- The main greenhouse gases defined within the context of the Kyoto Protocol are:

• Carbon Dioxide (CO2);
• Methane (CH4);
• Nitrous Oxide (N2O);
• Hydrofluorocarbons (HFCs);
• Perfluorocarbons (PFCs); and
• Sulphur Hexafluoride (SF6)

These gases are called greenhouse gases because they produce greenhouse effects - trapping heat that leads to increase in average Earth temperature (global warming) (refer to section on Greenhouse Gases).

The major atmospheric constituents (nitrogen, N2 and oxygen, O2) are not greenhouse gases. A greenhouse gas is one that is able to absorb infrared radiation. Only molecules that have at least three atoms can absorb infrared radiation. Homonuclear diatomic molecules such as N2 and O2 neither absorb nor emit infrared radiation, as there is no net change in the dipole moment of these molecules.

Water Vapour (H2O) is the most abundant of the atmospheric constituents of molecules containing three or more atoms. Consequently, Water Vapour (H2O) is a natural greenhouse gas. However, regarding global warming, Water Vapour (H2O) is not an issue because, unlike other ghgs, its concentration is dependent on the atmospheric temperature rather than the emissions from the Earth's surface.

The next most abundant 3-atom molecule atmospheric constituent is Carbon dioxide (CO2). CO2 in the atmosphere results from both natural and human sources and causes about the 9-26% of the greenhouse effect. Human CO2 emissions are thought to come from burning of fossil fuels for energy supply for such things as manufacturing engines, automobiles, aviation industries etc, and from deforestation.

Another ghg is methane gas resulting from industrial activities as well as from landfill operations, agricultural activities etc. Methane is believed to provide close to 4-9% greenhouse effect of the atmosphere.

The percentage of the GHGs has been increasing since the start of industrial revolution in 1750. This is demonstrated by the Intergovernmental Panel on Climate Change (IPCC) list of greenhouse gases shown below. IPCC was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to assess the "risk of human-induced climate change".

GHGs & Concentration Changes Post-Industrial Revolution (1750) (Source: IPCC)

Since CO2 is the most abundant of the ghgs, the concentration of ghgs in the atmosphere is measured as CO2 or CO2 equivalents. Works by several scientists, climatologist and related professions have been reviewed by IPCC and the following conclusions have been made:

• For all of human history until about 200 years ago, our atmosphere contained 275 parts per million of carbon dioxide. This amount was useful to keep our planet warm enough for habitation.
• However, with the industrial revolution of the 18th century, our planet now has 390 parts per million CO2 – and this number is rising by about 2 parts per million every year.
• The highest safe level for humanity, of CO2 in our atmosphere is 350 parts per million.

(Source - 350 Science)

Other Sources of Information

• Causes of Climate change - Edugreen
• Intergovernmental Panel on Climate Change (IPCC)
• United Nations Framework Convention on Climate Change
• World Meteorological Organization (WMO)
• United Nations Environmental Programme (UNEP)