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Efforts in Japan to mitigate the Urban Heat Island effect

Last modified October 01, 2008 10:42

The Urban Heat Island (UHI) effect describes a phenomenon in which temperatures in urban areas are markedly higher than those in surrounding areas. Isotherms on a map show an urban area with higher temperatures emerging like a warm island floating on a cooler sea. This is the origin of the name of "Urban Heat Island."



The Urban Heat Island (UHI) effect describes a phenomenon in which temperatures in urban areas are markedly higher than those in surrounding areas. Isotherms on a map show an urban area with higher temperatures emerging like a warm island floating on a cooler sea. This is the origin of the name of "Urban Heat Island."

The UHI effect results in higher average temperatures and a greater number of sweltering summer nights, defined in Japan as nights when the temperature stays above 25 degrees. This negatively impacts the natural environment and urban citizens' daily lives and health. It also causes serious weather events such as localized torrential rains. Recently, urban areas in Japan including Tokyo have experienced more incidents of unpredictable, torrential rains, called "guerrilla downpours;" one such event this year caused an accident in which workers in an underground sewage system were swept away and drowned.

UHI phenomena can occur any place with a concentrated population where countermeasures are not being taken. Problems caused by the UHI effect are becoming a common concern among big cities around the world, including those in developing countries. For areas with accelerating urbanization, urban planning is needed to create cities with less UHI effect, and mitigation measures are needed for areas that have already been developed and are facing UHI effects. In Japan, various efforts to deal with the UHI effect have been carried out.

Over the past 100 years, Tokyo's average temperature has increased by about three degrees Celsius, and that of Osaka has increased by two degrees Celsius (C). Since it is said that global warming has raised the Japan's average temperature by about one degree C, the temperature increase due to the UHI effect is probably about two degrees in Tokyo and about one degree in Osaka.

Along with the UHI effect, an increasing number of patients suffering from heat stroke and other heat disorders have recently been admitted to emergency rooms. In Tokyo, the number of such patients brought to hospitals by ambulances increased to 1,300 persons in 2007 from 200 in 1996. Some studies show a correlation between deaths from heat stroke and the heat experienced during extremely hot days and sweltering summer nights.

What causes the UHI phenomenon? Briefly speaking, promoting urbanization in itself leads to an increase of anthropogenic heat emissions in urban centers, while water, wind and greenery that can help to cool down an urban area have concomitantly decreased.

Urbanization changes the nature of the ground cover, and as the area of green spaces, water, and farmland decrease, the effects of transpiration also decrease. Also, due to a greater area of asphalt roads and concrete buildings, more heat is absorbed and retained, while the heat reflection ratio decreases.

There are still more causes of the UHI effect. They include the changing of urban forms, for example constructing a forest of high-rise buildings that block or weaken the wind, and eliminating types of ground cover that can cool urban heat such as large-scale green spaces and/or water surface. The amount of heat emitted from residential and other buildings, business activities, and vehicles has also been increasing - artificial heat emission is one of major causes of the UHI effect.

There is a vicious cycle between the UHI phenomenon and global warming. Rising temperatures due to the UHI effect create increased demand for air conditioning, which increases the amount of exhaust heat vented, which in turn leads to further temperature rise. In addition, the more electricity is consumed with the increasing use of air conditioners, the more carbon dioxide (CO2) is emitted and temperatures continue to rise further as global warming worsens.

Under global warming conditions, if the UHI effect accelerates further in urban areas and there is scarcely any greenery or waterfronts to provide relief from the heat, heat island-related problems will have significant or sometimes even life-threatening impacts on human health, for example, greater incidence of heat stroke.

Responding to this problem, in 2002 the Japanese government established a liaison committee to promote mitigation of the UHI effect, which drew up an "Outline of the Policy Framework to Reduce Urban Heat Island Effects" in 2004. The policy outline stipulates that the progress of measures undertaken to reduce temperatures in urban areas should be monitored annually, and the liaison committee meets every year and releases a progress report.

Outline of the Policy Framework to Reduce Urban Heat Island Effects

UHI measures include reducing artificial heat emissions, trying to avoid heat build-up by creating wind paths, promoting greening, improving pavement surfaces, and so on.

One of the most well-known examples of creating wind paths is the initiative taken by the city of Stuttgart, Germany. The city induced cool winds blowing down from the mountains to flow into the city center by creating green belts of forests. Projects to create wind paths are also ongoing in the central part of the Tokyo Metropolis, as part of an urban renewal project. Exhaust heat from air conditioners and automobiles tends to accumulate in certain areas, and the urban structure of high-rise commercial buildings prevents wind from Tokyo Bay from passing through the city.

