About 250 million years ago, large parts of the continents were covered with swamps (soft, wet land). Thick forests of fern (a beautiful plant with slender leaves) grew in these swamps. These fern plants carried on the process of photosynthesis during which they combined carbon dioxide of the atmosphere and water in the presence of solar energy (sunlight energy) to form a bio-mass of carbohydrates and liberated oxygen gas into the atmosphere. These carbohydrates collected in the body of the fern plants and they grew further.
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Thus, when fern forests grew on earth, they trapped a lot of solar energy (sun’s energy) by converting it into the chemical energy of carbohydrates. And the,carbon required for this purpose was extracted from the atmosphere which contained it in the form of carbon dioxide gas. When the fern forests became dense and fully developed,the earth opened up chasms (a gap or wide space) in its crust. The thick fern forests were buried in these chasms, away from the reach of oxygen of atmosphere.
In due course of time, more and more sand collected over the buried forests and under the combined effect of pressure and heat, and in the absence of oxygen, the buried plant material was slowly converted into coal. It is this coal which we take out by digging coal mines into the earth’s surface. Thus, by a series of favourable earth system processes, the plants and trees of the ancient forests were converted into a valuable resource called coal. Our earth can be compared to an engine in its organization. The earth behaves as an engine because it is transferring heat from the interior into energy, recycling matter and redistributing resources.
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We shall now describe what changes take place in the earth and its atmosphere by the formation of coal. Now, when the plants (like ferns) die under normal circumstances, then the biomass (carbon compounds) present in them react with the oxygen of air to form carbon dioxide and water vapour which go back to the atmosphere. So, if the dense forests of plants like ferns had not been buried deep under the earth, away from oxygen, then they would have just reversed the process of photosynthesis which created them initially. A process similar to respiration would have taken place consuming oxygen and giving out carbon dioxide and water.
In that case, the carbon dioxide extracted from the atmosphere would have been returned to the atmosphere. However, as a result of the formation of coal, the carbon dioxide extracted from the atmosphere of long ago was never returned. It was converted into the carbon of coal. Since the formation of coal decreased the amount of carbon dioxide in the atmosphere of long back, the green-house effect was also reduced accordingly. This prevented the excessive heating of the earth and its atmosphere. If, however, all the coal on earth which was formed over a period of millions of years is burnt up in a few hundred years time, then the excessive amount of carbon dioxide produced as a result of the rapid burning of coal will increase the green-house effect tremendously.