2.0 THE GREENHOUSE GASES

The Earth's atmosphere primarily consists of oxygen and nitrogen, but neither play any significant role in what is called the greenhouse effect [see Figure 1.0], as both are essentially transparent to terrestrial radiation. The gases currently known to cause the greenhouse effect include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs), and two CFC substitutes, hydrochlorofluorocarbon (HCFC-22), and perfluoromethane (CF4). Of these carbon dioxide, methane, nitrous oxide and ozone are the naturally occuring greenhouse gases. Certain human activities however add to the levels of most of these naturally occurring gases. Carbon dioxide is responsible for over half the enhancement of the greenhouse effect. The greenhouse effect is primarily a function of the concentration of water vapour, carbon dioxide, and other trace gases in the atmosphere that absorb the terrestrial radiation leaving the surface of the Earth, and act like a blanket over the earth's surface, keeping it warmer than it would otherwise be. Changes in the atmospheric concentration of these gases can alter the balance of energy transfers between the atmosphere, space, land, and the oceans. A guage of these changes is called the radiative forcing, which is a measure of changes in the energy available to the Earth-atmosphere system. Holding everything constant, increases in greenhouse gas concentrations in the atmosphere will produce positive radiative forcing (i.e net increase in the absorption of energy by the Earth).

The global carbon cycle consists of the various stocks of carbon in the earth system and the flow of carbon between these stocks. Carbon in the form of inorganic and organic compounds, notably CO2, is cycled between the atmosphere, oceans, and terrestrial biosphere. The largest natural exchanges occur between the atmosphere and terrestrial biota and between the atmosphere and ocean surface waters.Figure 2.0 illustrates the carbon stocks and its flows associated with the different ecosystems. The carbon stocks associated with the different ecosystems are stored in above ground and below ground biomass, dead organic matter, and soils. Carbon is withdrawn from the atmosphere through photosynthesis, and returned by oxidation process that include plant respiration, decomposition, and combustion. Carbon is also transferred within ecosystems and to other locations. Both natural processes and human activities affect carbon flows. Mitigation activities directed at one ecosystem component generally have additional effects influencing carbon accumulation in, or loss from, other components.

Figure 2.0: The global carbon cycle, showing the carbon stocks in reservoirs (in Gt C = 1015 g C) and carbon flows (in Gt C yr-1) relevant to the anthropogenic perturbation as annual averages over the decade from 1989 to 1998. Net ocean uptake of the anthropogenic perturbation equals the net air-sea input plus runoff minus sedimentation (source: Schimel et al., 1996)

Table 1: Greenhouse gases affected by human activities

Note : (a) Part per million by volume, (b) Part per billion by volume, (c) Part per trillion by volume(d) No single lifetime by CO2 can be defined because of the different rate of uptake by different sink processes.(e) This has been defined as an adjustment time that takes into account the indirect effect of methane on its own lifetime.** estimate from 1992-1993 data.[Source: IPCC 1996a]