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Mauna Loa Observatory. Hawaii Monthly Average Cartoon Dioxide Concentration

Mauna Loa Observatory. Hawaii Monthly Average Cartoon Dioxide Concentration

FIGURE 1.3. Atmospheric CO2 concentrations observed at Mauna Loa, Hawaii (19.5° N, 155.6° W). Note the seasonal cycle superimposed on the long-term trend. The trend is due to anthropogenic emissions. The seasonal cycle is thought to be driven by the terrestrial biosphere: net consumption of CO2 by biomass in the summertime (due to abundance of light and heat) and net respiration in wintertime.

FIGURE 1.3. Atmospheric CO2 concentrations observed at Mauna Loa, Hawaii (19.5° N, 155.6° W). Note the seasonal cycle superimposed on the long-term trend. The trend is due to anthropogenic emissions. The seasonal cycle is thought to be driven by the terrestrial biosphere: net consumption of CO2 by biomass in the summertime (due to abundance of light and heat) and net respiration in wintertime.

at which Earth radiates energy back out to space (see Chapter 2). The CO2 concentration in the atmosphere is controlled by such processes as photosynthesis and respiration, exchange between the ocean and the atmosphere, and, in the modern world, anthropogenic activities.

It is important to note that the proportion of some constituents (especially chemically or physically active species, such as H2O) is variable in space and time. Moreover, several crucially important constituents (e.g., H2O, CO2, O3) are present in very small concentrations, and so are sensitive to anthropogenic activity. For example, Fig. 1.3 shows the CO2 concentration measured at the Hawaiian island of Mauna Loa. Atmospheric CO2 concentration has risen from 315 ppm to 380 ppm over the past 50 years. Preindustrial levels of CO2 were around 280 ppm; it is thought that over the course of Earth's history, CO2 levels have greatly fluctuated. Atmospheric CO2 concentrations were probably markedly different in warm as opposed to cold periods of Earth's climate. For example, at the last glacial maximum 20,000 years ago, CO2 concentrations are thought to have been around 180 ppm. Reconstructions of atmospheric CO2 levels over geologic time suggest that CO2 concentrations were perhaps five times the present level 220 million years ago, and perhaps 20 times today's concentration between 450 and 550 million years ago, as we shall see in Section 12.3 and Fig. 12.14. If the curve shown in Fig. 1.3 continues its exponential rise, then by the end of the century, CO2 concentrations will have reached levels—perhaps 600 ppm—not seen since 30 million years ago, a period of great warmth in Earth history.

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