Heat impacts and heat-island effects

2.4 Heat impacts and heat-island effects

Cities will not only face risks from floods and rising sea levels, but also significant increases in temperatures and the frequency of heat waves. According to the IPCC A2 scenario, average annual temperatures projected for the period 2070-2100 indicate that urban population in European cities will feel as if the weather of the city had moved southwards. London will feel more like Bordeaux, Paris much more like Marseilles and Madrid and Rome will be as hot as North African cities (Figure 2.2). However, these changes could be even more acute if action is delayed. With atmospheric GHG stock already at around 430

ppm CO 2 , delaying action will raise GHG stock levels beyond 500 ppm in less than 25 years (Dietz & Stern, 2008). The implication is that such levels increase significantly the chances of a three degree increase in temperature.

Figure 2.2. Apparent southward shift of European cities due to climate change, 2070-2100

Source: Hiederer et al. (2009a) cited in EEA (2009), Ensuring quality of life in Europe's cities and towns, EEA Report No 5/2009, EEA, Copenhagen.

Heat waves are likely to increase in severity and duration in the future, contributing to heat mortality in both developed and developing countries. These increases will likely be more strongly felt in urban areas, as cities tend to have higher air and surface temperatures compared to rural areas. This is known as the urban heat island (UHI) effect, which is due to combined effects of structural interference with thermal radiation, low albedo of impervious surfaces and reduced transport of water into the atmosphere, known as Heat waves are likely to increase in severity and duration in the future, contributing to heat mortality in both developed and developing countries. These increases will likely be more strongly felt in urban areas, as cities tend to have higher air and surface temperatures compared to rural areas. This is known as the urban heat island (UHI) effect, which is due to combined effects of structural interference with thermal radiation, low albedo of impervious surfaces and reduced transport of water into the atmosphere, known as

decade. 22 The temperature differences between urban and surrounding rural areas can reach up to 10° C for large urban agglomerations. The built environment, including buildings and roadways that absorb sunlight

and re-radiate heat, combined with less vegetative cover to provide shade and cooling moisture, all contribute to cities being warmer and susceptible to dangerous heat events (OECD, 2009b).

The UHI effect can have negative public health effects in urban area as the impacts of heat waves can

be worse in urban areas. For example, in the 2003 European Heat Wave, a higher percentage of the causalities in France came from urban areas (Hallegatte et al., 2008). Increasing temperatures can affect mortality in a number of ways, including heat-induced mortality, famine, exacerbation of non-infectious health problems and spread of infectious disease (Ruth & Gasper in 2008a). Climate change can also exacerbate the effects of urban air pollution. UHI effects can generate changes in local atmospheric cycles. Changes in solar influx and chemical composition of near-ground air masses can cause formation of photochemical smog and reduce air circulation, which would otherwise diffuse the concentration of air pollutants (Hallegatte et al., 2008). Warmer temperatures due to climate change and UHI effects, all other things held constant, may increase concentrations of conventional air pollutants, such as ozone and acid

aerosols, as well as emissions of particulates and allergens. 23 Moreover, higher temperatures due to climate change may actually make it more difficult to control the formation of some pollutants, such as ozone,

which can exacerbate chronic respiratory diseases and cause short-term reductions in lung function. 24 One study estimates these effects in the New York metropolitan area to increase mortality rates in the 2050s due

to ozone-related acute climate change impacts alone (OECD, 2009b). 25 By aggravating heat-related climate change impacts, UHI effects are likely to increase future energy

demand (Box 2.2). 26 In the United States, for example, an estimated 3% to 8% of annual electricity use is required to offset UHI effects (Ruth & Gasper in OECD, 2008a). 27 Adaptation to rising temperatures by

increasing air condition can also further increase UHI effects. For instance, massive air conditioning has been shown to increase UHI effects up to 1 °C (Hallegatte et al., 2008).

21. Oke (1982) cited in Ruth & Gasper in OECD (2008a).

22. Voogt (2002) cited in OECD (2009b).

23. Aron & Patz (2001) cited in OECD (2009b).

24. Bernard et al, (2001) cited in OECD (2009b).

25. Knowlton et al., (2004) and Hunt & Watkiss (2008) cited in OECD (2009b).

26. McPherson (1994) cited in Ruth & Gasper in OECD (2008a).

27. Grimm et al., (2008) cited in Ruth & Gasper in OECD (2008a).

Box 2.2. The urban heat island effect

The built environment, which is concentrated in cities, reflects less sunlight, absorbs more heat and retains it longer than vegetation does. In addition, the concentration of energy use leads to a concentration of waste heat. Because of this, cities are consistently several degrees warmer than their surroundings, particularly at night. This exacerbates higher temperatures due to global warming, and participates in several feedback cycles with it.

The vicious cycle of business as usual:

A more virtuous cycle of mitigation and adaptation can be activated by policies that limit the urban heat island effect, such as green roofs, tree cover and permeable and light-coloured surfaces:

Source: OECD (2008a), Competitive Cities and Climate Change: OECD Conference Proceedings, Milan, Italy, 9-10 October 2008, OECD, Paris.