Keywords:Carbon emissions, Urban sprawl, Land use, Land form, Green urbanism, Eco-city, Population density, Urban design, Climate change
Soils are a source of greenhouse gases when surface vegetation is destroyed and soil organic matter decomposes into carbon dioxide. When new houses are constructed, surface vegetation is destroyed and soil organic matter decays into the greenhouse gas, carbon dioxide.
Surface soils are sealed by houses, driveways, paths, paving, swimming pools, sheds, roads and concrete/asphalt footpaths. Plants do not grow in sealed soils and plants do not synthesise organic matter from atmospheric carbon dioxide during photosynthesis. There is no plants to add organic matter to sealed soils.
Vigorous plant growth in a garden will help to store carbon in soils.
In this report calculations are made of carbon dioxide emissions from vegetation and soil organic matter during house construction. A typical house block and surroundings releases 12,000 kg of carbon when virgin forests are cleared for housing in the Sydney region. (see appendix for calculation)
Many variables exist in new housing developments and this report uses typical values of carbon content of forests and soils in Australian ecosystems. (1) The size of new house blocks is decreasing in Sydney and the size of new houses is increasing. The size of new house blocks have decreased from 800 sq m in 1993 to 750 sq m in 2003. Many new freestanding houses are now 2 stories and back yard gardens are very small.
When buildings and roads cover the soil surface plants cannot grow and the surface soil is sealed.
Impervious soils do not allow rain water to enter the surface soil and penetrate deep into the subsurface. Drainage engineers measure the extent of impervious soils when calculating the surface runoff.
Area of sealed soils preventing plant growth are generally the same as the area of impervious spoils. Plant growth does not occur when soils are impervious with no water infiltration.
An estimation of the carbon dioxide released into the atmosphere from the land when building a house is calculated by multiplying the carbon in vegetation and soil by the affected area of land.
Dead plants are not all converted immediately into carbon dioxide after the land is cleared. If dead plants are burnt, carbon dioxide is immediately released into the atmosphere. Dead plants will slowly decompose into carbon dioxide with the aid of microorganisms. Some wood may be used to make furniture and other long lasting products. In this report it is assumed all of the vegetation is burnt or decomposes into carbon dioxide.
Soil organic matter will slowly decompose into carbon dioxide. If the soil is anaerobic, methane is produced. Methane is a more powerful greenhouse gas than carbon dioxide. Parts of the soil organic matter will be resistant to decomposition and humus could remain in soils for many years and even hundreds of years. In this report it is assumed all soil organic matter decomposes into carbon dioxide.
Carbon emissions are calculated from after the clearing of virgin forest. Many housing developments are on agricultural land where the forest was cleared many years ago and a high portion of the carbon emissions occurred many years ago. Emissions are lower in areas of lower rainfall and poorer soils where growth of virgin forest is lower.
Road surfaces sealed by tar and concrete contributes a significant amount carbon dioxide to atmosphere from soils. Area sealed by streets and footpaths can equal 1/3 of total area of residential suburbs. (2)
Generally the surface area of streets increase when house blocks are larger. In many new developments streets are narow and curving with many cul-de-sacs producing denser developments with more house blocks per hectare and less road surface per house. Simple economic forces generate more house blocks and less roads per hectare.
Recent developments have a higher density and a smaller area of roads and this is a positive factor reducing carbon emissions. Provision of public transport often rsults in less land uptake than roads and expressways. Parking spaces also seals soils.
High densities and high rise developments will reduce the land uptake by an expanding population.
High density urban developments have a number of factors helping to reduce energy use and pollution from transport. Trip length is reduced and public transport is more viable. It is easy to walk to shops and public transport.
On individual house blocks, the area of soil sealed will depend on size of driveways, extent of concrete paths and paving and swimming pools.
In water sensitive urban design many techniques are used to reduce surface water runoff by increasing the area of permeable soils. Often the increase of permeable soils will be a positive factor reducing the extent of sealed soils and carbon emissions.
