Several stations around the world monitor this and the concentrations of the other greenhouse gases. 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. 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 186,000 years ago. The ice record shows that for many thousands of years the CO 2 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. The increase in CO 2 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 1994, CO 2 accounts for 73 per cent of Australia's greenhouse gas emissions.