FIGURE 14.10 Intrinsic infrared absorption band strengths of some potential greenhouse gases in the atmospheric "window" (from Ramanathan, 1988a, 1988b).

the Arctic, which they attribute to increased uptake of C02 by land vegetation during periods of warmer temperatures. [It is interesting that coverage by snow does not appear to terminate the exchange of C02 (Sommerfeld et al., f 993).]

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 fOO 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.

Figure 14.12 shows what has become classic data illustrating the increase in C02 concentrations at Mauna Loa, Hawaii, where continuous measurements have been made since 1958 (Keeling et al., 1995). The concentrations have risen from approximately 315 ppm in the late 1950s to 358 ppm in 1994. The cyclical pattern superimposed on the continuous increase reflects decreased C02 concentrations during summer and increased C02 during winter in response to seasonal differences in uptake during plant growth. The amplitude of the cyclical pattern in the Northern Hemisphere is largest at the most northerly locations, decreasing from ~15-20 to ~3 ppm near the equator, where plant growth is less dependent on season (Keeling et al., 1995).

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 f, f959, and January f, 1979; the match between the two curves shows that

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