Parks Helping Climate in Cities
2007 - June -
British scientists looking at the effect global warming will have on major cities say a modest increase in the number of urban parks and street trees could offset decades of predicted temperature rises. The parks and trees also would help retain rainwater that otherwise drains away into streams and rivers, eventually returning to the sea.
The University of Manchester study has calculated that a mere 10 percent increase in the amount of green space in built-up centers would reduce urban surface temperatures by as much as four degrees Celsius, or 7.2 degrees Fahrenheit.
This drop in temperature, which is equivalent to the average predicted rise through global warming by the 2080s, is caused by the cooling effect of water as it evaporates into the air from leaves and vegetation through a process called transpiration.
"Green space collects and retains water much better than the built environment," explained Dr. Roland Ennos, a biomechanics expert in Manchesterís Faculty of Life Sciences and a lead researcher in the team.
"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," he said.
The research, published in the journal "Built Environment," also examined the effect increased green space would have on the amount of rainwater urban areas capture and retain.
"By the 2080s, our summers will be hotter and drier but winters are predicted to become wetter," said Ennos. "An extreme wet winterís day by the 2080s will deliver almost 50 percent more rain than is currently experienced.
The Manchester teams has calculated that these more powerful storms would increase the amount of runoff from urban areas by more than 80 percent.
Once a year winter daily storms - that is, the biggest average storm in any given year - presently produce 18 millimeters of rainfall. By the 2080s, the team calculated, once a year winter daily storms will deliver 28 mm of rainfall.
"Unfortunately," Ennos said, "increasing the amount of green space only has a limited effect in reducing runoff and so flash flooding will become an increasing problem in our cities."
While winters will be wetter, 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," Ennos said.
He worked on the study with Professor John Handley and Dr. Susannah Gill in the Manchester University's School of Environment and Development.
Taking Greater Manchester as their model, the team used Geographic Information System, GIS, mapping to build up a picture of land use in the metropolitan area.
The team then worked out the impact that increasing the amount of green space would have on the urban climate and on water retention.
They found that urban areas can be up to 12 degrees Celsius, or 21 degrees F, warmer than more rural surroundings due to the heat given off by buildings, roads and traffic, as well as reduced evaporative cooling.
An increase of 10 percent green space reduced surface temperatures enough to overcome temperature rises caused by global warming over the next 75 years, effectively "climate proofing our cities," the team said.
The researchers advised that cities increase green space cover wherever structural changes are occurring within urban areas, as well as planting street trees and developing green roofs.
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