Page "United Nations Framework Convention on Climate Change" Paragraph 90

All relevant GHGs need to be considered if atmospheric GHG concentrations are to be stabilized.

Human activities result in the emission of four principal GHGs: carbon dioxide ( chemical formula: CO < sub > 2 </ sub >), methane ( CH < sub > 4 </ sub >), nitrous oxide ( N < sub > 2 </ sub > O ) and the halocarbons ( a group of gases containing fluorine, chlorine and bromine ).

Carbon dioxide is the most important of the GHGs that human activities release into the atmosphere.

At present, human activities are adding emissions of carbon dioxide into the atmosphere far faster than they are being removed.

This is analogous to a flow of water into a bathtub.

So long as the tap runs water ( analogous to the emission of carbon dioxide ) into the tub faster than water escapes through the plughole ( the natural removal of carbon dioxide from the atmosphere ), then the level of water in the tub ( analogous to the concentration of carbon dioxide in the atmosphere ) will continue to rise.

To stabilize the atmospheric concentration of carbon dioxide at a constant level, emissions would essentially need to be completely eliminated.

It is estimated that reducing carbon dioxide emissions 100 % below their present level ( i. e., complete elimination ) would lead to a slow decrease in the atmospheric concentration of CO < sub > 2 </ sub > by 40 parts-per-million ( ppm ) over the 21st century.