As the global rate of urbanisation is increasing the cumulative effect of all the negative impacts of urbanisation like the production of pollution, production of waste heat from human activity – most notably air conditioners and internal combustion engines, the modification of the physical and chemical properties of the atmosphere, and the covering of the soil surface are becoming more clear which is named as UHI (Urban Heat Island), defined as the rise in temperature of any man-made area, resulting in a well-defined, distinct “warm island” among the “cool sea” represented by the lower temperature of the area’s nearby natural landscape.
Though heat islands may form on any rural or urban area, and at any spatial scale, cities are favoured, since their surfaces are prone to release large quantities of heat. Nonetheless, the UHI negatively impacts not only residents of urban-related environs, but also humans and their associated ecosystems located far away from cities. In fact, UHIs have been indirectly related to climate change due to their contribution to the greenhouse effect, and therefore, to global warming.
Causes of Urban Heat Island
The UHI is an ‘inadvertent’ modification of the climate, caused by changes to the form and composition of the land surface and atmosphere. When a land cover of buildings and roads replaces green space, the thermal, radiative, moisture and aerodynamic properties of the surface and the atmosphere are altered. This is because urban construction materials have different thermal (heat capacity and thermal conductivity) and radiative (reflectivity and emissivity) properties compared to surrounding rural areas, which results in more of the sun’s energy being absorbed and stored in urban compared to rural surfaces. In addition, the height of buildings and the way in which they are arranged affects the rate of escape at night of the sun’s energy absorbed during the day by building materials. The result is that urban areas cool at a much slower rate than rural areas at night, thus maintaining comparatively higher air temperatures. Urban areas also tend to be drier than their rural counterparts because of the lack of green space, a predominance of impervious surfaces and urban drainage systems, which quickly remove water from the urban surface. This combination of effects alters the energy balance of the urban environment. Consequently in urban compared to rural areas, more of the sun’s energy absorbed at the surface goes into heating the atmosphere and thus raising the air temperature than into evapotranspiration (water uptake and loss by plants), which is a cooling process.
Additional factors such as the scattered and emitted radiation from atmospheric pollutants to the urban area, the production of waste heat from air conditioning and refrigeration systems, as well from industrial processes and motorized vehicular traffic (i.e. anthropogenic heat), and the obstruction of rural air flows by the windward face of the built-up surfaces, have been recognized as additional causes of the UHI effect.
Impacts of Urban Heat Island
It has been largely demonstrated that cities with variable landscapes and climates can exhibit temperatures several degrees higher than their rural surroundings (i.e. UHI effect), a phenomenon which if increases in the future, may result in a doubling of the urban to rural thermal ratio in the following decades. Hence, assessment of the UHI and strategies to implement its mitigation are becoming increasingly important for government agencies and researchers of many affected countries.
As it would be expected, the characteristic inclination towards warming of urban surfaces is exacerbated during hot days and heat waves, which reinforces the air temperature increase, particularly in ill-ventilated outdoor spaces or inner spaces of residential and commercial buildings with poor thermal isolation. This increases the overall energy consumption for cooling (i.e. refrigeration and air-conditioning), hence increasing the energy production by power plants, which leads to higher emissions of heat-trapping greenhouse gases such as carbon dioxide, as well as other pollutants such as sulfur dioxide, carbon monoxide and particulate matter. Furthermore, the increased energy demand means more costs to citizens and goverments, which in large metropolitan areas may induce significant economic impacts. On the other hand, UHIs promote high air temperatures that contribute to formation of ozone precursors, which combined photochemically produce ground level ozone.
A direct relationship has been found between UHI intensity peaks and heat-related illness and fatalities, due to the incidence of thermal discomfort on the human cardiovascular and respiratory systems. During extreme weather events such as heat waves, the urban heat island has the potential to prevent the city from cooling down, maintaining night-time temperatures at a level that affects human health and comfort. Heatstroke, heat exhaustion, heat syncope, and heat cramps, are some of the main stress events, while a wide number of diseases may become worse, particularly in the elderly and children. In a similar way, respiratory and lung diseases have shown to be related to high ozone levels induced by heat events. Several of the above mentioned impacts have been addressed by the US. Living in high-density urban areas, such as London, may be an important risk factor for heat related mortality and morbidity. The effects of the 2003 heat wave were greatest in London in terms of the number of deaths per head of population, especially amongst the elderly (overall there were approximately 2000 all-age extra deaths in London). There is emerging evidence that UK urban populations show greater sensitivity to heat effects compared to rural regions. Many of the 2003 summer excess deaths that occurred across London during the August heat wave event may be attributable to the urban heat island effect.
The anomalous warm of the city creates relatively low air pressures that cause cooler, rural air to converge on the urban centre, thus forcing warm air to ascend (i.e. convection), which at higher altitudes condensates and precipitates. Studies carried out in several cities of the United States such as Atlanta, New York, Chicago and Washington, have shown that urban-induced precipitation and thunderstorm events are mainly initiated by the UHI. Other meteorological impacts of the UHI are associated with reductions in snowfall frequencies and intensities, as well as reductions in the diurnal and seasonal range of freezing temperatures. Lastly, high temperatures may produce physiological and phonological disturbances on ornamental plants and urban forests.
Although in wintertime the UHI can result in energy savings (i.e. winter penalty), there is a great consent among researchers that this benefit is outweighed by the detrimental effects that occur in summertime.