Part of ASCCUE
Built Environment, Vol 3, No 1, pages 115-133
Evidence type: Research
Organisation: University of Manchester
Author(s): Gill, S., Handley, J., Ennos, A., and Pauleit, S.
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.
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.
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.
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.
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).
- 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.
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).
The effect is greatest where there is high building cover.
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.
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.
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.
Engineering and Physical Sciences Research Council & The UK Climate Impacts Programme