Sources and Trends
John R. Justus
The four most important variable greenhouse gases, whose atmospheric concentrations can be influenced by human activities, are CO2, methane, nitrous oxide, and chlorofluorocarbons-CFCs. Historically, CO2 has been the most important, but over the past several decades other gases have assumed increasing significance. Collectively, they are projected to contribute, directly, about as much to potential global warming over the next 60 years as CO2. The 1997 Kyoto Protocol would regulate three other trace gases: Hydrofluorocarbons (HFCs), Perfluorocarbons (PFCs), and Sulfur Hexafluorane (SF6).
The amount of carbon cycling from naturally occurring processes each year through the biosphere as CO2 is enormous -- some 700 billion tons. As evidenced by the general long-term stability of the global climate, the amounts generated by natural processes are thought to be about equal to the amounts absorbed by natural processes. However, human activity, primarily in the form of burning fossil fuels, is now generating an estimated 24 billion tons of CO2 per year and is disrupting that natural equilibrium. Available evidence suggests that only about half this amount is being absorbed by natural processes and the ocean. Atmospheric CO2 concentrations are now about 30% higher than they were 100 years ago. Some scientists have suggested that a significant amount of CO2 may be stored in northern latitude soils, and in temperate and tropical forests, thereby focusing greater importance on the role of human land-use practices including burning of biomass and deforestation.
Human activities are believed to be responsible for half or more of the total annual generation of methane and nitrous oxide, but there has been much less study of sources and sinks (both natural and human-related) of these gases. Nonetheless, available data show that atmospheric concentrations of methane and nitrous oxide are also growing steadily; however, the rate of growth rate of methane appears to be decreasing. Methane that results from human activities comes from cattle-raising, rice paddies, and trash dumps, and from losses to the atmosphere of natural gas during its production, transportation, and use. Natural sources include ruminant (grazing) animals, wetlands, and, to a lesser extent, termites. Most people-related nitrous oxide is a byproduct of fossil fuel combustion and industrial production of nylon, while most naturally occurring nitrous oxide emissions are from biological processes in soil.
Production of chlorofluorocarbons (CFCs) used for refrigeration, solvents, and aerosol propellants was banned in 1996, but climatic effects are expected for many more years. These gases, which may reside in the atmosphere for a century or more after release, all result from human activity. Their effects on the atmosphere, such as their capacity for depleting stratospheric ozone, were totally unforeseen when they entered commerce. IPCC scientists found CFCs to have both a direct global warming effect and an indirect cooling effect, the latter occurring because chlorine in CFCs deplete stratospheric ozone -- itself a greenhouse gas -- reducing the amount of heat radiated down into the troposphere. CFCs begin to break down on sulfate aerosols in the stratosphere and release ozone-depleting chlorine. However, taken together, the radiative forcing (global warming potential) of the CFC molecules and the climate-cooling effect of ozone depletion are believed to be similar in magnitude, opposite in sign, and, indeed, might even be offsetting in some regions where there are large emissions of pollutants containing sulfur.
Three other trace gases, HFCs, PFCs, and SF6, would be regulated under the 1997 Kyoto Protocol because of their global warming potential and for their potential growth of concentrations in the atmosphere. HFCs have been widely approved as substitutes for CFCs. PFCs are a byproduct of aluminum smelting, and have an extremely long atmospheric lifetime (up to 1,000 years). SF6 is widely used in insulation of electrical equipment.
If all these interactions among pollutants and the
atmosphere sound complicated, that is because they are. Atmospheric pollution might be
causing global warming, to the extent that it might be occurring, but some form of
atmospheric pollution might act to hold it in check, too -- if only transiently and near
highly industrialized regions.
Page last updated January 12, 2001.