Carbon Dioxide and the Climate
A 1956 American Scientist article explores climate change; two contemporary commentaries illuminate its relevance to the present
The climate must continually oscillate from a glacial to an interglacial period until the total carbon dioxide amount is again increased by a change in one of the factors in the carbon dioxide balance. When the total carbon dioxide amount is reduce slightly below its present value, there is no stable state for the climate; it must continually oscillate. On the other hand, if some event should greatly reduce the total carbon dioxide amount (perhaps by 30 per cent or more), a permanent period of glaciations without these oscillations would be possible. In order to explain the various states in this cycle more clearly, specific numbers have been assumed. However, it may be verified easily that none of the conclusions that have been reached depend in a critical way on the particular numbers that were chosen. It should also be pointed out that, if there is sufficient time in the various stages of the cycle for the oceans to come to equilibrium with calcium carbonate, the form of the curves in the figure is somewhat changed, but none of the conclusions reached above is essentially altered.
In addition to lower temperatures, increased precipitation is also necessary for the formation of extensive glaciation. Most theories of climatic change have found it very difficult to explain this increased precipitation. For example, in the variable sun theory, a decrease in the sun’s radiation reduces the surface temperature. However, this also reduces the energy available to drive the general circulation of the atmosphere. A decreased circulation presumably means decreased cloud formation and precipitation. In order to account for the increased precipitation an ingenious, but unconvincing modification of the variable sun theory states that glacial periods result from an increase in the sun’s radiations. The slightly increased average temperatures are supposed to be compensated by the greater precipitation.
The carbon dioxide theory provides a simple, straight-forward explanation for the increased precipitation during a glacial epoch. One of the parameters that determines the amount of precipitation from a given cloud is the radiant loss of heat energy from the upper surface of the cloud. If this radiation loss increases, the temperature at the upper surface of the cloud decreases. This increases the temperature difference between the upper and lower surface of the cloud. Because of these more vigorous convection currents, it is more likely that rain will fall from the cloud. Thus on the average there is more rainfall from a given cloud if the radiation loss from its upper surface increases.
According to the carbon dioxide theory there is a smaller than normal amount of carbon dioxide in the atmosphere when glaciers are beginning to form. Not only the surface of the Earth, but also the upper surface of a cloud is cooler, since they can lose heat energy more rapidly to space. Recent calculations show that the upper surface of a cloud at a height of 4 kilometers is 2.2 degrees cooler when the carbon dioxide pressure is half the present value. Further the upward flux of radiation that strikes the lower surface of the cloud is larger when the carbon dioxide amount is reduced; thus the lower surface of the cloud is warmer than before. Thus, the larger temperature difference between the upper and lower surfaces of the cloud causes increased convection in the cloud; the level of precipitation should increase appreciably. Thus, according to the carbon dioxide theory, colder and wetter climates occur together.
There is considerable geological evidence that extensive outbursts of mountain building occurred several millions of year before each of the last two major glacial epochs. Again the carbon dioxide theory seems to be the only theory that suggests a reason for the time lag between these two events. During a major period of mountain building, tremendous quantities of igneous rock are exposed to weathering. In mountainous country the zone for the active disintegration of rock extends much farther beneath the surface than it does in flat country. The weathering of igneous rock changes it into carbonates, thus removing carbon dioxide from the atmosphere.