PUBLIC SECTOR SUSTAINABILITY CARBON CAPTURE CITY
Dr Susanne Charlesworth, Reader in Urban Physical Geography, Low Impact Buildings Grand Challenge Initiative, Coventry University
he ‘greening’ of urban areas will need to become an essential feature of development and regeneration if the UK and countries around the world are to hit the all-important 2050 carbon reduction targets. Figures show that urban areas – making up just 2.4% of the global landmass – are responsible for more than 70% of worldwide emissions of carbon dioxide. Cities are also increasingly suffering from their own brand of climate change problems which are a serious risk to health. The ‘urban heat island effect’ was first noted in 1819 in London and is peculiar to cities where, even in winter, urban areas can be several degrees warmer than the surrounding countryside. The causes of heat ‘islands’ include: lack of vegetation, transport, heating, cooling and the thermal properties of the fabric of urban structures which store and then release heat. Warm air and associated pollutants such as ozone can become trapped in these warmer areas. Energy is therefore used to cool building interiors so that people can live and work in them in comfort, but the very act of trying to reduce temperatures increases outside temperatures, as excess energy is released from the building and into the environment.
T
A feature of schemes among the UK’s local authorities so far has been a preference for tree-planting and open space as confirmation of their green credentials. But what really works in terms of climate change and carbon capture? The stage has been reached where planners and developers need to be focused on the hard facts of carbon reduction and the implementation of a more integrated range of approaches – a green infrastructure with multiple benefits. Our research at Coventry University, as part of the Low Impact Buildings Grand Challenge Initiative, has examined how greening methods linked to sustainable drainage (SUDS) has the potential to capture and store carbon the city of Coventry.
SUDS is an alternative to the conventional drainage approach which goes far beyond simply mitigating flooding, and has a role in improving water quality, greening and cooling urban
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areas, reducing the urban heat island effect and having a positive impact on human health. Conventional ‘hard’ drainage tends to concentrate on managing water quantity by gathering all the runoff water from streets and pavements into storm sewer systems which pass via gullypots, pipes and water treatment facilities into the receiving watercourse. SUDS devices include above and below ground structures, essentially ‘hard’ constructions such as ‘porous paving’ (which slowly filters water) and rainwater harvesting (storing and making use of water, for example through underground tanks and water butts) which can usefully be combined with ‘soft’ ones – using vegetation such as green walls and roofs and constructed wetlands whereby water is infiltrated into the ground or allowed to dissipate slowly through ponds and ditches. We examined carbon capture rates of SUDS greening methods from previous research, and applied them to the city of Coventry to assess the potential for carbon capture and storage over the next 15 years. Two scenarios were considered: scenario 1, optimistic and considered more achievable, although still challenging; and scenario 2, more optimistic, and reliant on a commitment to wide-ranging SUDS implementation. The calculations were made on the basis of a detailed and realistic picture of the land cover, down to the different surface coverings in gardens.
of wooded habitat, of which 14,000 trees are associated with roads. Coventry has 370 active tree preservation orders covering a further 2000 trees. Under the city’s climate change strategy, 10,000 trees were planted per year during 2007-08 and 2008-09 to absorb and store carbon.
The carbon stored in trees in residential areas of Merseyside has been estimated at more than 16 tonnes of carbon per hectare. If a standard family car releases 1 tonne carbon a year, one hectare of urban tree cover, or approximately 160 trees, could account for the yearly emissions of 16 family cars. Estimates suggest Coventry contains about 3.6km2
A study of large-scale green roofing has calculated that if the city of Detroit, USA, greened its total of nearly 15,000
PUBLIC SECTOR SUSTAINABILITY • VOLUME 1 ISSUE 3
hectares of rooftop, then potentially 55, 252 tonnes of carbon could be sequestered. Roofs cover 14% of the city of Coventry in total, but in practical terms only large ones (more than 200m2
) would
be more suitable for greening in the short to medium term. There are 3,863 large roofs in Coventry, covering 4.4km2 of the land area.
(4.5%)
The wide variety of farm ponds around the world are claimed to capture and trap more carbon than by the oceans each year. According to
Pondconservation.org a 15m2
pond could trap as much carbon as 100m2 of
trees. Ponds do this by incorporating the carbon in their bottom sediments, where they remain essentially trapped for the lifetime of the pond. Retention and detention ponds, and to a certain extent swales and constructed wetlands could therefore provide additional carbon stores. At present, ponds cover around 0.33% of the land surface of Coventry. Large car parks cover 0.27% of the
city. Mainly owned by the city council or large operators, they are likely to be the sites most suitable for early SUDS implementations. There is also potential for impermeable road sides and traffic island roundabouts (which, along with paved areas, form 8.1% of existing land cover) to be converted to vegetated surfaces. In Coventry, regeneration zones and previously developed land are prioritised for development, only then would greenfield land be released. This is in support of national government policy that at least 60% of housing should be provided on previously developed land. In the current exercise, only previously developed (brownfield) land was taken into account, since existing greenfield sites already perform a carbon mitigation function. Using only brownfield sites would result in about 10% of the city being redeveloped in the period 2006-2026, an annual change of about 0.5% of the total area. With all new developments assumed to incorporate above-ground, vegetated SUDS to maximise carbon capture, there is the potential for 8.5% of the land area of developments to be used for carbon capture (excluding features installed in private gardens, such as trees). There is no research on the carbon capture abilities specifically of porous
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