Creating more parks and green spaces in urban areas could cool cities by up to 4°C - possibly enough to offset the warming from climate change - say researchers.

"If you look at infrared maps of cities, the woodland areas are 12°C cooler than city centres with no trees," says Roland Ennos at Manchester University in the UK, who carried out the study with colleagues.

Ennos's team used the city of Manchester as a template for their study. With two computer models - one to calculate changes in temperature and one to calculate changes in rainwater run-off - they investigated how the urban climate would change if world greenhouse-gas emissions continue to rise at the current rate.

"We found that the temperature in Manchester will go up by 4°C by 2080 if the amount of green area remains unchanged," says Ennos.

Green houses effect

But, by altering the amounts of green cover in the city, the researchers found that the temperature rise could effectively be cancelled out. "Adding 10% of green cover could reduce surface temperatures by 4°C by 2080," Ennos told New Scientist.

Vegetation cools local temperatures when the water it has absorbed is evaporated from its leaves - much like the cooling effect of perspiration. The researchers say that the increased greenery would not have to involve building new parks. For instance, green roofing - roll-out strips of soil planted with succulents, commonly used in Germany - would have a similar effect.

"Even if a fraction of city's buildings had green roofs, this could have a big impact," says Ennos.

Chris Huntingford of the Centre for Ecology and Hydrology in the UK says that unless people undergo significant lifestyle change, heat stress is likely to be the biggest health issue facing city dwellers in the UK as a result of climate change.

Heat of the night

"These new results are dramatic and, if correct, indicate that relatively little alteration to our cities would be needed to combat the adverse effects of future temperature extremes," he told New Scientist.

But he cautions that the findings are so dramatic that they need to be verified through further studies. He suggests this could be done by altering the amount of parkland in an existing city "to see if temperatures can be changed now".

Huntingford also points out the importance to human health of finding out if night-time temperatures would also be affected by the additional greenery. "This period is critical in allowing the body to rest from the effects of excessive day-time warmth," he says.

Studies have suggested that a great number of deaths in France during the 2003 heatwave were due to the fact that people were not given any night-time respite from the high day-time temperatures.