Download Low Carbon Cities: Ecological processes in the eThekwini Open

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
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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’).