QUARTERLY REVIEW No.18 / January 2006 6 Measures to Mitigate Urban Heat Islands YOSHIKA YAMAMOTO Environment and Energy Research Unit “the urban heat island effect as air pollution” 1 Introduction in August 2001, mitigation measures suddenly emerged as a political issue. In response, the The “Outline of the Policy Framework to Cabinet decided in March 2002 to “set up a Reduce Urban Heat Island Effects”, which general task force comprising the ministries was laid down in March 2004, stipulates that concerned and draw up guidelines to implement lifestyles must be reformed as part of measures comprehensive approaches to urban heat island to mitigate urban heat islands. One of these mitigation” in accordance with the “Three-Year measures is the promotion of the wearing of Program for Promoting Deregulation (Revised).” light clothing in summer. For instance, with the The Ministry of the Environment, the Ministry Kyoto Protocol now taking effect, the Ministry of of Land, Infrastructure and Transport, the the Environment encouraged its staff members Ministry of Economy, Trade and Industry, and to participate in the “Cool Biz” dress code the Cabinet Secretariat subsequently established campaign by not wearing neckties or jackets in the Inter-Ministry Coordination Committee to summer beginning in June 2005. This enabled Mitigate Urban Heat Islands (hereinafter “the the temperature of its air-conditioned workplaces Liaison Council”) in September 2002. This was to be kept at 28°C. In addition, an annual event followed by the establishment of the “Outline dubbed the “Grand Water Sprinkling Campaign” of the Policy Framework to Reduce Urban Heat was carried out across the country on August 10. Island Effects” in March 2004. Urban heat island mitigation measures are making The “Guideline of Measures to Prevent Global steady headway. Aside from lifestyle issues, Warming” was laid down in 2002. It aims to however, another perspective from which to “promote global warming mitigation measures in debate urban planning itself is necessary. order to achieve a 6% reduction in greenhouse Japan’s approaches to the urban heat island gas emissions.” One of these measures concerns effect trace their origins to the 1980s, when it the “promotion of comprehensive approaches to became a topic of study in the field of physical the urban heat island effect.” science, including meteorology and geography. The “Basic Policies for Urban Renaissance,” That is where most early progress was made. adopted by the Cabinet in July 2002, frames In the 1990s, the phenomenon also became urban heat island mitigation measures as a means a research topic in engineering fields such to revitalize urban areas. Heat island mitigation as architecture and civil engineering, which measures have thus become a major political issue addressed urban heat and energy problems. from an urban renewal perspective. Study of the urban heat island effect from an The urban heat island effect was thus originally urban planning perspective thus began [1]. studied in the fields of physical science and Political approaches, meanwhile, have also made engineering, from which various preventive significant headway over recent years, playing policies gradually developed. It is too complex catch-up with more-established approaches. a problem to be solved by a single ministry, as is When the Ministry of the Environment defined acknowledged in the establishment of the Liaison 65 SCIENCE & TECHNOLOGY TRENDS QUARTERLY REVIEW No.18 / January 2006 Council. Furthermore, it is an interdisciplinary subject that involves meteorology, geography, 2-1 Status and causes architecture, civil engineering and the like. of the urban heat island effect A range of studies is underway to elucidate (1) Status of the urban heat island effect the effect, develop and implement mitigation (i) Long-term upward trend in average measures, and so on. There is a pressing need temperatures to mobilize a wide range of findings from The third Intergovernmental Panel on these studies to come up with comprehensive Climate Change (IPCC) report*1 points out that mitigation measures. average global temperature rose by some 0.6°C This article explores urban heat island during the 20th century. Six big cities in Japan mitigation measures primarily from the (Sapporo, Sendai, Tokyo, Nagoya, Kyoto, and perspective of urban planning. Fukuoka) have seen average temperature rises of 2-3°C. The urban heat island effect has been more pronounced in these cities than have changes due 2 The Urban heat island effect to global warming. Urbanization involves concentration of population, loss of natural surface, and expansion (ii) Sweltering nights and rising daytime of living space above and below ground. All of temperatures these factors alter the balance of radiation, heat, Temperatures are on the rise, particularly in big and water, generating a climate typical of urban cities. In fact, the temperature now stays above areas[2]. 