Endnotes
1 CFC-12 and HCFC-22 along with all other CFCs and HCFCs, are being phased out under the Montreal Protocol of 1987 due to their role in depleting stratospheric ozone. Other industrially produced gases - perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), perfluoromethane (CF4) and sulphur hexafluoride (SF6) - absorb energy at wavelengths at which naturally occurring atmospheric gases do not. Thus, prior to the advent of the industrially produced gases, energy radiated at these wavelengths from the earths surface was able to escape directly back into space and did not contribute to the warming effect.
2 Since gases remain in the atmosphere for different lengths of time (Table 1, fifth column), a time period over which the cumulative effect of the gas is to be measured must be specified. The Intergovernmental Panel on Climate Change (IPCC) developed three alternative sets of GWPs based on 20-, 100-, and 500-year time frames. Estimations of GWPs require considerable knowledge of atmospheric processes and are dependent on relative concentrations of gases. As knowledge increases and relative concentrations change, GWPs will be modified. At present, GWPs are considered accurate within +/- 35%.
3 Table 4 does not show emissions for the other three groups of gases covered by the Kyoto Protocol: PFCs, HFCs, and SF6.
4 Annex I comprises all Annex B countries shown in Table 4 plus Turkey and Belarus.
5 Extensive records of the amount of fossil fuels used for energy are available and the correlations between fossil fuel consumption and resultant CO2 emissions are well documented. This contrasts markedly from the situation for CH4 and N2O. Attempts to document sources and to establish correlations between activities and emissions for these gases are relatively recent. There are relatively little data, and models to correlate emissions with activities are limited. In general, emissions from controlled industrial processes are known to a much greater degree of accuracy than emissions from other sources. The majority of CH4 and N2O emissions are from landfills and agriculture rather than from controlled industrial processes, while the majority of CO2 emissions are from industrial processes. The result is that, taken in total, CO2 emissions estimates are accurate to within ± 10 percent for much of the world while emissions estimates for CH4 and N2O are considered to be accurate to within ± 50 percent.
6 By comparing the IPCC and UN projections, it can be seen that population projections from the IPCC A2 scenario are higher than the most recent high UN projections. The A2 scenario population projections are based on the International Institute for Applied Systems Analysis (IIASA) 1996 high population projections. Population projections are extremely sensitive to small changes in human fertility rates, particularly over the long term. In recent years fertility rates have declined more quickly than anticipated in a number of developing countries and countries in transition. These declines, coupled with mortality increases due to AIDS, have resulted in downward revisions of previous population projections. The population projections of the B2 scenario assume that world average fertility rates will stabilize roughly at replacement levels (approximately 2.1 children per woman) around 2050. The A1 and B1 scenarios represent fertility rates stabilizing somewhat below replacement levels, but not at rates as low as the low UN projection that assumes an average fertility rate of approximately 1.6 children per woman.
7 U.S. forests cover 737 million acres or approximately 33 percent of U.S. land and contain some 78,000 MMTC, approximately 4 percent of the carbon stored in forests worldwide (USDA, 1992). Approximately half of the carbon stored in U.S. forests is in the soil. In addition, pastureland, rangeland, grassland, and agricultural soils also store and release carbon. Carbon stored by management of these lands is believed to amount to less than 10 percent of the carbon storage that can be stored by management of forests.