Essay on the Theories Involving Change in the Composition of Atmosphere

These theories include the carbon dioxide and volcanic dust theories. An appreciable change in the carbon dioxide content of the atmosphere or the widespread distribution of volcanic dust changes the insulating effects of the atmosphere.

Carbon dioxide theory:

This theory was advanced by T.C. Chamberlin in the last decade of the nineteenth century. According to this theory variations in the carbon dioxide content of the atmosphere play dominant role in causing worldwide climatic changes.

This gas is transparent to incoming solar radiation, but absorbs outgoing long-wave terrestrial radiation. The absorbed terrestrial radiation is radiated back to the earth’s surface.

Carbon dioxide and water vapour contents of the atmosphere being the most important heat absorbers produce what is called the greenhouse effect. Thus, it is clear that any appreciable change in the carbon dioxide content would bring about changes in temperature in the lower atmosphere.

The present industrialization has been directly responsible for adding large supplies of carbon dioxide gas to the atmosphere. Some of the increased gas is consumed by vegetation and part of it is dissolved in the oceans. However, about 50 per cent is present in the atmosphere.

According to a moderate estimate, between 1860 and 1970 the increase in the carbon dioxide content of the atmosphere was about 10 per cent. This would have led to the warming up of the atmosphere during this period, but surprisingly enough the rise in global temperature continued up to the middle of the 1940s only.

After that there has been recorded a gradual decline in the temperature of the world. Contrary to it, a section of the scientists believes that in the absence of the greenhouse effect produced by the increasing amount of carbon dioxide in the atmosphere, the cooling trend might have been much greater.

According to the current estimates, the present carbon dioxide content of the atmosphere is 330 ppm (parts per million). It is likely to reach 400 ppm by the end of the century. If the present rate of increase of carbon dioxide continues, this atmospheric gas may double by the year 2040. Consequently the global temperature may increase by about 2°C by 2040 A.D.

However, studies of oxygen isotope records in the Greenland ice core suggest that the present cooling trend that has started since 1940s is just a temporary phase-a sort of climatic fluctuation.

After one or two decades, the global temperatures will again start increasing. As the scientists predict, by the first decade of the next century the temperatures all over the world would be higher than ever during the last 1,000 years. Thus, it is evident that increasing carbon dioxide content would lead to a rise in global temperatures.

Possibilities also exist that the increasing carbon dioxide content in the atmosphere may lead to lowering down of the temperature. Higher temperature means increased evaporation and more clouds in the atmosphere.

Greater cloudiness would naturally reduce the amount of insolation reaching the earth’s surface resulting in reduced temperature. The atmosphere being a very complex interactive physical system, numerous possibilities exist about the outcome of the increased carbon dioxide content.

This hypothesis is subjected to a number of criticisms. The most important point raised against this hypothesis is that the content of the atmosphere in the geologic past cannot be ascertained. This hypothesis does not explain minor climatic fluctuations such as the post-glacial periods and historic pulsations.

According to Huntington and Visher, keeping in view the complexities of the problem this hypothesis should be regarded as a tentative rather than a final explanation of glaciation. However, the carbon dioxide content of the atmosphere appears to be an important factor in producing the longer fluctuations of climate from one geographical era to another.

Volcanic dust theory:

Volcanic dust, because of the size of its particles, deflects light of the short wavelengths coming from the sun. But long-wave terrestrial radiation can easily pass through volcanic dust without any loss. It is, therefore, natural that large-scale volcanic dust may lower down the earth’s temperature to a certain extent.

The large amount of volcanic dust present in the atmosphere is possibly one of the causes of the ‘Little Ice Age’. According to this theory, the ice ages are supposed to be initiated during epochs of frequent volcanic eruptions.

In 1883, the East Indian volcano Krakatoa erupted so violently that most of the island was doomed to destruction, and the explosion was heard from a distance of 4,800 kilometers.

The amount of dust injected into the atmosphere was so large that for two years the sky appeared redder at the time of sunsets. Calculations revealed that global temperature declined during this period because of the presence of dust in the atmosphere.

When the dust particles settled to the ground, the temperatures became normal. There is evidence from the historical records that during the last century and a half, after the occurrence of explosive volcanic eruptions the global temperatures dropped by a few degrees.

When put to geological test, the volcanic dust theory does not hold good. If volcanic dust can initiate an ice age, the evidences of it should be preserved in sedimentary layers, in existing glaciers or in layers of mud deposited in lakes and oceans at the time of violent volcanic eruptions.

Theoretically the theory can be tested by comparing the records of ice age climates with sedimentary record of volcanic activity. But practically it seems impossible to collect accurate measurements over a large area to test the validity of this theory.