Methane

Although its atmospheric abundance is less than 0.5 percent that of CO2, on a molecule by molecule basis, a molecule of CH4 is approximately 50 times more effective as a greenhouse gas in the current atmosphere than CO2. When this is combined with the large increase in its atmospheric concentration, methane becomes the second most important greenhouse gas of concern to climate change. Based on analyses of ice cores, the concentration of methane has more than doubled since pre-industrial times. In the year 1997, the globally averaged atmospheric concentration of methane was about 1.73 ppmv (Dlugokencky et al., 1998).

Continuous monitoring of methane trends in ambient air from 1979 to 1989 indicates that concentrations had been increasing at an average of about 16 ppb (—1 percent per year). During much of the 1990s, the rate of increase in methane appeared to be declining. Although the cause of the longer-term global decline in methane growth is still not well understood, it may be that much of the earlier rapid increase in methane emissions from agricultural sources is now slowing down. However, in 1998 the CH4 growth rate increased to about 10 ppb per year (Figure 1.3b). There are some indications that this increase in the growth rate may be due to a response of emissions from wetlands in the Northern Hemisphere responding to warm temperatures. In 1999, the growth rate decreased to about 5 ppb per year (Dlugokencky, NOAA CMDL, private communication, 2000).

Methane emissions come from a number of different sources, both natural and anthropogenic. One type of human related emission arises from biogenic sources from agriculture and waste disposal, including enteric fermentation, animal and human wastes, rice paddies, biomass burning, and landfills. Emissions also result from fossil fuel-related methane sources such as natural gas loss, coal mining, and the petroleum industry. Methane is emitted naturally by wetlands, termites, other wild ruminants, oceans, and by hydrates. Based on recent estimates, current human-related biogenic and fossil fuel-related sources of methane are approximately 275 and 100 TgCH4/yr while total natural sources are around 160 TgCH4/yr.

1.3.3 Sulfuric and other aerosols

Emissions of sulfur dioxide and other gases can result in the formation of aerosols that can affect climate. Aerosols affect climate directly by absorption and scattering of solar radiation and indirectly by acting as cloud condensation nuclei (CCN). A variety of analyses indicate that human-related emissions of sulfur, and the resulting increased sulfuric acid concentrations in the troposphere, may be cooling the Northern Hemisphere sufficiently to compensate for a sizable fraction of the warming expected from greenhouse gases. As the lifetime in the lower atmosphere of these aerosols is typically only about one week, the large continual emissions of the aerosol precursors largely determines the impact of the aerosols on climate. Large volcanic explosions can influence climate for periods of one to three years through emissions of sulfur dioxide, and the resulting sulfate aerosols, into the lower stratosphere.

Figure 1.3 Globally averaged atmospheric CH4 concentrations (ppbv) derived from NOAA Climate Monitoring Diagnostic Laboratory air sampling sites (Dlugokencky et al., 1998). The solid line is a deseasonalized trend curve fitted to the data. The dashed line is a model (that accounts for CH4 emissions and loss in the atmosphere) estimated calculated trend that is fitted to the globally average values. (b) Atmospheric CH4 instantaneous growth rate (ppbv/year) which is the derivative with respect to the trend curve shown in (a).

Figure 1.3 Globally averaged atmospheric CH4 concentrations (ppbv) derived from NOAA Climate Monitoring Diagnostic Laboratory air sampling sites (Dlugokencky et al., 1998). The solid line is a deseasonalized trend curve fitted to the data. The dashed line is a model (that accounts for CH4 emissions and loss in the atmosphere) estimated calculated trend that is fitted to the globally average values. (b) Atmospheric CH4 instantaneous growth rate (ppbv/year) which is the derivative with respect to the trend curve shown in (a).

Over half of the sulfur dioxide, SO2, emitted into the atmosphere comes from human-related sources, mainly from the combustion of coal and other fossil fuels. Most of these emissions occur in the Northern Hemisphere. Analyses indicate that anthropogenic emissions have grown dramatically during this century. Other SO2 sources come from biomass burning, from volcanic eruptions, and from the oxidation of di-methyl sulfide (DMS) and hydrogen sulfide (H2S) in the atmosphere. DMS and H2S are primarily produced in the oceans. Atmospheric SO2 has a lifetime of less than a week, leading to formation of sulfuric acid and eventually to sulfate aerosol particles. Gas-to-particle conversion can also occur in cloud droplets; when precipitation doesn't soon occur, the evaporation of such droplets can then leave sulfate aerosols in the atmosphere.

Other aerosols are also important to climate. Of particular interest are the carbonaceous aerosols or black carbon (soot) aerosols that are absorbers of solar and infrared radiation, and can thus add to the concerns about warming.

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