The Main Greenhouse Gases

Table 1 shows that concentrations of some GHGs have increased since the late 18th century. Atmospheric concentrations of carbon dioxide (CO₂) have continued to climb since the 1994 values given in the table and stood at 368,000 parts per billion volume (ppbv) in 1999 (CDIAC, 2000).

The first three GHGs shown in Table 1 - CO₂, methane (CH₄), and nitrous oxide (N₂O) - are emitted to the atmosphere by both natural and anthropogenic sources, while the last four occur only as a result of industrial processes.1

Table 1

Note: ppbv = parts per billion volume; 1 ppbv of CO₂ in the earth's atmosphere is equivalent to 2.13 million metric tons of carbon (www.cdiac.esd.ornl.gov [1], accessed on December 10, 2000.)
* No single lifetime for CO₂ can be defined because of the different rates of uptake by different sink processes.
** GWP for 100-year time horizon.
*** Includes indirect effects of tropospheic ozone production and stratospheric water vapor production.
**** Net global warming potential (i.e., including the indirect effect due to ozone depletion).
Source United Nations Environment Programme's Introduction to Climate Change, accessed at www.grida.no/climate/vital/intro.htm [2] on April 17, 2001.

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