Sources, properties and emission trends of the important greenhouse gases

The buildup of carbon dioxide, methane, nitroux oxide, ozone, and halogenated compounds (CFCs, HCFCs) in the atmosphere intensifies the Earth's naturally-occurring greenhouse effect.

rial activities have led to the buildup of CO2 and other greenhouse gases, including methane, nitrous oxide, ozone, and the halogenated compounds (including the CFCs).

Carbon dioxide (C02)

The buildup of carbon dioxide accounts for about two-thirds of the human sources of excess greenhouse warming from long-lived gases.[5] (Figure 1 shows the history of global CO2 emissions.)

Each year, the burning of fossil fuels and biomass, along with other activities such as cement making, releases over seven billion metric tons of carbon to the atmosphere in the form of CO2, raising the atmospheric concentration of the gas by about half a percent annually. Figure 2 shows CO2 emissions for the United States for the period 1980 to 1996.

Figure 3 shows the source of emissions for 1994 by sector and fuel.

Note that electric-power production and transportation are by far the largest and fastest-growing sources. With current global emissions of some 6.3 billion metric tons per year (1995), fossil fuel combustion is the best quantified and the largest source of CO2 from human activity. Since preindustrial times, the global CO2 concentration has increased almost 30 percent, from 280 parts per million (ppm) to about 360 ppm today. (See Figure 4.) Carbon dioxide stays in the atmosphere 50 to 200 years.[6]

Methane

Methane (the main component of natural gas) has both natural and human sources. Natural sources include peat bogs, termites, swamps, and other wetlands. Human sources include rice paddies, domestic animals, landfills, biomass burning, and the production and burning of fossil fuels. Fossil fuels account for about 20 percent of the total, including leakage from natural gas pipelines, oil wells, and coal seams.[7] About 60 to 80 percent of all methane emissions are of human origin. Atmospheric methane concentrations have been increasing in the atmosphere at about 0.6 percent per year[8] and have more than doubled from pre-industrial levels. Methane stays in the atmosphere 12 to 17 years and accounts for about 20 percent of greenhouse warming from human sources.

Nitrous oxide (N20)

The principal sources of nitrous oxide from human activities are the application of nitrogen fertilizers to agricultural lands, the burning of biomass and fuels, and industrial chemical production. Human sources are about a third of total global emissions. Nitrous oxide is a very stable molecule (lifetime of 120 years) and also contributes to stratospheric ozone depletion.[9] This gas accounts for about 5 percent of the human sources of greenhouse warming.

Halogenated compounds

Halogenated compounds contain fluorine, chlorine, bromine, or iodine, and many such compounds are strong greenhouse gases. Halocarbons containing carbon and either chlorine or bromine, such as the CFCs, halons, and HCFCs, also cause stratospheric ozone depletion. CFCs are made for use as aerosol propellants, blowing agents for plastic foams, refrigerants, and solvents. They are particularly damaging as greenhouse gases because of their long lives in the atmosphere and their effectiveness in trapping heat, approximately 20,000 30,000 times that of CO2. Hydrofluorocarbons (HFCs) -- substitutes for some CFC applications -- are also potent greenhouse gases.

Estimates of the net warming effect of CFCs have recently been reduced. This reduction stems from the cooling effect from the CFC destruction of ozone (a greenhouse gas) in the lower stratosphere.[11] CFCs and some other ozone-depleting compounds have been phased out in the industrialized countries by the Montreal Protocol on Substances that Deplete the Ozone Layer but production for essential uses and to meet developing-country needs continues.

Also included in the halocarbon family are perfluorocarbons (PFCs) and sulfur hexafluoride (SF6), which have lifetimes measured in thousands of years. PFCs are a byproduct of aluminum making and are also manufactured for use in the semiconductor industry. They are exceedingly powerful greenhouse gases. Though presently low, concentrations of the PFCs and SF6 are increasing. Because of their long lifetimes, these compounds represent an essentially irreversible threat to the climate.[12] The halocarbons contribute about 10 percent of the warming from human sources.

Ozone in the lower atmosphere

Ozone (O3) in the upper atmosphere (stratosphere) acts as a crucial filter that protects life on the planet from many of the harmful effects of ultraviolet radiation. However, ozone in the lower atmosphere (troposphere) is both a greenhouse gas and, in many polluted urban areas, a threat to public health. Nitrogen oxides (NOx) and volatile organic compounds (VOCs) are both necessary for ozone formation. VOCs and NOx have both natural as well as man-made sources, but in urban areas, which are prone to ozone formation, combustion is a major source of both.[10] As NOx are formed, they combine with VOCs in the presence of sunlight to form tropospheric ozone. Background tropospheric ozone concentrations, the evidence suggests, have approximately doubled in the northern hemisphere since preindustrial times.