Atmospheric trace gases that have sources related to human activities
and are of significance to global environmental change.

	Carbon Dioxide CO2	Methane CH4	Nitrous Oxide N2O	Chlorofluorocarbons CFCs	Tropospheric Ozone O3	Carbon Monoxide CO	Water Vapor H2O
Greenhouse Role	Heating	Heating	Heating	Heating	Heating	None	Heats in air; cools in clouds
Effect on Stratospheric Ozone Layer	Can increase or decrease	Can increase or decrease	Can increase or decrease	Decrease	None	None	Decrease
Principal Anthropogenic Sources	Fossil fuels; deforestation	Rice culture; cattle; fossil fuels; biomass burning	Fertilizer; land use conversion	Refrigerants; aerosols; industrial processes	Hydrocarbons (with NOx); biomass burning	Fossil fuels; biomass burning; deforestation	Land conversion; irrigation
Principal Natural Sources	Balanced in nature	Wetlands	Soils; tropical forests	None	Hydrocarbons	Hydrocarbon oxidation	Evapo-transpiration
Atmospheric Lifetime	50 - 200 years	10 years	150 years	60 - 100 years	Weeks to months	Months	Days
Present Atmospheric Concentration in Parts per Billion by Volume at Surface	356,000	1709	310	CFC-11: 0.28 CFC-12: 0.53	20 - 40	100	3,000 - 6,000 in stratosphere
Preindustrial Concentration (1750 - 1800) at Surface	280,000	790	288	0	10	40 - 80	Unknown
Annual Rate of Increase (1980s)	0.5%	0.9%	0.3%	4%	0.5 - 2.0%	0.7 - 1.0%	Unknown
Relative Contribution to the Anthropogenic Greenhouse Effect	60%	15%	5%	12%	8%	None	Unknown

Mackenzie, F.T. and J.A. Mackenzie (1995) Our changing planet. Prentice-Hall, Upper Saddle River, NJ, p 288-307.
(After Graedel and Crutzen, 1990, and UCAR/OIES, 1991c.)