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Keeping the lid on urban overheating

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International research indicates the phenomenon of 'urban heat islands' could affect UK and European cities within the next few decades. Dr Jacqueline Glass outlines ongoing work by the Oxford Centre for Sustainable Development that focuses on strategies to mitigate the effects of urban overheating

At a time when the uk government is emphasising the need for sustainable cities and lifestyles, there are many issues which will need to be addressed before these objectives can be fully realised. Among these issues are localised climatic conditions. Variations in temperature, air quality and ultimately human well-being between urban and rural areas are a key concern for those seeking to achieve sustainability. In particular, the so-called 'urban heat island' phenomenon is an issue.

A heat island arises where air temperatures in towns and cities exceed those in rural areas by several degrees Centigrade. The results of this urban overheating have been well documented by researchers in the us, Australia and elsewhere. In recent years, heat-island temperature intensity has been linked to a rise in the number of air-conditioned buildings and to an increase in the conversion rate of vehicle emission gases into harmful no2.

The principle of heat islands is simple. The physical environment of towns and cities can act as a heat sink. Exposed surfaces of buildings and roads contribute to a rise in air temperature, whereas rural or landscaped areas with trees and vegetation experience the opposite effect. Absorption of the sun's heat is accelerated by dark-coloured, highly conductive materials which absorb and store heat during the day and re-radiate it at night. The cities worst affected by the heat-island phenomenon are typically highly populated, with few 'green' areas, and are located in the mid-latitudes, such as Los Angeles and Mexico City. However, incidences of air temperatures rising, and recommended pollution levels being exceeded, are increasing in the cities of industrialised nations. Hence, there is growing concern that even the temperate European climate is set to change over the coming decades. For this reason, the uk needs to address the issue of urban overheating as an important part of the public debate about sustainable urban development.

Many organisations and community groups have already begun to address urban overheating by testing and implementing their own 'mitigation strategies' which range from tree-planting initiatives to recommendations on transport use. Although appropriately-scaled local solutions may be a short-term response, one of the most respected figures in the field of human thermal comfort, Baruch Givoni, has reasoned that our efforts should focus on the manageable aspects of the urban environment. These include: building materials, energy use, vegetation and building density.

The Lawrence Berkeley National Laboratory at the University of Berkeley in California has been instrumental in providing research and design guidance on heat-island mitigation strategies. The focus of much of the work has been on surface reflectivity, otherwise called 'albedo'. Researchers at Berkeley have studied African cities where highly reflective building materials prevent much of the heat absorption taking place, and so there is not such a noticeable heat-island effect. Pale-grey, white or painted concrete is said to be one of the most effective materials for reducing such heat gains as it is highly reflective and so has a useful albedo value.

The pale-coloured finish and high thermal-mass properties of concrete makes the rationale for using concrete in roofs more powerful. Indeed, the Lawrence Berkeley Laboratory recommends the use of reflective materials as a means of reducing solar heat gains. The research team at Berkeley estimates the net saving in energy costs could be $100 million for the Los Angeles area alone.

For buildings, there is clearly an energy advantage to be gained in the combination of high thermal capacity and high surface reflectivity offered by concrete construction, but other factors such as durability are often more relevant to highway and pavement construction. An interesting development from Japan is a concrete surface treatment which uses the uv rays from daylight to remove harmful no2 from vehicle exhaust fumes by photocatalysis, currently being trialed on concrete blocks. The blocks do not need to be exposed to full sun to perform this 'green' function; research shows they operate very effectively even on the cloudiest days.

To fulfil landscaping needs, nasa has carried out research into the benefits of 'urban forests' in which heat-mapping flights over Sacramento, Baton Rouge and Salt Lake City showed that areas of asphalt can be twice as hot as those of vegetation (71degreesC compared to 36degreesC). It claims 'urban forests are important for keeping cities cool', and that 'asphalt parking lots and roofs soak up virtually all of the radiation that falls on them'. nasa's research involved local schoolchildren who, in other areas, have been responsible for 'plant a tree' initiatives to provide practical long- term shading, while raising awareness of urban overheating problems.

The issue of urban intensification is linked closely to the heat-island phenomenon. As mentioned above, the uk government is committed to sustainability, and as a result is encouraging redevelopment of brownfield sites to satisfy future housing needs. Where such brownfield redevelopment occurs in larger cities, there is potential to exacerbate urban overheating unless mitigation strategies are implemented. While it is feasible for us communities to press for more 'green' space in new development, the availability of cheap suitable land in many uk cities is a rarity. Urban intensification is therefore clearly a complex issue involving the often conflicting demands of sustainable development and economic viability.

Although the uk is not likely to endure the same degree of urban overheating as its more southerly ec neighbours, there is considerable evidence that highly-trafficked, densely-populated cities lacking green spaces and reflective concrete surfaces are vulnerable to the urban heat-island phenomenon. We should also not be complacent about the ability of our chilly winters to cool the streets. If recent figures released by the Meteorological Office are accurate, then temperatures in the uk will rise, in all seasons.

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