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<br /> 360 <br /> 350 '" <br /> 340 '" <br /> 330 ,.. <br />'i' 320 '" <br />ll. 310 ,.. <br />e:. 300 .. <br /> ... <br />w 290 no <br />0 280 ... <br />X <br />0 270 <br />0 260 - <br />z 250 <br />0 240 <br />lD <br />a: 230 <br />.0: <br />0 220 - <br /> 210 <br /> 200 <br /> 190 <br /> 180 <br /> .'60 -'40 -120 -100 -80 -60 -40 -20 0 <br /> THOUSANDS OF YEARS TO PRESENT <br /> <br /> <br />Figure 2. - Concentration of CO2 in the atmosphere over <br />the past 160,000 years. <br /> <br />International Geophysical Year. The concentration varies <br />with both time and place. The continuous measurements <br />since 1958 at Mauna Loa, Hawaii, show small diurnal <br />variations, annual variations, which extend about 6 ppm <br />from peak to trough, arid irregular fluctuations, some of <br />which can be related to large-scale weather disturbances, <br />particularly the El Nino events. All of these are <br />superimposed on a rising trendline, which was at 315 ppm <br />in 1958 and reached 352 ppm in 1989 (Keeling et al. <br />1989). Land vegetation, which draws down the <br />atmospheric supply of CO2 in the Northern Hemisphere <br />spring and releases CO2, in the fall, accounts for about 90 <br />percent of the annual fluctuation (Sellers and McCarthy <br />1990). The seasonal warming of the North Atlantic and <br />North Pacific Oceans during Northern Hemisphere spring <br />and summer tends to raise the atmospheric CO2 <br />concentration by degassing from the sea surface, but the <br />biological processes predominate. <br /> <br />Mixing of air throughout the global atmosphere, <br />including transequatorial exchanges, is accomplished on a <br />time scale of about a year. As a result, the concentration <br />of CO2 is fairly uniform over the whole world, although <br />minor variations exist. Ground-level concentrations over <br />the Northern Hemisphere now exceed those over the <br />Southern Hemisphere by almost 3 ppm, and the difference <br />increased from 1958 to 1989. The size of the excess is <br />correlated with annual global consumption of fossil fuels, <br />which is concentrated in the industrialized countries of the <br />Northern Hemisphere, although the correlation is not exact <br />(Keeling et al. 1989). <br /> <br />The present rate of increase in the mean global <br />concentration of CO2 in the atmosphere far exceeds <br />anything detected in the geologic record (fig. 2). <br />Comparison of the sizes of the different carbon reservoirs <br />(table I) shows that the atmospheric concentration could <br />continue to rise for some time as a result of fossil fuel <br />consumption. Nordhaus and Yohe (1983) projected future <br />levels under various assumptions about economic trends <br />and associated fuel consumption; their median scenario <br />suggests a doubling of the atmospheric CO2 concentration <br />from the 300 ppm that prevailed at the beginning of the <br />twentieth century to 600 ppm around the year 2070 (fig. <br />3). Because of increases in other greenhouse gases, <br />notably methane (CH4) and chlDrofluorocarbons (CFCs), <br /> <br /> 2000 <br />E / /440 <br />a. <br />a. <br />Z <br />0 <br />fi <br />a: 1000 <br />I- 910 <br />z <br />w <br />u 770 <br />z <br />0 <br />u <br />~ <br />a: 580 <br />w <br />J: 540 <br />ll. <br />III 500 <br />0 <br />~ <br />~ <br /> <br /> <br /> <br />Figure 3. - Observed and predicted concentrations of <br />CO2 from 1975 to 2100 for indicated <br />percentile runs of Nordhaus and Yohe <br />(1983). The numbers on the right-hand side <br />indicate concentrations in the year 2100. <br /> <br />the earth will experience the equivalence in radiative terms <br />of a CO2 doubling somewhat earlier, say around 2050. <br /> <br />3. PHYSICAL BASIS FOR THE GREENHOUSE <br />EFFECT <br /> <br />Apart from a negligible amount of heat generated by <br />radioactivity in its crust and other even smaller sources, all <br />of the earth's heat comes from the sun. The incoming <br />solar energy at the top of the atmosphere varies only by <br />0.1 percent or so; averaged over all latitudes, times of day, <br />and seasons, it amounts to about 342 watts per square <br />meter (yY m'~. Figure 4, which is based on Ramanathan et <br />al. (1989), shows the disposition of the solar energy. <br />About 105 W m'2 are reflected or scattered back into space, <br />so the albedo of the earth is calculated as 105/342, or 0.31. <br />The absorbed energy is transformed into sensible heat and <br />latent heat. Eventually, some of it appears as the kinetic <br />energy of the winds and much of it is reradiated from the <br />surface or by the atmosphere as long-wave (infrared) <br />radiation.2 <br /> <br />Application of the Stefan-Boltzmann Law shows that <br />the average outgoing IR at the top of the atmosphere, 237 <br />W m'2, is equivalent to that from a black-body radiator at <br />255 oK. The 33 DC difference between that number and <br />the actual, average, surface temperature of the earth, which <br />is about 288 OK, is due to the atmosphere's greenhouse <br /> <br />21n accordance with the Glossary of Meteorology, <br />the abbreviation IR will be used to mean both "infrared" <br />and "infrared radiation." <br /> <br />110 <br /> <br />---0: <br />