Trends in Trace Gas Concentrations

FIGURE 14.9 Wanning of troposphere by increased greenhouse gas concentrations (adapted from Mitchell, 1989). 

FIGURE 14-10 Intrinsic infrared absorption band strengths of some potential greenhouse gases in the atmospheric window (from Ramanathan, 1988a, 1988b). 

There are different time scales associated with the various emissions and uptake processes. Two terms that are frequently used are turnover time and response or adjustment) time. The turnover time is defined as the ratio of the mass of the gas in the atmosphere to its total rate of removal from the atmosphere. The response or adjustment time, on the other hand, is the decay time for a compound emitted into the atmosphere as an instantaneous pulse. If the removal can be described as a first-order process, i.e., the rate of removal is proportional to the concentration and the constant of proportionality remains the same, the turnover and the response times are approximately equal. However, this is not the case if the parameter relating the removal rate and the concentration is not constant. They are also not equal if the gas exchanges between several different reservoirs, as is the case for C02. For example, the turnover time for C02 in the atmosphere is about 4 years because of the rapid uptake by the oceans and terrestrial biosphere, but the response time is about 100 years because of the time it takes for C02 in the ocean surface layer to be taken up into the deep ocean. A pulse of C02 emitted into the atmosphere is expected to decay more rapidly over the first decade or so and then more gradually over the next century. 

Superimposed on the C02 concentration measurements in Fig. 14.12 are the concentrations expected if 55.9% of the cumulative C02 emissions from fossil fuel combustion and cement production remained in the atmosphere (Keeling et al., 1995). This percentage was chosen to match the atmospheric observations for the 20-year period between January 1, 1959, and January 1, 1979 the match between the two curves shows that 

FIGURE 14.11 Summary of global carbon cycle. Amount (in gigatons of C = 109 metric tons = fO1 g of C). Reservoirs are shown in parentheses, and fluxes (gigatons of C per year) are indicated by arrows. Note that the time scales associated with the various processes vary (adapted from IPCC, 1996). 

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