Scientists at the University of Manchester have conducted a study looking at the effect global warming will have on our major cities, and say a modest increase in the number of urban parks and street trees could offset decades of predicted temperature rises. The study has calculated that a mere 10% increase in the amount of green space in cities would reduce average urban surface temperatures by as much as 4°C.
This 4°C 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 spaces collect and retain water much better than concrete, and as the water evaporates from the leaves of plants and trees the surrounding air is cooled. This process, called transpiration, is similar to the human cooling effect of perspiration.
“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 Roland Ennos, who worked on the project with Professor John Handley and Dr Susannah Gill in the School of Environment and Development.
“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.
“Such a reduction has important implications for human comfort and health within urban areas and opportunities need to be taken to increase green space cover wherever structural changes are occurring within urban areas, as well as planting street trees or developing green roofs.”
Increased green spaces in urban areas would have multiple other benefits, such as increased rainwater retention and carbon capture. Currently most of the rainwater that falls on urban areas is lost as “run-off” through storm drains, which increases the city’s sewage treatment load as well as increasing the need for irrigation. A 10% increase in green space will only have a minimal impact on precipitation capture however, as the overall climate model predicts that towards the end of this century, our summers will be hotter and drier but winters are expected to be wetter. This results in insufficient water during the time of the year when the plants need it the most, which leads to reduced transpiration; effectively cancelling out that benefit of the green spaces. Winters, on the other hand, are expected to become much wetter, producing an excess of precipitation when the trees are unable to use it to their best advantage. In order to maximize the benefits of green spaces, cities would require an infrastructure to store water in winter months to irrigate the green spaces in warmer months. Given the advantages of the cooling effects of the green spaces as well as the air purification benefits, the cost of updating urban infrastructure becomes very minimal.
Additionally, buildings could divert greywater to irrigate green roofs and nearby green spaces, which would lessen the need for city water piping changes, and provide an additional source of nutrients to the plants.
Source: Built Environment, University of Manchester
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