Albedo Effect

'Summers by 2050 will be 1.5 - 3.50C hotter...in central London the urban heat island currently adds 5 - 6oC to summer night time temperatures and will intensify in the future.'

London Climate Change Partnership, London's Warming. The Impacts of Climate Change on London www.london.gov.uk

Urban areas have a higher average temperature than surrounding rural areas; this Difference in temperatures is called the urban heat island effect [UHIE] and is caused by the reduction in green space through urbanization and the large amount of hard surfaces that provide high thermal mass. The dark surfaces of roofs exacerbate the UHIE by absorbing summer heat and reflecting it back to the atmosphere during the night. As the UHIE increases there is greater need for air conditioning and thus energy, as buildings heat up.

Furthermore increases in the UHIE effect can lead to increased levels of air pollution in cities, which combined with an increase in temperatures can exacerbate health problems, especially in the old, young and vulnerable.
The Green Roof effect of evaporation and evapotranspiration acts to cool air. Furthermore by providing a cooler surface at roof level the green roof reduces the need for air conditioning during periods of higher than normal temperatures. The Combined effect is to reduce the UHIE.

One study recent study concludes that 'Sustainable Urban Futures' demand the use of vegetation, with the greatest benefit to be had when both green roofs and green walls are used, enveloping the whole building fabric in vegetation. Such an approach could lead to an 84%reduction in cooling demand.

A modeling scenario undertaken in New York by the New York Heat Island Initiative determined that providing 50% green roof cover within the metropolitan area would lead to an average 0.1- 0.80C reduction in surface temperatures. For every degree reduction in the UHIE roughly 495million KWh of energy could be saved.

http://www.ccsr.columbia.edu/cig/greenroofs/index.html

In response to the modeling New York has recently launch a tax rebate for building owners who install green roofs within the metropolitan district.

http://www.nyc.gov/html/dof/html/property/property_tax_reduc_taxreductions.shtml

In Toronto Canada it has been estimated that the effect of greening the cities rooftops would lead to a 0.5 - 20C decrease in the UHIE. A reduction of this magnitude would, lead to indirect energy savings citywide from reduced energy for cooling of $12million, equivalent to2.37kWh/m2 per year and that this would reduce peak demand at rate of 0.0023kW/m2.

http://www.toronto.ca/greenroofs/index.htm

In Japan, many cities suffer from the severe effects of the UHIE. The average annual temperature in Tokyo has risen 30C in the last century. This is four times higher than what could be explained due to the effects of global warming.

http://www.gnla.ca/library.htm

'The Tokyo based Organisation for Landscape and Urban greenery Technology Development estimates that if half of the roofs in the city were planted with gardens, daytime temperatures in summer would fall by 0.84ºC, would save 110 million Yen on air conditioning costs.'

http://metropolis.co.jp/tokyo/485/feature.asp

The city has introduced policies that require green roofs to be installed on 20% of all new flat surfaces on government buildings and 10% of all flat roofs on private dwellings.