As part of a renovation plan in the Tokyo Station vicinity, there is an ongoing project to construct twin skyscrapers located about 246 meters apart and connected by a pedestrian deck. After these buildings are completed, an existing old 12-story building that now blocks the wind from Tokyo Bay will be demolished. It is expected that creating a wind path will make this area much cooler than before.

Greening is also an effective approach to mitigating the UHI effect. According to surveys by the Tokyo Metropolitan Government (TMG), when the temperature of concrete surfaces rose to 55 degrees C in mid-summer, the surface in green areas was as low as about 30 degrees C. The national government and local governments have been promoting greening through subsidies, reductions in fixed property taxes, bonus plot ratios and so on. Nagoya City set up an ordinance that requires new houses and office buildings of more than 300 square meters to have green spaces covering 10 to 20 percent of their lots.

Another new idea that has cropped up is to plant grass along tram lines. Greening on rooftops and walls has been a common measure, and recently more and more people have begun to grow climbing plants such as morning glory and bitter gourd on nets or frames outside the windows of their homes as green curtains of vegetation.

As measures for dealing with pavement surface heat, water-retentive and/or insulating pavements are being adopted more widely. The TMG conducted an experiment in which a total of four kilometers of water-retentive pavement was installed, and the results showed that this type of pavement cooled down the road surface temperature by about 10 degrees C. In addition, applying thermal barrier coating to roofs to reflect sunlight throws off heat to a remarkable extent. The TMG, together with the governments of seven Tokyo wards and seven other organizations, established a "Committee to Promote Cool-roof" that promotes measures to deal with both the heat island effect and global warming through rooftop greening and thermal barrier coating. (Japanese only)

We would like to introduce some further measures being taken by the Tokyo Metropolitan Government (TMG) as an example of the way Japan's local governments are combating the UHI.

Measures to address Heat Island

The TMG set a target of reducing the number of sweltering summer nights to around 20 days a year by 2015. It is trying to promote a flexible range of measures that match the UHI factors affecting each area. To this end it has established a network of 120 monitoring sites and is creating a heat environment map that shows the distribution of area factors and scale of the heat load, atmospheric impacts from artificial exhaust heat and ground-surface conditions.

Tokyo Uses Heat Map to Combat Heat Island Effect

Other TMG initiatives include; creating more shady spaces under thick-trunked street trees, adopting water-retentive pavement, experimenting with insulating pavement and conducting a feasibility study involving the spraying of treated sewage water over pavement. For further cooling effects, it is also trying to create green areas that will total about 300 hectares in all by building a large park and improving existing parks, greening the rooftops of local government buildings and high schools, and planting grass on the grounds of elementary and junior high schools in Tokyo.

The TMG passed an ordinance in fiscal 2001; the nation's first of its kind, requiring rooftop greening on newly constructed buildings. Under this ordinance, over 20 percent of the total site must be set aside for greening in the case of construction of large facilities over 1,000 square meters (over 250 square meters in case of public facilities). The TMG has developed its own "Guidelines for Heat Island Control Measures," which offers visual instructions for feasible countermeasures suited to each building type in order to promote those measures among private builders.

Guidelines for Heat Island Control Measures

New at Tokyo City Hall: Rooftop Greenery, Solar Power

Finally, we would like to introduce a unique civil movement, the "Mission Uchimizu". This campaign invites people to sprinkle secondary used water such as leftover bath water on streets on hot summer days, to utilize the cooling effect of water vaporization.

Mission Uchimizu

Let's Cool Down Tokyo! -- Edo-Period Sprinkling Campaign

In former eras, Japanese people had various customs to ease the heat, such as hanging up tinkling wind chimes, placing woven reed blinds over the windows to block the strong sunshine, and sprinkling water on the streets and yards around houses. Campaigners of Mission Uchimizu are trying to rekindle the old wisdom of sprinkling water, and have organized water-sprinkling activities in some other countries such as Spain.

Technologies to ease the UHI effect will probably be developed and introduced now and in the future. At the same time, or even before these technologies come into use, there are other questions to be considered: What kind of cities do we want to create? What is true comfort? Do we really need to pave all our streets for cars? Which factors have higher priority? Planning with clear vision will create cities where people can live comfortably.

(Written by Junko Edahiro, Japan for Sustainability)

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