Atmosphere changes induced by man fall into four categories with respect to basic causes:
(1) Changes in concentrations of the natural component gases of the lower atmosphere;
(2) Changes in the water vapour content of the troposphere and stratosphere;
(3) Alteration of surface characteristics of the lands and oceans in such a way as to change the interaction between the atmosphere and those surfaces;
(4) Introduction of finely divided solid substances into the lower atmosphere, along with gases not normally found in substantial amounts in the unpolluted atmosphere.
Atmospheric carbon, as carbon dioxide (C0²), and molecular oxygen (O²) are both involved in continuous cycles of interchange between atmosphere and oceans, soil and rock layers, and the biosphere. Carbon is linked with oxygen in CO² as a free gas in the atmosphere and in solution in ocean water and fresh water of lakes and streams, and in soil water. Both land and marine plants withdraw and use CO² to create carbohydrate compounds of plant tissues. Animals consume plants, releasing CO¹ back to the atmosphere in the process of biological oxidation (respiration). Decomposition of plant matter also releases CO² to the atmosphere.
During the past 110 years, there has been an increase in atmospheric CO² from a 295 parts per million (ppm) to a 1970 value of about 320 ppm, an increase of about 10 percent. Moreover, the rate of increase in this period, while slow at first, has become much more rapid toward the end of the period. Now, we also have a fairly good evaluation of the quantity of hydrocarbon fuel burned during the same period, and from this we can calculate the increase in atmospheric CO² that would have resulted in the same period, if all of the additional CO² had remained in the atmosphere. It is estimated that 40 to 50 percent of the CO² produced has remained in the atmosphere. The excess CO² has been removed from the atmosphere, most of it going into solution in ocean waters, but some of it perhaps going into increased rates of plant growth.
We turn next to consider the environmental effects to be anticipated from an increase of atmospheric CO². Because CO² is an absorber and emitter of long-wave radiation, its presence in larger proportions will raise the level of absorption of both incoming and outgoing radiation, changing the energy balance so as to raise the average level of sensible heat in the atmospheric column. Thus, a general air temperature rise is the anticipated result.
When we calculate in terms of complete combustion of all of the estimated world supplies of hydrocarbon fuels, and make the assumption that 50 percent of the CO² plus produced is added to the atmospheric quantity, an average air temperature increase of between 2F° and 4F° (1.1C° and 2.2C°) is indicated, assuming this cause alone is acting. Recent calculations show that as CO² increases, its temperature-raising influence falls off, so that no really serious runway heating effect need be feared.
Two feedback mechanisms may come into play as temperature rises. First, as seawater temperature increases, more CO² is released into the atmosphere. This mechanism would tend to accelerate the air temperature rise. Second, higher temperatures would increase evaporation and raise water vapour content of the atmosphere, in turn increasing cloudiness and the earth's albedo. With more solar radiation turned back into space, atmospheric temperatures would be lowered. These two mechanisms tend to counteract each other's effects in a self-regulatory, thermostat-like mechanism and it is very difficult to predict the outcome.
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