Green Infrastructure to combat Climate Change

part of the North West Climate Change Action Plan

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Adapting Cities for Climate Change: The Role of The Green Infrastructure

Part of ASCCUE

Built Environment, Vol 3, No 1, pages 115-133

Date: 2007

Evidence type: Research

Organisation: University of Manchester

Author(s): Gill, S., Handley, J., Ennos, A., and Pauleit, S.

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Description

The urban environment has distinctive biophysical features in relation to surrounding rural areas. These include an altered energy exchange creating an urban heat island, and changes to hydrology such as increased surface runoff of rainwater. Such changes are, in part, a result of the altered surface cover of the urban area. For example less vegetated surfaces lead to a decrease in evaporative cooling, whilst an increase in surface sealing results in increased surface runoff. Climate change will amplify these distinctive features. This paper explores the important role that the green infrastructure, i.e. the greenspace network, of a city can play in adapting for climate change. It uses the conurbation of Greater Manchester as a case study site. The paper presents output from energy exchange and hydrological models showing surface temperature and surface runoff in relation to the green infrastructure under current and future climate scenarios. The implications for an adaptation strategy to climate change in the urban environment are discussed.

Key Messages

Urban greenspace has potential to help adapt cities for climate change, through the provision of cooler microclimates and reducing surface water run-off.

Urbanisation replaces vegetated surfaces (which provide shading, evaporative cooling, and rainwater interception, storage and infiltration functions) with impervious built surfaces. However, urban greenspaces provide areas within the built environment where such processes can take place.

Green infrastructure type

Non-specific green infrastructure

Climate change role/function

Managing high temperatures; Managing surface water

Modelling showed that adding 10% green cover to areas with little green, such as town centres and high density residential areas, keeps maximum surface temperatures below 1961-1990 baseline temperatures up to (but not including) the 2080s High.

This is for an extreme summer day (the 98th percentile, or the type of day expected on average twice per summer) using UKCIP02 climate scenarios (N.B. 2080s High refers to a high greenhouse gas emissions scenario in 2071-2100). The maximum surface temperature is very dependent on the proportion of green cover.

Green infrastructure type

Non-specific green infrastructure

Climate change role/function

Managing high temperatures

Modelling suggested that adding green roofs to all buildings can have a dramatic effect reducing maximum surface temperatures.

Roof greening makes the biggest difference in areas where there is a high proportion of buildings and a low proportion of green and blue cover.

Green infrastructure type

Green roofs

Climate change role/function

Managing high temperatures

Modelling highlights that if green cover in high density residential areas and town centres is reduced by 10%, maximum surface temperatures will be 7-8 Degrees C warmer by the 2080s High (compared to the 1961-1990 baseline case).

Green infrastructure type

Non-specific green infrastructure

Climate change role/function

Managing high temperatures

Without irrigation, the evaporative cooling effect of vegetation may be diminished during droughts. Trees will be important for continued shade provision, whilst water surfaces continue to provide evaporative cooling.

- Irrigation will need to be sourced sustainably to ensure that it does not conflict with other uses at a time when restrictions may be placed on its use. This could invlude storing and harvesting excess rainwater for this purpose (thereby also helping to reduce flooding), tapping into lower grade or rising aquifers, and re-using greywater.

- A pilot study undertaken by the ASCCUE project suggests that the shade provided by mature trees can keep surfaces cooler by as much as 15.6°C.

- Xeriscaping or drought-resistant plantings may also be an option in some circumstances, but they may not provide evaporative cooling in the same manner.

Green infrastructure type

Non-specific green infrastructure; Trees; Water courses; Water bodies

Climate change role/function

Managing high temperatures; Managing water supply; Managing surface water

Modelling showed that increasing green cover by 10% in residential areas reduces runoff from them by about 5% (this is for a 28mm precipitation event, an extreme event in the 2080s High).

The 28mm event is typical of the 99th percentile winter precipitation event (expected on average one day per winter) in the 2080s High (using UKCIP02 scenarios).

Green infrastructure type

Non-specific green infrastructure

Climate change role/function

Managing surface water

Modelling showed that adding green roofs to all the buildings in town centres, retail and high density residential areas significantly reduces runoff from these areas.

The effect is greatest where there is high building cover.

Green infrastructure type

Green roofs

Climate change role/function

Managing surface water

Modelling showed that greenspace is most effective at reducing surface runoff on sandy, faster infiltrating soils.

There may be a case for adapting to climate change through preserving and enhancing vegetated surfaces on such soils, for example, through the creation of Conservation Areas. Infill development could be restricted in lower density residential areas where soils have a high infiltration capacity.

Green infrastructure type

Non-specific green infrastructure

Climate change role/function

Managing surface water; Reducing soil erosion

Modelling suggests that, whilst greenspace reduces surface runoff, especially at a local level, the increase in winter precipitation brought by climate change is such that runoff increases regardless of changes to surface cover.

Thus, in order to adapt to the increased winter precipitation expected with climate change, greenspace provision will need to be considered alongside increased storage. There is significant potential to utilize sustainable urban drainage (SUDS) techniques, such as creating swales, infiltration, detention and retention ponds in parks. There is also an opportunity to store this excess water and make use of it for irrigating greenspaces in times of drought. This would ensure that their evaporative cooling effect is continued when it is most needed.

Green infrastructure type

Non-specific green infrastructure; Parks and public gardens; General amenity space; Water bodies

Climate change role/function

Managing high temperatures; Managing water supply; Managing surface water

Adaptation strategies need to preserve and enhance exiting green infrastructure, and increase it where possible, especially taking opportunities in re-structuring and new developments to create significant new spaces.

In many existing urban areas where the built form is already established, it is not feasible to create large new greenspaces. Thus, greenspace will have to be added creatively by making the most of all opportunities, for example through the greening of roofs, building façades, and railway lines, street tree planting, and converting selected streets into greenways. Priority should be given to areas where the vulnerability of the population is highest.