Carbon dioxide is present in small amounts in the earth’s atmosphere being about 0.033 per cent of all the atmospheric gases. Though carbon dioxide is present in small proportion in the atmosphere, but it performs very important functions. Some of the important functions performed by carbon dioxide gas in the atmosphere are given below :
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- Carbon dioxide gas produces green-house effect, leading to the heating of earth and its atmosphere, which is essential for our survival.
- Carbon dioxide takes part in the photosynthesis process carried out by green plants to prepare food for plants and animals.
- Carbon dioxide dissolves in oceans to form carbonate rocks.
Some of the processes taking place in earth and its atmosphere release carbon dioxide gas whereas other processes consume carbon dioxide gas. So, the proportion of carbon dioxide gas in the atmosphere is maintained by a number of different processes. The major suppliers of carbon dioxide gas to the atmosphere are : (i) Eruption of volcanoes, (ii) Respiration of living organisms, vim) Decay of dead organisms, (iv) Burning of fossil fuels, and (v) Cultivation of lands.
The carbon dioxide gas given to the atmosphere through volcanoes comes from the interior of the earth. The major consumers of carbon dioxide gas from the atmosphere are : (i) Green plants, and (ii) Oceans. The green plants use carbon dioxide gas to prepare their food through the process of photosynthesis whereas oceans dissolve the carbon dioxide gas to form carbonate rocks. In this way, both, the atmosphere and oceans continuously exchange carbon dioxide with rocks, plants and higher organisms.
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Before we discuss the green-house effect in detail, we should once again recall the composition of sun-light. The sun-light consists of three types of radiations : ultra-violet radiation, visible light and infra-red radiation. About one-third of the sun-light consists of infra-red radiation or infra-red rays which are actually heat rays and cause heating of earth and its atmosphere. ‘It should be noted that the infra-red rays emitted by the extremely hot bodies like the sun are of very short wavelength which are not absorbed by either carbon dioxide or even a glass sheet.
So, the short wavelength infra-red rays emitted by the sun can pass through a blanket of carbon dioxide gas or even a glass sheet. Actually, all the hot bodies emit heat rays or infra-red rays. But the infra-red rays emitted by less hot objects like the earth’s hot surface or the various objects on it, are of long wavelength which can be absorbed by carbon dioxide molecules, and hence not allowed to escape by a blanket of carbon dioxide in the atmosphere. The long wavelength infra-red rays cannot go out even from a glass sheet.Keeping these points in mind, it will now be easier for us to understand the green-house effect.