Page - 35 - in Biodiversity and Health in the Face of Climate Change

35 some of the biodiversity metrics underpinning how ecosystems influence health out- comes examined in the previous sections. The Urban Heat Island (UHI) effect is the well-recognised phenomenon whereby cities and towns are often much warmer than surrounding rural areas, particularly at night after calm, sunny days (Oke 1982). The effect can exacerbate the potential for human exposures during periods of high temperature (Wilby 2003). The UHI effect  is primarily generated as a result of the physical properties of urban materials, their structure and  – to a lesser extent  – their use, e.g. through anthropogenic heat emissions (Smith et  al. 2009). Built materials have different radiative and thermal storage properties compared to natural surfaces, with the former tending to absorb direct and diffuse short-wave radiation during the day and later re-radiate stored energy back to the atmosphere as long-wave radiation. Where there is higher sky- view factor (the amount of sky which is visible from a point on the ground) stored energy can be re-radiated quickly. However, geometries in cities are complex and low sky-view factor tends to inhibit the loss of long-wave radiation leading to a heating of overlying air during periods of low wind speeds and/or due to inhibited wind flows (Lindberg 2007). In urban areas there is also a relative lack of vegetation and water, which provide cooling functions through evapotranspiration and surface shading  in the case of large vegetation stands (Sproken-Smith and Oke 1999). Due to their cooling properties, large areas of vegetation and water within cities play an important role in offsetting urban temperatures, with even modest amounts having an effect (Bowler et  al. 2010). An analysis of temperature records for Manchester has shown that UHI intensi- ties have been increasing over time (Levermore et  al. 2017). If trends continue to the end of the century, increases will be similar to those expected with climate change (medium emissions scenario). Increased UHI intensities are likely to be associated with more severe heat-wave events in the future. In the north west of England, a heat- wave is defined as a period of time where the maximum temperature exceeds 30  °C for 2  days with a minimum temperature of ≥  15  °C in the intervening night. Using this definition, the number of heat waves is not expected to increase dramatically by the 2050s (according to the central estimate of the UKCIP09 projections (high emis- sions scenario)) (Cavan 2010). However, estimates based on climate projections do not explicitly consider the additional UHI effect on temperatures (Jenkins et  al. 2009). Even without the UHI effect being considered, the number of days exceeding 30  °C is expected to be around three per  annum by the 2050s (Cavan 2010). Monitoring of the UHI carried out between May and August 2010 demonstrated that the UHI effect can add up to 6  °C (day) and 8  °C (night) in some locations in Greater Manchester (Cheung 2011). The conurbation could also see up to a 3.4  °C (2.4  °C) increase in the temperature of the warmest summer day (night), according to the central estimate of the UKCIP09 projections (high emissions scenario) with these highest increases expected for the upland Pennine fringe (Cavan 2010). Archival studies show that high temperatures in Manchester, even those that could be considered relatively modest elsewhere, are associated with increased hos- pital admissions rates and excess mortality. In July 2006, an estimated 140 excess deaths in the region were associated with elevated temperatures which reached a 2 Biodiversity, Physical Health and  Climate Change: A  Synthesis of  Recent Evidence