The emission of 12,000 kg of carbon during land clearing for new houses is significant.
In comparison an average house emits each year 2,000 kg carbon. The one-off emission of 12,000 kg is equivalent to 6 years of emissions from households.
The extent of land uptake and the clearing of vegetation should be taken into account for different types of developments. Higher density housing estates will result in lower carbon per head of population. Construction of units several stories high reduces land uptake and carbon emissions per resident.
Urban sprawl contributes to many environmental problems. The clearing of vegetation and the formation of sealed soils contributes to greenhouse gas emissions. Urban sprawl is a symptom of poor planning in the inner suburbs. The population of the inner suburbs should increase so as to reduce pressures on the growth of urban sprawl.
(1) National Greenhouse Gas Inventory 1988 and 1990, Australian Government 1994.
(2) Port Jackson South Stormwater Management Plan. EPA, NSW 1999.
|Area of house block||500 square meters (sqm)|
|Area of House||200sqm|
|Paving, paths sheds||30sqm|
|Total sealed area house block||300sqm|
|Area surrounding house block|
|Sealed Roads||300sqm sealed|
|Permeable Open spaces||100sqm permeable|
|Permeable Drainage, schools, electricity etc||100sqm permeable|
|Area surrounding house||500sqm|
|House block + area surrounding house =||1,000sqm|
|Sealed House block||300sqm|
|Number people per house 2.5|
|model house 500 + surrounding area 500sqm|
|= 10 houses per hectare|
|or 25 people / hectare|
Carbon content of existing vegetation 20 kg carbon / sq m
|House block||300 sqm|
|x 20 kg|
|= 6,000 kg carbon|
|Roads||300 sq m|
|x 20 kg|
|= 6,000 kg|
|Total carbon emissions||= 12,000 kg per house block + roads|
(Tonnes carbon per hectare)
|Total C||Above Ground C||Soil C||% Soil Carbon|
|Woodland and scrub||93||23||70||75%|
Carbon stored in typical forest
Assumed 200 tonnes carbon stored / hectare of house development
= 20 kg carbon stored / sq m
60% carbon in soil
120 tonnes C / ha in soil
80 tonnes C / ha above ground
1 hectare = 10,000sqm
200 tonnes carbon stored / hectare = 200 / 10,000 tonnes carbon stored / sqm
= 1,000 x 200 / 10,000 kg C / sqm
= 20 kg carbon stored / sqm
C (Carbon) = 12
O (Oxygen) = 16
Molecular Weight of CO2 = 12 + (16 x 2) = 44
Carbon content of CO2 = 27.3%
To Calculate C from Carbon Dioxide x by 0.273
To Calculate CO2 from Carbon x by 3.66
Soil organic matter contains 58% carbon
to convert carbon to organic matter x 1.72
to convert organic matter into carbon x 0.58
Bulk density of a typical dry soil equals 1.4
This value depends on soil structure and soil compaction density of wet soil depends on moisture content
Soil carbon and soil organic matter are essentially the same. All organic matter contains carbon. Nearly all soil carbon is organic except for a few salts eg Calcium carbonate and charcoal.
Organic matter and the carbon cycle in soils are major factors in maintaing soil fertility and health. (Soil organic matter and Carbon cycle in soils). Carbon storage in soils is a greenhouse gas sink and is a viable method for the reduction of atmospheric carbon dioxide. (Carbon storage in parks and gardens). Soil organic matter is essential in preventing soil degregation and erosion. (Peak soil).
Population density in the outer urban fringe of Sydney can be as low as 16 people per hectare while population density in some inner suburbs can reach 40 people per hectare. The New South Government is attempting to increase the population of the inner suburbs. This policy has gained an active opposition from many residents and local councils. It is essential to reduce urban sprawl and the production of greenhouse gases from soil organic matter to help obtain a sustainable Sydney.