30°C for longer, over a larger area (see the upper The urban heat island effect is a phenomenon and middle color maps on the front cover and whereby cities become warmer than the Figure 1). Accordingly, the number of sweltering surrounding suburbs. In other words, there is a nights is increasing. temperature difference between cities and the areas surrounding them. The effect was first (2) Causes of the urban heat island effect observed in London and other European cities The following four factors are the major causes in the 1830s, followed by big cities such as New of the urban heat island effect[5] (see Figure 2). York and Chicago in the USA. The phenomenon is now becoming a major problem in Asia as (i) Increased anthropogenic heat release well. Indeed, the urban heat island effect exists • Heat release resulting from energy wherever there are large cities. While elimination consumption in urban areas of the phenomenon is not feasible, the key issue (ii) Changes in surface cover is how best to mitigate it. A variety of factors, • Reduced surface evapotranspiration such as surface cover, anthropogenic heat release, capacity due to less green area and urban characteristics including geographic • The heat storage effect of construction features and climatic conditions interact with materials such as concrete and asphalt one another to create the effect. Its generating (iii) Urban structure mechanism is complex and yet to be fully • Heat stagnation due to densely packed elucidated. Currently, therefore, each mitigation buildings measure, such as energy-saving technologies and • Expansion of urban areas greenification, is being separately implemented[3]. (iv) Other Under these circumstances, elucidation of each • The greenhouse effects of fine-particulate contributing phenomenon to establish a scientific air pollution in the urban atmosphere background and development of quantitative assessment techniques are imperative. 66 67 SCIENCE & TECHNOLOGY TRENDS QUARTERLY REVIEW No.18 / January 2006 Figure 1 : Distribution of cumulative hours with temperatures above 30°C (Tokyo) (see the color maps on the front cover) Based on data provided by AMeDAS (July to September in 1981 and 1999). Cumulative hours with temperatures above 30°C are tabulated, and their distribution is shown by means of an isochrone. Source: Reference[4] Figure 2 : Causes of the urban heat island effect Source: Reference[6] (2) Winter impacts 2-2 Impacts of the urban heat island effect Inversion layers*3 form by radiative cooling*2 on (1) Summer impacts clear, calm winter nights. Ascending air currents Urban areas are becoming uncomfortable created by warm urban areas are trapped under places to live because of higher temperatures inversion layers, forming mixed layers (dust during daytime and an increasing number domes * 4 ) that exacerbate air pollution (see of sweltering nights. Higher temperatures Figure 3). boost demand for air conditioning, resulting in increased energy consumption. They also (3) Other contribute to localized torrential downpours and Changes in surface cover cause decreased the production of photochemical oxidants. evaporation, making urban areas drier. 66 67 SCIENCE & TECHNOLOGY TRENDS QUARTERLY REVIEW No.18 / January 2006 Figure 3 : Atmospheric conditions inside and outside urban areas in winter (when inversion layers are formed) Source: Reference[4] encouraging building owners to adopt proactive 3 Measures to mitigation measures (see Figure 4.). In July 2005, mitigate urban heat islands the CASBEE-HI system to assess the overall environmental performance of buildings was 3-1 Status of urban heat island completed. CASBEE-HI is a tool for evaluating mitigation measures the effects of mitigation measures. It maintains a Since 2000, local governments made comfortable thermal environment in pedestrian remarkable strides in implementing systematic spaces and other areas inside buildings. It approaches to urban heat island mitigation uses a five-level rating system to assess the measures. Typical systems aim primarily environmental performance of buildings by to promote the greening of urban areas by evaluating reduction of thermal impacts on their mandating the promotion of greening, subsidizing surroundings. the cost of greening, and incentivizing rooftop In December 2004, the Urban Renaissance greening by granting higher floor-area ratios to Headquarters of the Cabinet Secretariat (headed buildings that implement it. by the Prime Minister) adopted the eighth Urban At the government level, in March 2002 the Renaissance Project: “Development of