Editor's Note: We are posting this article from our July 1959 issue to offer an historical perspective on some of the issues being discussed at the United Nations Framework Climate Change Conference in Poznan, Poland, which began December 1 and runs through December 12.
The theories that explain worldwide climate change are almost as varied as the weather. The more familiar ones attribute changes of climate to Olympian forces that range from geological upheavals and dust-belching volcanoes to long-term variations in the radiation of the sun and eccentricities in the orbit of the earth. Only the so-called carbon dioxide theory takes account of the possibility that human activities may have some effect on climate. This theory suggests that in the present century man is unwittingly raising the temperature of the earth by his industrial and agricultural activities.
Even the carbon dioxide theory is not new; the basic idea was first precisely stated in 1861 by the noted British physicist John Tyndall. He attributed climatic temperature-changes to variations in the amount of carbon dioxide in the atmosphere. According to the theory, carbon dioxide controls temperature because the carbon dioxide molecules in the air absorb infrared radiation. The carbon dioxide and other gases in the atmosphere are virtually transparent to the visible radiation that delivers the sun's energy to the earth. But the earth in turn reradiates much of the energy in the invisible infrared region of the spectrum. This radiation is most intense at wavelengths very close to the principal absorption band (13 to 17 microns) of the carbon dioxide spectrum. When the carbon dioxide concentration is sufficiently high, even its weaker absorption bands become effective, and a greater amount of infrared radiation is absorbed [see chart on page 42]. Because the carbon dioxide blanket prevents its escape into space, the trapped radiation warms up the atmosphere.
A familiar instance of this "greenhouse" effect is the heating-up of a closed automobile when it stands for a while in the summer sun. Like the atmosphere, the car's windows are transparent to the sun's visible radiation, which warms the upholstery and metal inside the car; these materials in turn re-emit some of their heat as infrared radiation. Glass, like carbon dioxide, absorbs some of this radiation and thus traps the heat, and the temperature inside the car rises.
Water vapor and ozone, as well as carbon dioxide, have this effect because they too absorb energy in the infrared region. But the climatic effects due to carbon dioxide are almost entirely independent of the amount of these other two gases. For the most part their absorption bands occur in different regions of the spectrum. In addition, nearly all water vapor remains close to the ground, while carbon dioxide diffuses more evenly through the atmosphere. Thus throughout most of the atmosphere carbon dioxide is the main factor determining changes in the radiation flux.
The 2.3 X 1012 (2,300 billion) tons of carbon dioxide in the earth's present atmosphere constitute some .03 per cent of its total mass. The quantity of carbon dioxide in the atmosphere is determined by the amounts supplied and withdrawn from three other great reservoirs: oceans, rocks and living organisms. The oceans contain some 1.3 X 1014 tons of carbon dioxide—about 50 times as much as the air. Some of the gas is dissolved in the water, but most of it is present in carbonate compounds. The oceans exchange about 200 billion tons of carbon dioxide with the atmosphere each year. When the equilibrium is disturbed, the oceans may engulf or disgorge billions of additional tons of carbon dioxide. This puts a damper on the fluctuations in the carbon dioxide content of the atmosphere: when the atmospheric concentration rises, the oceans tend to absorb much of the excess; when it fails, the oceanic reservoir replenishes it.