The frequent occurrence of extreme climate conditions is threatening the life of urban dwellers. Currently, more than 50% of the world's population lives in urban areas. By 2050 this will increase to 70%. With rapid growth of urbanisation comes rapid changes in the landscape that affect the climate and air quality in urban areas, leading to higher temperatures -- or "heat islands" -- higher emissions, and more ambient pollution. During the summer, the higher urban temperatures may lead to more frequent health problems, and actually increase the mortality rate among the most vulnerable urban dwellers including elders and less economically fortunate, for example.
Unfortunately for the urban dwellers, there is also the problem of global warming. It is widely predicted that by the end of century, the temperature of the earth will increase by 3 to 5 degrees; when combined with urban heat islands, it adds as much 8C to summertime city temperatures. Simply, human physiology is not capable to adapt to such a rapid increase in ambient temperatures in a short time of a century.
The consequences may be grave, particularly among those who cannot afford air conditioning. Of course, there is an international effort among progressive nations to stabilise the global warming to 2C by developing aggressive programmes to reduce the emission of greenhouse gases (GHG).
Even with that positive view, there remains two problems for urban dwellers: (1) the higher temperatures in urban areas may only increase by 5C (rather than 8C, still a very large change) and (2) some elected officials are not committed to participate in providing solutions to the problem of global warming.
For most cities in the world, urbanisation leads to darker surfaces (roofs and pavements) and decreased vegetation. Darker urban surfaces absorb most of incoming solar radiation, making them hot and, in turn, heating the air. Most urban surfaces need to be changed (resurfaced) every 10-30 years; old roofs are replaced with new roofs; damaged pavements will be resurfaced. At the time of installing a new roof or resurfacing a pavement, typically choices are available to select surfaces with lower solar absorptance (higher reflectance) at no incremental cost. Most roofs in North America are made of fibre-glass asphalt shingles, clay and concrete tiles, metals and membranes; for all these materials more reflective options are available at no incremental cost. Similarly, there are several options available for more reflective pavement materials.
A reflective roof, or "cool roof", will have a lower temperature than a dark roof, conducting less heat into the building, hence reducing air conditioning energy use by 10-20%. Similarly, trees shade buildings and reduce cooling energy requirements. Cool roofs and cool pavements when coupled with urban vegetation collectively cool urban areas, leading to lower urban temperatures, reduced urban pollution, and improved outdoor comfort.
Imagine that. Solutions that save energy in air-conditioned buildings, improve comfort in non-conditioned buildings, cool urban areas, reduce pollution, improve outdoor comfort, potentially reduce heat-stress related mortality, and counter global warming. All that while putting money back in the pocket of ratepayers by saving energy in reduced air conditioning bills.
The opportunity has been too good to ignore.
Practically all states in the US are encouraging (and some requiring) installation of cool roofs. For instance, California Building Energy Efficiency Standards prescribes a reflective roof for both commercial and residential buildings. Cool roofs are installed in an increasing rate even in the colder climates -- think Montreal, Toronto or New York. Even the American Society of Heating, Refrigeration and Air-Conditioning Engineers prescribes cool roofs for all buildings in many climate zones in the US. Many other countries in the world are also having effective programs to promote installation of cool roofs. In Japan for example, cities are focusing on greening existing infrastructure and buildings, setting up large installations which mist large open areas in hopes of reducing city centre temperatures.
Unfortunately, there are not similar progressive programme for cool pavements.
The Global Cool Cities Alliance (a non-profit organisation) has been working diligently with many cities and countries to encourage and assist them in developing programmes for installation of cool roofs and cool pavements. The Cool Roof Rating Council, a non-profit organisation, which celebrated its 20-year anniversary in May, provides labels for the solar reflection of roofing materials. The European Cool Roof Council is following the same path as CRRC in Europe.
We still have a long way to go. With all these efforts, only a small fractions of cities are cool surfaced. We need leaders -- our mayors, city councillors, business leaders, community groups -- to step up, roll up their sleeves and prepare for permanent transitions to cool cities. Help may not arrive from your federal leaders. - The Mark News
Hashem Akbari, PhD, is a Professor of Engineering at Concordia University.