Future Anthropogenic Emissions of CO₂

We've looked at atmospheric carbon dioxide from the past up to the present, and now it might be informative for us to look to the future. Anthropogenic emissions of carbon dioxide to the atmosphere in the future, which occur mainly due to burning of fossil fuels, are very closely tied to economic development. Strong economic activity in developed countries and modernization of developing countries both relate closely to the production of electrical energy, use of fossil-fuel burning machines, and the use of cement. These agents of growth all produce carbon dioxide as byproducts. Presently the anthropogenic production of carbon dioxide increases about 2% per year. We can use different economic growth rates to project future anthropogenic production of carbon dioxide, as is shown in Figure 7a. Here it can be seen that by continuing on our present rate of growth, atmospheric carbon dioxide levels will reach 600 ppm by about 2050. Reducing our growth in production to zero (keeping emissions constant at current levels) reduces the level to 440 ppm by 2050. If our goal were to limit atmospheric levels to less than 400 ppm, we would have to reduce emissions by 2% per year. If we turned off all fossil-fuel burning power plants, stopped using automobiles, and eliminated all other anthropogenic emissions, the atmospheric carbon dioxide concentration would return to about the 1980 level by 2050 (Figure 7b).

This figure reveals the dilemma that we face if we seek to limit the growth of atmospheric carbon dioxide. We seem destined to have a very high level of carbon dioxide in the earth's atmosphere, compared with levels of the last 160,000 years, by the middle of the next century.

Let's briefly summarize what we have learned about the carbon cycle to this point. During the period of 1860-1994 there has been about 241 gigatons of carbon emitted to the atmosphere by fossil fuel combustion, and the rate in 1990 was 6.0 gigatons of carbon into the atmosphere due to deforestation, and the present rate is about 1.6 gigatons per year. Apparently deforestation is on the increase again in South America because of the renewed demand for farm land. Atmospheric carbon dioxide concentrations have increased from about 275 ppm in the middle of the last century to the present value of about 370 ppm in 2001. We understand the basic features of the carbon cycle quite well. It is possible to construct quantitative models to use as a guide in projecting the CO₂ concentrations. The uncertainties of the projections of likely future CO₂ changes on the basis of a given emission scenario are considerably less than those of the emission scenarios themselves. We cannot project our economic growth very well, but if we could, we probably could project our CO₂ levels quite accurately. It will require some reduction in emissions growth rate to keep from doubling our atmospheric carbon dioxide level over the next 50 years.

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