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Human activity is increasing atmospheric concentrations of existing greenhouse gases such as carbon dioxide and methane and adding new gases such as chlorofluorocarbons (CFCs). Many experts believe that these gases are linked to global warming and climate change by way of an enhanced greenhouse effect - the process by which water vapour, carbon dioxide and other gases form a blanket around the earth, trapping heat. Projections indicate annual average temperatures in Australia could be 0.4-2.0 degrees celcius (°C) higher by 2030 and 1.0-6.0 °C higher by 2070 (CSIRO 2004a). These estimates are based on world emissions scenarios produced by the Intergovernmental Panel on Climate Change.
As amounts of greenhouse gas emissions continue to increase they are being reflected in the findings from atmospheric measuring stations. In the past 25 years a steady increase in the level of carbon dioxide has been recorded at the Cape Grim Baseline Air Pollution Station in Tasmania (graph 24.33) (CSIRO 2004b).
Carbon dioxide is the highest contributor to total net greenhouse gas emissions (68.8% in 2002, down from 70.4% in 1990). A large increase in nitrous oxide emissions has led to this gas increasing its share of total emissions from 5% to 7% between 1990 and 2002. Perfluorocarbons and sulphur hexafluride were the only greenhouse gases to record a decrease in emissions over the period (14% lower in 2002 than in 1990). Carbon dioxide increased by 2.3% and methane by 1.9% over this period (table 24.34) (AGO 2004).
Greenhouse gas emissions and the Australian economy
The Australian economy is highly dependent on energy consumption. The combined energy sectors were the largest source of Australia's greenhouse gas emissions, comprising 68.9% (371.4 Mt CO2-e) of emissions. This proportion is less than many other countries, however, due to the relatively large contribution from the agriculture (19.6%) and land use, land use change and forestry sectors (3.4%) in Australia (graph 24.35) (AGO 2004). The stationary energy sector is the highest contributor to total emissions (48.6%) and recorded the biggest increase (34%) between 1990 and 2002. The transport sector is the third largest contributor (14.7%) and it shows the second highest increase (27.7%) over the 12-year period. The agriculture sector had an 11% increase over the period. The land use change and forestry sector is the only sector that shows a considerable decrease (80.5%) in emissions. Carbon removals through the growth of forest (21.8 Mt CO2-e) have contributed to this change. Forest and grassland conversion sub sector is the major contributor in this reduction by reducing its emissions in 1990 (120.4 Mt CO2-e) by 65% in 2002 (42.1 Mt CO2-e). Emissions from other sectors have changed very little over the period (AGO 2004). More information on energy use in the Australian economy is provided in Chapter 17 Energy.
24.35 GREENHOUSE GAS EMISSIONS (CO2-e), By sector
Vegetation plays an important role in reducing the level of greenhouse gases in the atmosphere, as trees and other plants absorb carbon dioxide from the air and store it as carbon. Under ideal conditions, one million hectares of new forest could absorb about 25 Mt of carbon dioxide a year, which would lower Australia's present carbon dioxide production by about 9% (CSIRO 2004c). The forestry sector (including commercial forestry) is an emitter (source) and an absorber (sink) for carbon dioxide. Emissions from the forestry sector are affected by both timber harvest and forest re-growth rates. In 2002 carbon removals through the growth of forests were 21.8 Mt CO2-e with forest and grassland conversion causing 42.1 Mt of emissions. Land use change and forestry provided a total of 3.4% of total net national emissions (AGO 2004).