12 May What are the issues associated with Urban Heat Islands (UHIs)?

Following on from my previous blog in which I briefly introduced the concept of urban heat islands (UHIs), in this blog I will outline the issues associated with them. I will focus on four main issues-higher energy consumption, higher greenhouse and air pollution emissions, impaired health and personal comfort and poorer water quality.

Most of us would be familiar with the elevated temperatures that characterise a typical summer day in our cities. One of the first ways that we tend to cope with these higher temperatures is by turning on air conditioning or fans in our homes and adjusting the air conditioning or HVAC system temperature control in our workplace. We may well feel justified in doing so, however many people may not give much of a thought to everyone else in our building or all of our neighbours doing the same thing at the same time. Research has shown that the demand for electricity for cooling increases approximately 2% for each 0.6°C increase in air temperature and up to 10% of the total electricity demand in a city can be tied to attempts to overcome the UHI effect.

UHIs increase both the peak and overall demand for electricity, and during extremely hot weather, the demand can overload the provider’s systems, ultimately resulting in controlled rolling brownouts or blackouts to avoid total power outages. While these aren’t pleasant, especially if you are under pressure to complete a document or presentation at work at the time, they are more palatable than complete outages across large areas of cities.

Given that UHIs increase the electricity demand in summer, and electricity suppliers typically rely on fossil fuel powered plants for the majority of the demand, it then follows that air pollutant and greenhouse gas emissions will increase. The most common pollutants emitted from power plants include Sulfur Dioxide (SO₂), Nitrogen Oxides (NO_x), Particles (PM) and Carbon Monoxide (CO). These pollutants are considered harmful to human health and are precursors for Ozone formation (smog) which residents of many cities would be familiar with. Emissions of greenhouse gases such as Carbon Dioxide (CO₂) also increase with higher throughput in fossil fuel powered plants. Ground level Ozone formation can also be sped up depending on the micro-meteorological conditions (eg. wind speed and direction) and layout/position of buildings relative to each other and nearby roads. The formation occurs as a result of a reaction between Nitrogen Oxides (NO_x) and Volatile Organic Compounds (VOCs) in the presence of sunlight and higher temperatures. Ground level Ozone concentrations can be enhanced further if the traffic on the nearby roads moves slowly for extended periods (eg. morning and afternoon peak) and the buildings that line the roads are spaced or designed such that wind isn’t able to move between them at a sufficient speed to “carry the ozone away”.

Higher daytime temperatures and air pollution concentrations can also contribute to discomfort, breathing difficulties, heat cramps/exhaustion, heat stroke and in extreme cases-heat related mortality. UHIs can also exacerbate the impacts of heat waves (unusually hot and humid weather). Children, the elderly and adults with existing health conditions are at risk from these events. The United States Centres for Disease Control and Prevention estimated that from 1979-2003, excessive heat exposure contributed to more than 8000 deaths, which was higher than the total from hurricanes, lightning, tornadoes, floods and earthquakes combined.

Higher footpath and rooftop temperatures can literally “heat up” the stormwater runoff. Reseach has shown that a footpath at 38°C can raise the temperature of rainwater from 21°C to more than 35°C. Such water becomes runoff which drains into storm drains, raising the water temperatures when it is ultimately released into streams, rivers, ponds and lakes. The metabolism and reproduction of a range of aquatic species is also affected by higher temperatures, and such temperatures can be fatal to these species.

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