Low Carbon Cities:Ecological processes in the

eThekwini Open Space system

Bob Scholes

4 October 2010

Carbon stores Glenday (2007)

• EThekwini open space has 6.6 + 0.2 MtC in an area of 64037 ha– Uncertainty range is optimistic– Estimate is feasible

• Annual uptake rate is 8.4 - 9.8 x 103 tC/y over the whole open space– could reach ~ 64 x 103 tC/y, and be sustained at this

level for a decade or two. – Even at this elevated level, the rate is small compared

to the eThekwini C emission, 4300 x 103t C/y

Afforestation in the city

• Real and lasting carbon benefits can accrue from increasing the cover and density of trees in the urban landscape, but also in vegetated by non-treed landscapes such as grasslands;

• Only a fraction of these benefits qualify under current UNFCCC accounting rules;

• The costs of verification may make such projects unprofitable if the purpose is sale of carbon credits into formal markets;

• The full set of ecosystem service benefits of a greener, better managed natural area system are often the main reason for undertaking these activities, with carbon sinks as a co-benefit.

Wetlands

• Wetlands are very important for ecosystem services, but have almost no potential as climate mitigation interventions – Emissions of CH4 almost exactly cancel

uptakes of CO2

C sinks and building• embodied energy of building materials is typically the equivalent of

many years of energy-efficient operation.– Low: locally-sourced natural materials such as grass, wood, mud and

stone– Middle: fired brick and tile intermediate and high-energy materials such

as glass, – High: plastic and metal

• Carbon content of the materials themselves– High for wood and thatch

• Insulation and thermal mass properties, which assist with the energy efficiency of the building over its lifetime. Foams, some types of particle boards and double glazing have good insulating properties. Mud, stone and brick have good thermal mass properties, stabilising the temperature fluctuations through the day and thus saving both cooling and heating energy.

Roofs, roads, parking lots and gardens

• If the vegetation is lighter (higher albedo) than the surface, it reflects sun energy back to the sky acting like a negative greenhouse gas. The reverse is true if the vegetation is darker than the surface it covers, which may be the case for unpainted concrete, aluminised surfaces, unpainted galvanised iron, light-coloured painted surfaces or beach sand. Green vegetation albedo is about 13%, which is higher than tar,dark painted surfaces, some bricked surfaces and deep open water.

• Transpiration from the green leaves cools the air, reducing the need for air conditioning. This comes at a water cost - a problem if water is scarce – and is less effective in humid environments such as EThekwini.

• A thick layer of soil on a roof adds to its insulation and thermal mass properties.

• The moist inside of the leaves acts as a trap for pollutants (notably ozone and its precursors) and the rough foliage traps dust. This, like transpirational cooling works best with leafy plants with high stomatal conductance, as opposed, for instance, to water saving, sparse succulent gardens (‘xeroscaping’).