Origin of Climatic Change

What causes climates to change? So many different factors influence climate that there is no shortage of possible explanations. One train of thought blames mankind for the temperature fluctuations of Fig. 5.13. The initial temperature rise is attributed to an increase in the carbon dioxide content of the atmosphere. Both the biologic and oceanic cycles are, on the average, balanced in their consumption and production of carbon dioxide. But there are also sources of carbon dioxide that have no absorption processes to counter their effects. The most significant of these sources is the burning of coal and

oil by man to produce heat for dwellings and mechanical energy for industry and transportation. At present our chimneys and exhaust pipes pour about 12 billion tons of carbon dioxide each year into the atmosphere, and this rate is rapidly increasing. Since 1880 the carbon dioxide content of the atmosphere has gone up by 12 percent (Fig. 5.14). Despite the relatively small proportion of carbon dioxide in the atmosphere—only 330 parts per million—it is a most significant constituent because of its ability to absorb solar energy reradiated by the earth and thus to contribute to the greenhouse effect that provides energy to the atmosphere.

The cooling of the atmosphere since 1945 must have a different explana­tion since the carbon dioxide content has continued to increase. The culprit here is thought by some scientists to be dust at high altitudes which scatters a portion of the incoming sunlight back into space. The chief natural source of airborne dust is volcanic eruptions. Man's contribution comes from the chimneys of industry, large-scale burning of tropical forests to clear land for agriculture, and soil particles blown away during mechanical cultivation. There is no question that a sufficiently large increase in atmospheric dust would lead to the observed general cooling of the atmosphere—but just how large an increase is needed and whether it has in fact occurred are not known, nor are the relative importances of the different dust sources.

Another point of view attributes climatic change to variations in the solar energy arriving at the top of the atmosphere, not to events within the atmos­phere. (Of course, the carbon dioxide and dust contents of the atmosphere play a role in climate: the issue is which influences are primary and which are secondary.) The sun's radiation is not constant but fluctuates through the 11-year sunspot cycle, and a number of weather phenomena apparently follow a similar cycle. Perhaps there are long-term variations in solar output as well. Also, periodic changes in the earth's orbit bring it exceptionally close to and far from the sun from time to time. But does the radiation reaching the earth vary enough when this happens to produce the drastic climatic changes known to have taken place in the past, notably the ice ages? The puzzle of climatic change remains one of the most challenging in earth science.