A modest increase in the number of urban parks and street trees in our major cities could offset decades of predicted temperature rise, a new study by researchers from the University of Manchester has revealed.
According to the team, a mere 10 per cent increase in the amount of green space in built-up centres would reduce urban surface temperatures by as much as four degrees Centigrade.
This 4°C drop in temperature is equivalent to the average predicted rise through global warming by the 2080s, and is caused by the cooling effect of water as it evaporates into the air from leaves and vegetation through a process called transpiration, said Dr Roland Ennos, the lead researcher in the team.
"Green space collects and retains water much better than the built environment. As this water evaporates from the leaves of plants and trees it cools the surrounding air in a similar way to the cooling effect of perspiration as it evaporates from our skin. Urban areas can be up to 12°C warmer than more rural surroundings due to the heat given off by buildings, roads and traffic, as well as reduced evaporative cooling, in what is commonly referred to as an ‘urban heat island’," said Dr Ennos.
For their study, the team took Greater Manchester as their model, and used Geographic Information System (GIS) mapping to build up a picture of the conurbation’s land use.
The team then worked out the impact of an increase in the amount of green space on the urban climate as well as on water retention.
The research also examined the effect an increased green space has on the amount of rainwater urban areas capture and retain; towns and cities lose a large proportion of rainwater through what is termed ‘run-off’ where precipitation quickly leaves the surface and drains away into streams and rivers, eventually returning to the sea.
"We discovered that a modest increase of 10% green space reduced surface temperatures in the urban environment by 4°C, which would overcome temperature rises caused by global warming over the next 75 years, effectively ‘climate proofing’ our cities,” said Dr Ennos.
"By the 2080s, our summers will be hotter and drier but winters are predicted to become wetter. An extreme wet winter’s day by the 2080s will deliver almost 50% more rain than is currently experienced. Based on an existing model, we have calculated that these more powerful storms would increase the amount of run-off from urban areas by more than 80%. Unfortunately, increasing the amount of green space only has a limited effect in reducing run-off and so flash flooding will become an increasing problem in our cities,” he said.
"Conversely, the warmer, drier summer months will reduce the amount of water available to plants and, during the longer droughts, this will reduce transpiration with its associated cooling effect.
"In order for the cooling effect of green spaces to work when it is most needed, cities would need to develop ways to store additional water, which could then be used to irrigate the green spaces during drier months,” he added.
Dr Ennos worked on the project with Professor John Handley and Dr Susannah Gill in the School of Environment and Development.
The findings appear in the journal Built Environment.