Atmospheric greenhouse gases

In the Kyoto Protocol four gases: carbon dioxide, methane, nitrous oxide and sulfur hexafluoride; and two groups of gases: perfluorocarbons and hydrofluorocarbons, have been identified as agents that can cause global warming because of human activities. The concentrations are building up in the atmosphere, and these gases have the right physical and chemical properties to be able to cause global warming. In this paper we discuss the cycles of these gases in the Earth's atmosphere and the mechanisms by which they act to cause global warming. Included is the current understanding of the global budgets and the changes in concentrations over recent times and over the last 100-200 years. A criterion is developed that can be used to classify greenhouse gases into three groups - gases that can cause substantial global warming, gases that are individually not very potent, but together may significantly affect the global climate and gases that, for one reason or another, are not likely to cause significant global warming, even when taken collectively, unless there are huge changes in the rates of emissions due to human activities.