Some periods of mountain-building have not produced glaciers, In these periods the output of carbon dioxide from volcanoes, which are especially active during the early stages of mountain-building, might have balanced the carbonate consumption of the newly exposed rocks, In fact, a landscape teeming with active volcanoes could easily release more carbon dioxide than the rocks could possibly absorb, so the temperature of the earth would rise sufficiently to prevent the expansion of glaciers.
The geological effects of volcanic action, coal formation or any other local disturbance of the carbon dioxide concentration are not restricted to the area in which they occur. If the amount of carbon dioxide in one hemisphere of the earth rises or falls sharply, the concentration in the other hemisphere changes rather quickly. In less than a few decades the concentration in both hemispheres becomes identical. According to the carbon dioxide theory, this rapid diffusion helps to explain the fact that glaciers advance and retreat simultaneously in both hemispheres.
During the past century a new geological force has begun to exert its effect upon the carbon dioxide equilibrium of the earth [see graphs on page 43]. By burning fossil fuels man dumps approximately six billion tons of carbon dioxide into the atmosphere each year. His agricultural activities release two billion tons more. Grain fields and pastures store much smaller quantities of carbon dioxide than the forests they replace, and the cultivation of the soil permits the vast quantities of carbon dioxide produced by bacteria to escape into the air.
Not all of this eight billion tons of surplus carbon dioxide remains in the atmosphere, Plants remove some of it. When the atmospheric concentration rises, plants use more carbon dioxide for photosynthesis. In a few years, however, the increase in the rate of photosynthesis is balanced by advances in the rate of respiration and decay processes. The net result is only a slight increase in the carbon dioxide content of the biosphere.
Most of the carbon dioxide added to the atmosphere by human activities will ultimately be absorbed by the oceans[see graph on page 46]. To predict the effect of human activities upon climate we must calculate just how rapidly this happens. Recent studies make it appear that volume of carbon dioxide dissolved in the oceans comes to equilibrium with the carbon dioxide pressure of the atmosphere in about 1,000 years, and that the oceans take up about half of any carbon dioxide added to the air. Over a longer period of time, perhaps several thousand years, the oceans take up much larger additional quantities of carbon dioxide in carbonate compounds before the system again reaches equilibrium. These equilibrium rates are quite significant, because they will govern the temperature of the earth as long as man burns large amounts of fossil fuels.
We have only to extrapolate existing records of temperature and fossil-fuel consumption to predict the climate of the future. Quite accurate records of the amount of fossil fuel consumed in the world each year show that in the past 100 years man has added about 360 billion tons of carbon dioxide to the atmosphere. As a result the atmospheric concentration has increased by about 13 per cent. The carbon dioxide theory predicts that such an increase should raise the average temperature of the earth one degree F. This is almost exactly the average increase recorded all over the world during the past century! If fuel consumption continues to increase at the present rate, we will have sent more than a trillion tons of carbon dioxide into the air by the year 2000. This should raise the earth's average temperature 3.6 degrees.
In less than 1,000 years, if consumption continues to increase at the current rate, we will have exhausted the currently known reserves of coal and oil. By that time we will have multiplied the carbon dioxide tonnage of the air 18 times. When the ocean–atmosphere system comes back to equilibrium, the concentration of carbon dioxide in the air will be 10 times greater than it is today, and the earth will be 22 degrees warmer. In another few thousand years, when the carbonate content of the oceans has reached equilibrium, the concentration will still be four times greater than it is today. The earth's temperature will then fall to about 12.5 degrees above its present average.