Scientists have been regularly measuring the air's carbon dioxide (CO2) content since about 1960. Several stations around the world monitor this and the concentrations of the other greenhouse gases. But how can we find out the CO2 concentrations that existed before this regular monitoring started? Evidence comes from a variety of sources, but one of the most straightforward involves taking ice samples from the polar ice caps. Ice in Antarctica builds up from the compression of each year's snowfalls. By drilling down into the ice cap (which is up to 4 kilometres thick), scientists can collect core samples of the annual snowfall going back over thousands of years. The deeper you go, the older the ice. This ice contains air bubbles, captured when the snow fell and sealed in ice since that time. Scientists can take a slice of a core and analyse the air trapped within the bubbles. This ice record can give us information about the air from as far back as 740,000 years ago. The ice record shows that for many thousands of years the CO2 concentration slowly fluctuated. It remained steady during the last few thousand years, but began to increase about the year 1800 – as did methane and nitrous oxide. (Human activity has caused no significant direct increase in water vapour.) Human activity causes increases in greenhouse gases Carbon dioxide The increase in CO2 is partly caused by fossil-fuel burning, cement manufacture, land clearing, forest harvesting and changes in agricultural practice. According to the National Greenhouse Gas Inventory of 2005, CO2 accounts for 74.3 per cent of Australia's greenhouse gas emissions. Methane Biomass burning, increased agricultural production in rice paddy fields, digestive fermentation (burps and farts) from cattle and other livestock, and leaks from natural gas pipelines and coal mines have lead to a steady increase in methane emissions. Methane production accounts for 20.2 per cent of Australia's greenhouse gas emissions. Nitrous oxide There are many small sources of this gas both natural and manufactured that are difficult to quantify. The main sources created by human activity are from agriculture (especially the development of pastures in tropical regions), biomass burning and number of industrial processes. Nitrous oxide production accounts for 4.3 per cent of Australia's greenhouse gas emissions. Halocarbons Chlorofluorocarbons (CFCs) are halocarbons which were widely used for propellants, refrigerants, and foaming agents. Their use rapidly increased after their invention in the 1930s. The realisation that they were responsible for ozone depletion in the stratosphere has led to their phasing out under the 1987 Montreal Protocol. Perfluorocarbons, another type of halocarbon, are produced during aluminium production. Halocarbon production accounts for 1.1 per cent of Australia's greenhouse gas emissions. (Despite their small concentrations, halocarbons have a significant greenhouse effect.) Related site
Other boxes Box 3. Global warming and climate change Box 4. International deliberations Box 5. Australia's policy response
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