Urban warming has a very significant impact on human life: it increases energy consumption, reduces comfort levels, increases pollution concentration, threatens human health and affects the urban economy. Mitigation technologies aiming to counter the impact of the phenomenon are rapidly being developed and applied in real projects.
Urban warming and the heat island effect
Each year sees more European funding given to projects that deal with the study of the urban climate and temperature regulation in contemporary cities, with the ultimate goal of helping cities and their citizens to adapt to climate change. Urban climate is affected by various local, regional and global parameters. Global climate change, the result of an increased concentration of greenhouse gases in the atmosphere due to human activity, also increases urban temperatures. The Intergovernmental Panel on Climate Change (IPCC) identified projections of the average ambient temperature increase in the region of 0.15 K to 0.3 K per decade between 1990 and 2005. Depending on the scenario, the most recent IPCC estimates suggest a worldwide increase of 1.8 K to 4 K to occur between 1990 and 2100. Urban temperatures, however, are also affected by the urban heat island effect. This phenomenon is strongly influenced by human activity as concluded also in IPCC's Fifth Assessment Report (AR5). It affects the urban heat balance and contributes to higher urban temperatures, including changes in land cover, increased heat storage by urban structures, higher anthropogenic heat liberated, lowered evaporative cooling and increased sensible heat released by the urban skin. These changes in the urban morphology are primarily due to the massive migration of rural populations into cities and the unprecedented growth of urban populations which tend to follow urban economic growth and industrialisation, local conflicts in rural areas, mechanisation of agriculture and a lack of resources in rural areas.
The impacts of increasing urban temperatures
The Urban Heat Island (UHI) effect increases the concentration of specific urban pollutants and affects air quality. It also results in increased energy consumption for cooling purposes as well as peak electricity demand, raised CO2 emissions and the ecological footprint of cities, while simultaneously having a serious impact on human health, thermal comfort and the economy. A review study on the impact of urban heat island and global warming on the power demand and electricity consumption of buildings reports that the increased peak electricity demand per degree of temperature is between 0.45% and 4.6%. Regarding the relationship between electricity consumption and ambient temperature in various cities worldwide, the same study shows increased hourly, daily and monthly electricity consumption of between 0.5% and 8.5%, per degree of temperature rise.
- More green urban space: Urban greenery provides solar protection and cooling of ambient air through evapotranspiration. A relative study on the contribution of urban green spaces showed that the evapotranspiration of plants increases relative humidity and contributes indirectly to reduction of temperature in cities. In parallel, urban greenery masks noise, prevents erosion and stabilises soil, filters ambient pollutants, decreases stress to visitors, increases property values and makes cities more attractive. Green roofs and walls can contribute significantly to a lowering of ambient urban temperatures. As shown in the Practical Guide to Cool Roofs and Cool Pavements, vegetation lowers the surface temperature of roofs and decreases the release of heat to the atmosphere. In parallel, apart from energy conservation and urban heat island mitigation, they present a variety of advantages including noise reduction, better air quality, and storm water management.
- Use of reflective materials: Reflective or cool materials present a very high reflectance to solar radiation together with a high emissivity coefficient. Lower absorption of solar radiation and increased infrared emission minimize the surface temperature of materials and substantially decrease the amount of heat released to the atmosphere. Four classes of reflective materials have been identified: (a) natural materials, such as white marble, that present high reflectivity to solar radiation (b) recently developed highly reflective white artificial coatings, (c) coloured coatings with high reflectivity in the infrared spectrum, and (d) intelligent coatings doped with nano-technological additives like thermochromic paints and Phase Change Materials (PCM) that display enhanced optical and thermal properties. Application of reflective materials on building roofs and paved surfaces contribute significantly to increasing the albedo (reflection coefficient) of cities and thus decrease surface temperatures and mitigate the urban heat island. Within the framework of the Cool Roofs project, five case studies in different European countries were implemented and the results showed energy savings of 10-40% and a reduction of indoor temperatures of 1.5-2°C, depending on climatic conditions. A study on the use of cool materials as a heat island mitigation strategy investigated the effect of modifying the albedo values of roofs in Athens to 0.63 and to 0.85. It found that in the first case the temperature depression at a height of 2m was as high as 1.5°C and in the second case it reached 2.2°C. The typical temperature change pattern is a decrease during daytime and little or no decrease at night.
- Decrease of anthropogenic heat levels: Waste heat from urban anthropogenic activities may play an important role in the formation and the magnitude of the heat island phenomenon. Anthropogenic radiant flux in the centres of large cities can create a heat island of up to 2-3oC, during day and night. A study on the impact of the heat released by air-conditioning units in a Tokyo office district showed an increase in air temperature of 1°–2°C or more during weekdays. A review of heat island impacts and mitigation techniques proposed that decreasing waste heat from transport and industry and improved energy efficiency in buildings can result in decreased release of anthropogenic heat, with a significant impact on ambient urban temperatures.
- Use of low temperature natural sinks: Excess urban heat may be dissipated to a natural heat sink presenting much lower temperatures than the ambient environment. Such techniques are well known as passive cooling dissipation strategies and technologies and are widely used to decrease building cooling loads. Two specific heat sinks, the ground and water bodies, have attracted the attention of urban mitigation designers because of low ground temperatures and the important latent heat released during the evaporation of water.