'Green roofs' could cool warming cities

Covering city buildings in vegetation – creating “green roofs” and walls – could substantially save energy by reducing the need for air conditioning on hot days, say researchers.

Green roofs and walls can cool local temperatures by between 3.6°C and 11.3°C, depending on the city, suggests their new study.

Eleftheria Alexandri and Phil Jones at the Welsh School of Architecture, at the University of Cardiff in the UK, mimicked the microclimate around and inside buildings using computer modelling. They compared local temperatures when buildings were made of bare concrete with when the concrete was covered in vegetation.

Such green surfaces are already in use – roofs that are strong enough to take the additional load can be covered with mosses, turf and even trees. In Switzerland, roofs covered in alpine plants that require little soil are becoming increasingly common. Walls can also be greened, often by climbing plants planted at ground level.

Temperature drop
The researchers compared the effects of green surfaces in nine cities around the world, including subarctic Montreal in Canada, temperate London in the UK, humid Mumbai (India), and tropical Brasília (Brazil). In all cases, they studied the month during which that city sees its hottest temperatures.

They found that green walls and roofs would cool the local climate around a building in all of the cities – and the hotter the climate, the greater the cooling effect.

If, for example, a group of buildings in Riyadh, Saudi Arabia, is entirely clad in vegetation, the gap between the buildings will become 9.1°C cooler during the day, according to the researchers' model. The gap's peak temperature of the day is brought down by 11.3°C. And in London and Montreal, the peak temperature drops by just over 4°C.

Greening the walls only, and not the roofs, results in smaller effects. Maximum temperatures in London and Montreal, for example, drop by between 2.5°C and just over 3°C between the buildings.

Green surfaces cool local temperatures in two ways. Firstly, the green surfaces absorb less heat from the sun. Hot surfaces warm the air around them, so by cooling the surface, the vegetation also affects air temperatures. Secondly, the plants also cool the air by evaporating water in a process known as evapotranspiration.

Lowering demand
Being dense regions of concrete and paved surfaces, cities and towns lose the cooling effects of vegetation. This generates what is known as the "urban heat island" effect.

Alexandri and Jones say their results suggest the urban heat island effect could be countered by introducing green roofs and walls in cities.

They point out that, other than making cities more comfortable and safer to live in, green roofs could also significantly reduce the demand for electricity – most of which is generated by burning fossil fuels and therefore contributing to man-made global warming.

In recent years, Europe and North America have been hit by severe heatwaves, the effects of which are often most extreme in cities. In 2003, a heatwave in Europe is thought to have killed 35,000 people and hundreds died this summer in Eastern Europe. Research has shown that the frequency of extremely hot days has nearly tripled in Europe since 1880.

Eliminating air-con
"In addition to the fact that they add a further insulation layer to the building, the green surfaces can decrease air conditioning demands inside the building," says Jones.

In Brasília and Hong Kong, he and Alexandri found that the need to air-condition a building during the hottest month of the year is eliminated if it is given a green roof and green walls. Buildings in these cities would normally need air conditioning in the afternoon and early evening.

In hotter cities, such as Riyadh, the number of hours when air conditioning is needed would be cut from 12 hours to just 5.

Some air conditioners still use chemicals that deplete the ozone hole and demand for air-conditioners is expected to rise as a result of global warming, so green buildings could help counter this demand.

Journal reference: Building and Environment (DOI: 10.1016/j.buildenv.2006.10.055)

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Catherine Brahic NewScientist.com
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