Warmer and wetter winters with more frequent and more intense storms; hotter and drier summers; and more extreme events such as heatwaves and torrential downpours - this is what the UK Climate Impacts Programme (UKCIP) scenarios are predicting with high confidence, although regional variations are expected. Average annual temperatures in the UK may increase by between 1 and 5°C by the 2080s, with summer temperatures expected to increase more than winter temperatures. And by the 2080s winters may be up to 30% wetter and summers up to 50% drier, depending on the region and the emission scenario.
Climate change is already with us. Its impact will be most significant where people are concentrated – in our towns and cities. But urbanisation already has marked effects on the environment. Urbanisation affects surface temperature (with risks associated with urban heat island effects), hydrology (with risks of flooding), carbon storage and sequestration and biodiversity. These deleterious effects of urbanisation are likely to increase significantly under future climate change unless climate-ameliorating measures are taken. The value of green open spaces in cities for offsetting, or even reversing, climate change and urbanisation effects is widely appreciated.
Urban greenspaces provide areas within the built environment where important processes can take place, including shading, evaporative cooling, and rainwater interception, storage and infiltration.
Thus urban greenspaces can mitigate urban heat island effects during hot summer days by reducing the air temperature in cities, and can improve public health and well-being. Several studies have announced promising figures regarding the potential for greenspaces to ameliorate climatic conditions locally. In Japan, comparison of current temperatures in city parks and surrounding urban areas has demonstrated differences of 2.5 to 4°C. According to other studies, large numbers of trees and urban parks could reduce local air temperature by 0.5 to 5°C. Simulation studies on Greater Manchester report that an increase of 10% in urban green cover in high-density residential areas (green roofs) would decrease the expected maximum surface temperature in the 2080s by around 2.5°C.
Similarly, some green structures have the potential to be sustainable urban drainage systems (SUDS) - such as green roofs or walls and permeable paving – which can mitigate risks associated with flooding. Their benefits are:
- Lower risk of flooding because run-off is reduced
- Reduced likelihood of flash flooding following heavy rainfall by slowing the rate of release
- Cheaper and easier maintenance than other traditional systems
- Improved water quality.
For potentially contaminated brownfield land, greenspace establishment is a sustainable solution to mitigate or break pollutant linkages that may develop during future rainfall and flooding events, for example by reducing soil erosion.
Green solutions for climate amelioration are needed at different spatial scales - neighbourhood, city, conurbation and catchment. This requires an integrated planning approach.
An increase in vegetated surfaces has the potential to ameliorate climate and urbanisation effects in the following ways:
- Various types of greenspace can be established: from traditional solutions such as grasslands, wetlands, parks, street trees and woodlands to more innovative or technical solutions such as vegetal walls, green roofs and grassed swales.
- Plant species will need to be selected for their resistance to stress factors, including both future climate conditions and potential contamination, to ensure solutions are suitable and sustainable in the long term. In general, deep-rooting varieties that can resist both flooding in winter and drought periods in summer will be selected, although the effects of tree roots on buildings and utilities must be considered.
- Green infrastructure with drainage potential will help mitigate the risk of future flooding. For example, surface run-off from green roofs, green walls, and permeable/porous paving can be captured into a series of retention ponds located in ‘storm water wetlands’.
Research will be central in:
- Quantifying the impacts of specific solutions in different cities
- Ensuring proposed solutions can solve both urban heat island and flooding issues.
This is generally made possible by the use of models:
- Energy-balance models to estimate future maximum surface temperatures (for urban heat islands)
- Surface run-off models (for flooding risks).
To simulate the impact of future climate change on urbanisation effects if green infrastructures are employed, modelling studies can be based on the climate-change scenarios published by the UKCIP. Based on climate-change scenarios the microclimate in urban areas can be estimated, as well as the benefits of various green infrastructures on surface temperature and flooding.
Adapting cities to climate change
Manchester University Centre for Urban Regional Ecology (CURE) conducted a study on the potential of green infrastructures to help cities adapt to climate change. Great Manchester was selected as the case study site because it represents a large conurbation (1300 km2, population 2.5 million).
