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Carbon Dioxide and the Climate


Gilbert N. Plass, James Rodger Fleming, Gavin Schmidt

What is the reason for the recent temperature rise that is found throughout the world? Will this trend toward warmer climates continue for some time? The carbon dioxide theory may provide the answer. We have discussed the burning of fossil fuels which is adding more than 6 x 109 tons per year of carbon dioxide to the atmosphere. If all of this extra carbon dioxide remains in the atmosphere, the average temperature is increasing at the rate of 1.1 degrees per century from this cause. Since 1900 a careful study of world temperature records shows that the average temperature has been increasing at roughly this rate. Of course, the agreement between these two numbers could be merely a coincidence.

As the concentration of carbon dioxide in the atmosphere increases, there are two factors in the carbon dioxide balance than can change. First the oceans absorb more carbon dioxide to come to equilibrium with the larger atmospheric concentration. However, only the surface waters can absorb this gas and because of the slow circulation of the oceans, it probably takes at least ten thousand years for this process to come to equilibrium. Whenever the carbon dioxide amount is increasing an upper limit for the amount absorbed by the oceans can be found at any time by assuming the atmosphere-ocean system is always in equilibrium. The actual amount absorbed by the oceans will be considerably less than the amount calculated in this manner for at least several centuries after a sudden increase in the atmospheric carbon dioxide amount. In the first few centuries the surface ocean waters can absorb only a relatively small fraction of the additional carbon dioxide.

The second factor that can change is the amount used in photosynthesis. A higher level of photosynthetic activity can be supported by the increased carbon dioxide amount. As previously discussed, this process temporarily withdraws some of the additional carbon dioxide from the atmosphere into the organic world. However, in a relatively few years the increased rates of respiration and decay bring this process back into equilibrium and only a relatively small amount of carbon dioxide is permanently lost from the atmosphere. Thus it appears that a major fraction of the additional carbon dioxide that is released into the atmosphere remains there for at least several centuries.

Even if there may be some question as to whether or not the general amelioration of the climate in the last fifty years has really been caused by increased industrial activity, there can be no doubt that this will become an increasingly serious problem as the level of industrial activity increases. In a few centuries the amount of carbon dioxide released into the atmosphere will have become so large that it will have a profound influence on our climate.

After making allowance for industrial growth, a conservative estimate shows that the known reserves of coal and oil will be used up in about 1,000 years. If this occurs, nearly 4 × 1013 tons of carbon dioxide will have been added to the atmosphere; this is seventeen times the present amount. The total amount in the atmosphere-ocean system will have increased from 1.32 x 1014 tons to 1.72 x 1014 tons. Even if the atmosphere-ocean system is assumed to be in equilibrium at the end of the thousand year period, the atmospheric carbon dioxide pressure will be 3 x 10–3 atmospheres, which is 10 times the present value; the corresponding increase in the temperature from this cause will be 13.4 degrees. If it is further assumed that there would be sufficient time for the calcium carbonate to dissolve and come to equilibrium in the oceans, the atmospheric pressure will be 1.1 x 10–3 atmospheres and the temperature rise 7.0 degrees. The last figure is a lower limit for the temperature rise that will occur because of man’s industrial activities; the actual temperature rise must be larger since there will be insufficient time for these various equilibria to be established. Our energy requirements are increasing so rapidly that the use of nuclear fuels will probably not change materially the rate of use of the organic fuels.

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