The term Greenhouse effect had its origin from the practice of encasing vegetation in glass chambers to protect them from frost particularly in cold countries. It was observed that there was a continued rise in temperature in such chambers even when the outside temperature remained low. This enabled the warming up of vegetation inside the chamber, resulting
in good plant growth.
WHAT HAPPENS IN A TYPICAL GREENHOUSE?
The transparent roof and wall of the glass chamber allow sun rays to pass through and heat up the soil and plants in the chamber. The warm soil and plants emit infra-red radiation. Since glass is opaque to IR radiations. It partly absorb and partly reflect these radiation back to soil and plants. In this way energy of sun remains trapped in greenhouse. This causes
rise in temperature inside the chamber.
GREENHOUSE EFFECT AROUND THE EARTH
An effect similar to the one in glass chamber is responsible for keeping the earth’s surface warmer than it would otherwise be. Atmosphere around the earth acts like a glass of the greenhouse chamber. Just as glass in a greenhouse holds sun’s warmth inside. The atmosphere around the earth traps sun’s heat near the earth’s surface and keeps it warm. This is called natural greenhouse effect which is responsible in maintaining the temperature and making earth perfect for life. The gases present in the atmosphere which cause greenhouse effect are referred to as greenhouse gases. The various greenhouse gases are: Carbon dioxide, Water vapours, Chlorofluorocarbons and Oxides of nitrogen, out of these, the most significant role is played by CO2.
The greenhouse gases in the atmosphere form a thick cover around the earth. The earth receives a large amount of energy from the sun. Sun emits u. v. radiations, visible light and infrared radiations. Of these, the harmful u.v. radiations are absorbed by ozone layer in the stratosphere. The visible and J.R. radiation pass through the atmosphere and reach the earth’s surface. About 75% of solar energy which reaches the earth’s surface is absorbed by it causing increase in its temperature. The rest 25% of energy is reflected back into atmosphere. The earth also radiate heat from its heated surface in the form of LR radiations. Carbon dioxide and water molecules present in air are transparent to u. v. radiation but they are opaque to LR radiation. Thus, they do not allow J.R. radiations emitted by earth to escape out and absorb them.
The heat thus, absorbed by the cover of carbon dioxide and water vapour around the earth does not go to upper atmosphere and is partly radiated back to the surface of earth making it warmer. In this way, the additional heat is kept within the lower atmosphere causing warming of earth.
Fig. 52.2. Greenhouse-like effect on the earth.
The heating of the atmosphere due to absorption of infrared radiations by carbon dioxide and other gases is called greenhouse effect. In fact, the atmosphere, and the natural greenhouse effect has kept the temperature of the earth constant for centuries.
HARMFUL EFFECTS OF GREENHOUSE EFFECT
If the amount of CO2 in the atmosphere becomes more than 0.03%. The natural greenhouse balance will get disturbed. In this situation more and more heat will be trapped by CO2 cover and will be subsequently radiated back to earth causing excessive heating of earth’s atmosphere. In the recent years, quantities of CO2 and other greenhouse gases in the atmosphere have increased significantly because of the use of chemical fertilizers, deforestation and excessive burning of fossil fuels. This excessive amount of CO2 cannot be
removed from the atmosphere by plants and ocean waters. This gradual increase in the concentration of CO2 and other greenhouse gases, has in turn increased the greenhouse effect
and it poses serious threat to mankind as it leads to slow and gradual increase in temperature of the earth’s environment which is referred to as global warming.
In the last 50 years the average temperature of earth has increased by 1°C. If the concentration of carbon dioxide in atmosphere continues rising, it is estimated that average temperature of atmosphere may rise by 3-6°C. This in turn will have following disastrous consequences:
- The high temperature of atmosphere may melt polar ice caps releasing enormous amount of water and the level of sea is likely to rise by 100 m thereby sinking most of the coastal areas and causing large scale destruction.
- The high temperature may reduce crop production, thereby causing famines.
- The high temperature will reduce work efficiency of human beings.
- Tropical rains and hurricans will become more frequent and also more stronger causing more devastation.
- The change in ocean temperature will adversely affect the aquatic life.
- Increasing concentration of greenhouse gases are expected to cause cooling of stratosphere. This may happen because most of the thermal IR radiation will be absorbed at lower altitute and little of them will be left to warm stratosphere. Cooling of stratosphere would mean enhanced greenhouse effect i.e., greater global warming.