The study focused on two urban issues that are likely to be enhanced under future climate conditions: the urban heat island effect, and flooding. The Greater Manchester urban environment was characterised in terms of urban morphology types (woodland, high-density residential, disused and derelict land, etc.) and their respective surface covers. Two models were used: an energy-balance model to estimate maximum surface temperatures (urban heat island issues); and a surface run-off model to estimate run-off rates (flooding risks). They used the UKCIP02 low- and high-emissions scenarios to scope out the effects of climate change on green interventions.
The interventions tested were both increasing and reducing green cover by 10%.
The maximum surface temperature expected in Manchester town centre by the 2080s is between 33.2 and 35.5°C, depending on the emissions scenario. They found that green interventions have a dramatic effect on maximum surface temperature, but there are variations depending on the urban morphology type (the following results are for high-density residential morphology types).
Urban heat island effects
Modelling results suggested that:
- Adding 10% green cover to areas with little green cover would reduce expected maximum surface temperature by 2.5°C by the 2080s
- Removing 10% green cover would increase expected maximum surface temperature by 7°C by the 2080s.
Roof greening made the greatest difference in urban morphology types with a high building proportion and a low evaporating fraction (town centres, manufacturing areas, high-density residential, etc.).
Modelling results suggested that:
- Adding 10% green cover would reduce run-off from a 28 mm rainfall event by 4.9% by the 2080s
- Adding 10% tree cover would reduce run-off from a 28 mm rainfall event by 5.7% by the 2080s
- Increasing green or tree cover by 10% in residential areas could not keep future run-off at or below levels expected for the current climate; run-off will still be approximately 65% higher than in the current climate by the 2080s (worst scenario).
However, adding green roofs to all the buildings in town centres, retail and high-density residential areas significantly reduces run-off from these areas (reduction of 11.8 to14.1% by the 2080s for a 28 mm rainfall event).
This modelling work suggests that green infrastructure offers significant potential for moderating urban heat island and flooding effects, and that modelling can help quantify interventions to plan future developments more successfully.
Gill, S.E., Handley, J.F., Ennos, A.R. and Pauleit, S. (2007). Adapting cities for climate change: the role of the green infrastructure. Built Environment 33: 115-133.
Forest Research is available to provide advice and recommendations on:
- How to prepare climate-change scenarios for your specific locations
- How to design and develop sustainable solutions for your cities to adapt to climate change
- What plant species should be used (including impacts of climate change on vegetation).
Their environmental modellers conduct modelling studies including:
- Resilience and adaptation of trees and other plant species to future climate conditions
- Development of urban microclimate scenarios
- Modelling microclimatic effects of vegetation (with a range of green infrastructure designs), with respect to urban heat islands and urban flooding.
CIRIA (undated). Sustainable Urban Drainage Systems: Promoting Good Practice - A CIRIA Initiative. CIRIA, London.
Goode, D. (2006). Green Infrastructure (PDF-172K). Report to the Royal Commission on Environmental Pollution. Available online.
POST (2006). Adapting to Climate Change in the UK (PDF-123K). Postnote No. 267. Parliamentary Office of Science and Technology, London.
POST (2007). Urban Flooding (PDF-116K). Postnote No. 289. Parliamentary Office of Science and Technology, London.
Publications from Adaptation Strategies for Climate Change in the Urban Environment (ASCCUE).
- Evaporative cooling
- Means of temperature reduction operating on the principle that water absorbs latent heat from the surrounding air when it evaporates.
- Grassed swale
- A graded and engineered landscape feature appearing as a linear, shallow, open channel with trapezoid or parabolic shape. The swale contains flood-tolerant, erosion-resistant plants. The design of grassed swales promotes the conveyance of storm water at a slower, controlled rate and acts as a filter medium, removing pollutants and allowing infiltration of storm water. When properly designed to accommodate a predetermined storm event volume, grassed swales show significant improvements over traditional drainage ditches in both slowing and cleaning water.
- Urban heat island
- The characteristic warmth of both the atmosphere and surfaces in cities (urban areas) compared with their non-urbanised surroundings. Heat islands are caused by urbanisation - buildings, roads and paved surfaces store heat during the day and then release it slowly during the evening, keeping urban areas hotter than surrounding areas. On hot summer days, this effect (summer urban heat island) leads to overheated conditions (heatwaves), and can be responsible for many deaths in the UK.