they have yet to gain favor in the United States.
3. Reduce traffic congestion
Automobile and truck traffic are a major contributor to UHIs. Reduction of their use produces obvious benefits to cities, reducing both UHI severity and pollutants in the air. Lighter colored cars also absorb less heat. Dark colored cars in bumper-bumper traffic obviously negates the benefit of lighter colored pavements.
But any attempt to reduce the use of the automobile is immediately controversial, especially in the United States. There, in major cities, it is common to sit in traffic for well beyond the length of the workday, and a decision of any governmental body to eliminate dark colors for cars would be met with howls of protests over infringements on personal liberty. Reducing automobile use would immediately reduce the temperatures of UHIs, but is unlikely to happen any time soon.
It is a scientific fact that automobile emissions contribute to both air pollution and UHIs and in hot weather conditions the contribution is multiplied. Reducing traffic congestion is therefore desirable if one wishes to reduce the impact of UHIs. There are other methods of reducing congestion without restricting the number of vehicles on the road. In the UK, these methods are being explored and, in some cases, implemented.
2. Create cooler storm water runoff to improve water quality
Stormwater runoff from hot roofs and pavements has been shown to increase temperatures of streams and lakes. Increased temperatures induce negative effects on aquatic life, both plant and animal. The result is unhealthy waters which become lifeless and lose much of their ability to cool and filter. Deteriorating water quality is another result of UHIs which affects the lives of those in the urban area, as well as the lives of those who live downstream.
One solution is the creation of bioswales, constructed dips in the features of the landscape which can hold water temporarily, allowing it to cool, before routing it to streams. Permeable pavements, which cool the surface of the urban area, also absorb and diffuse more water, rather than directing it to storm drains. All of the mitigation efforts so far listed have a positive impact of cooling storm runoff, which would benefit streams and lakes.
Because water in rivers flows downstream, the effects of UHIs on water quality extends beyond the urban area, impacting aquatic biodiversity far from the heat center. The adverse impact of UHIs on water quality is a major incentive for cooling urban environments.
1. More efficient air conditioning could reduce UHIs
Air conditioning cools interiors by absorbing heat which is released to the outside air. Cooling interiors are heating the city where the interiors are located. In essence the heat (and humidity) internal to a building is ejected outside. In a densely populated area on a hot day, massive amounts of heat are added to the air outside, creating a dome which traps sunlight creating heat.
Yet air conditioning and refrigeration are facts of modern life. More efficient methods of cooling the air within buildings and vehicles would go a long way toward mitigating the effects of UHIs. Research to attain more efficient air conditioning is underway. If and when attained it will be just one of the several weapons deployed by future generations to combat the dangers of UHIs.
None of the strategies in this list will succeed in eliminating UHIs alone. But in varying combinations, depending on local politics, community involvement, and environmental factors, combinations of several could have a mitigating effect on UHIs, making cities more comfortable, and thus livable, across the globe.