paving using vegetation. However, in structure they are not unlike turf and there are some studies relating to use of turf grass in the USA, pointing to “significant” potential of between 320 and 780 kg per hectare per year, with greater potential dependent on the type of turf used. There is therefore some potential for vegetated porous paving although this surface is installed far less, and in smaller areas than hard paving systems. Hard porous paving is constructed in part with cement. Standard Portland cement releases up to 7% of worldwide CO2
of the material made, 1 tonne of CO2
such that a tonne could absorb up to 0.4 tonnes of CO2
using magnesium-based cement, buildings could become net absorbers of CO2
. While there doesn’t
appear to be any move towards using such material in the manufacture of hard paving, there is an opportunity to research using CO2
-absorbing porous surfaces in cities of the future.
Widespread use of vegetation-based SUDS in new developments provides the second highest carbon storage by 2025 (under scenario 1, 35.7%, and under scenario 2, 31.8% of the value of techniques investigated). Green roofs provided the largest potential (scenario 1
and for every tonne . By
= 53.3%; scenario 2 = 55.7%), largely due to the scale of possible implementation based on the number of roofs in the city. All other retrofit options evaluated contributed less than 13% together in both scenarios 1 and 2. Ponds and wetlands proved the most valuable of these less effective options. The city council’s aim to plant 10,000 trees each year is seen to have the least mitigation potential in terms of carbon absorption alone (0.3%), although there will be other, adaptive, benefits from tree planting, such as reduction of the urban heat island effect and potential for shading, as well as an increase in biodiversity. Even with a doubling of the number of trees to 20,000 per year, trees on public land remain the lowest mitigation option (0.3%). Calculations from the research suggest that the cumulative effect of mitigation actions is important, so, therefore early implementation is also important as it can ensure a greater role for a vegetated SUDS to play in carbon mitigation over the medium to long term. The SUDS approach is flexible and provides multiple benefits, one of those being that a green infrastructure made up of vegetated devices can offer carbon
capture in cities attempting to mitigate the changes brought about by increased greenhouse gas emissions. Whilst the actual amounts sequestered and stored are not large, this ability is in addition to the other benefits SUDS offers. New build projects continue to be given priority in legislation. But since the large majority of the current building stock will still be standing in 2050, retrofitting of property – despite the perceived high cost and difficulty – needs more attention and investment. Green roofs, for example, offer great potential for both new build and retrofit, and unlike approaches like constructed wetlands, are an option for mass adoption in concentrated urban areas.
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PUBLIC SECTOR SUSTAINABILITY • VOLUME 1 ISSUE 3 19
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