Greenhouse gases

Global warming is caused by a number of gases, each with its own potential to trap heat in the Earth's atmosphere. This section provides information about the sources of six major greenhouse gases in the atmosphere emitted by human activities and their respective global warming potentials:

• carbon dioxide (CO₂)
  
  
• methane (CH₄)
  
  
• nitrous oxide (N₂O)
  
  
• hydrofluorocarbons (HFCs)
  
  
• perfluorocarbons (PFCs)
  
  
• sulphur hexafluoride (SF₆)
  
  

Carbon dioxide (CO₂)

Carbon dioxide (CO₂) is released in large quantities from natural processes, notably respiration by living organisms. Releases from respiration are balanced by a similar quantity taken up by photosynthesis. Other natural sources of CO₂ include volcanoes, forest fires and evaporation from seawater. Man-made CO₂ emissions are produced by combustion processes and released to the atmosphere via chimney stacks from power stations, tail-pipe emissions from motor vehicles, as well as other processes in which fuels containing carbon are burnt.

The majority of CO₂ emissions in Australia arise from the combustion of fossil fuels. The largest single contributor to CO₂ emissions is electricity generation (which relies mainly on coal), followed by road transport.

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Methane (CH₄)

Methane (CH₄) occurs naturally in the environment mainly through the decomposition of plant and animal matter by methane-producing bacteria. Methane forms when organic material is broken down in the absence of oxygen.

On a global scale, the human activities that result in the most methane emission, in descending order of importance, are:

1. livestock farming
2. extraction of fossil fuels
3. wet rice cultivation
4. biomass burning
5. landfill and
6. domestic sewage.

The agriculture sector accounts for the majority of methane emissions in Australia, from livestock emissions through enteric fermentation and the decomposition of animal wastes. Smaller quantities are generated through rice cultivation, burning of savannas to increase grass production and field burning of crop residues.

Fugitive emissions from fuels also contribute to national methane emissions mainly from the mining of coal for domestic use and export. Another significant contributor to methane emissions is the waste sector, generated from anaerobic decomposition of organic matter in landfills.

The global warming potential of methane is 21 times that of carbon dioxide (CO₂).

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Nitrous oxide (N₂O)

Agriculture accounts for the majority of nitrous oxide (N₂O) emissions in Australia. Most of this N₂O is produced from the application of fertiliser and animal wastes to agricultural soils. Burning of savannas to increase grass production, field burning of agricultural residues and manure management account for the balance.

The transport sector also contributes to emissions of N₂O. Emissions from transport have more than doubled between 1990 and 2003 with additional vehicles using three-way catalytic converters.

N₂O is an important greenhouse gas as it breaks down very slowly – over about 120 years. N₂O also exists in the atmosphere at only minute levels compared to carbon dioxide (less than 1000th the concentration of CO₂) but N₂O has a high global warming potential of about 310 times that of CO₂.

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Hydrofluorocarbons (HFCs)

Major releases of hydrofluorocarbons (HFCs) are from leakage from refrigeration equipment during operation and its end-of-life destruction. Minor releases arise from the use of HFC-containing aerosols, air conditioners and metered dose inhalers. HFC 23 is also produced as a by-product in HCFC 22 manufacture. There are no natural sources of releases to the environment.

HFCs have very high global warming potentials (100 to 3800 times that of carbon dioxide), however these are lower than the chlorofluorocarbons (CFCs) and HCFCs they replace. Due to their stability HFCs have fairly long atmospheric lifetimes (tens to hundreds of years).

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Perfluorocarbons (PFCs)

Most emissions of PFCs in Australia are generated during aluminium production. Better management of the aluminium smelting process and a production shift to smelters with lower PFC emission rates have resulted in a general decline in emissions. The semiconductor industry is a relatively minor source and there are other small emissions from the refrigeration sector (leakage during operation and at the end-of-life destruction of equipment) and fire extinguishing systems.

PFCs have extremely high global warming potentials (5000 to 10,000 times that of carbon dioxide). However, because they are only released in relatively small amounts, their contribution to global warming is minor. Due to their stability they have very long atmospheric lifetimes (thousands of years).

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Sulphur hexafluoride (SF₆)

Sulphur hexafluoride (SF₆) is a man-made chemical. The major sources of SF₆ release include leakage from electrical switchgear, from magnesium smelting processes and use in semiconductor manufacture. There are no natural sources of release to the environment.

In Australia, emissions of SF₆ from the electricity supply industry are estimated to be constant in the last few years. A small quantity has been used as a cover gas in experimental work on magnesium casting.

It has by far the highest global warming potential (23,900 times that of carbon dioxide), however it is only released in relatively small amounts. Due to its stability it has a very long atmospheric lifetime.

Source: Australia's Fourth National Communication on Climate Change – United Nations Framework Convention on Climate Change (UNFCCC) report 2005 –, European Pollutant Emission Register, the United Kingdom Pollutant Inventory and the Cooperative Research Centre for Greenhouse Accounting.

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