Urban greening can help climate

Plants transpire to cool cities, say scientists

By Rachel Sixsmith Increasing the number of urban parks and street trees can reduce city temperatures by as much as 4°C, claim scientists from the University of Manchester. Researchers for the Adaptation Strategies for Climate Change in the Urban Environment project found increasing green space in cities by 10 per cent could reduce surface temperatures by 4°C. The cooling effect would be caused by transpiration — when water evaporates into the air from leaves and vegetation. Biomechanics expert Dr Roland Ennos said: “Opportunities need to be taken to increase green space cover in urban areas, plant street trees or develop green roofs.” The team made its calculations by mapping out a picture of Greater Manchester’s land use and working out the impact that more green space would have on its climate. But it concluded cities would need to find ways of storing rain water. Ennos said: “Increasing the amount of green space only has a limited effect in reducing run-off so flash flooding will become an increasing problem in our cities. “Conversely, the warmer summer will reduce the amount of water available and this will reduce transpiration with its associated cooling effect.” In March, RHS horticultural advisory services head Guy Barter told Association for Public Service Excellence conference delegates: “Rain is going to evaporate that much faster so there’ll be less water available for the urban areas, which means trees will not be able to transpire.”

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