Analysis of these bubbles shows that there is a close relationship between carbon dioxide levels and world temperatures. The ice core record also shows that carbon dioxide concentrations are now higher than at any time in the past 160,000 years. The most recently trapped air from ice cores drilled in the Australian Antarctic Territory dates from the late 1970s. Ruminant animals such as cows and sheep rely on bacteria to decompose the cellulose they eat. These bacteria generate methane.
Bacteria also play a role in releasing methane from rice paddies.
Other sources of methane are land fills, burning vegetation, coal mines and natural gas fields. The growth rate of methane has slowed during the past 20 years. In 1992, methane concentrations briefly stabilised but have since been growing at about 8 ppbv per year. Nitrous oxide is an important greenhouse gas as it does not easily break down. It has an atmospheric lifetime of more than a century.