Study Notes on Green House Effect

The green house effect is a naturally occurring phenomenon which is responsible for heating the earth surface and atmosphere.

Due to green house effect, the average temperature of earth surface is 15°C and without green house effect the average temperature would have been – 18°C.

A green house (also called as glasshouse) is a building in which plants are grown. These structures range in size from small sheds to industrialized buildings.

A greenhouse has different type of covering materials, such as glass or plastic roof and walls. It accumulates temperature and heats up because incoming visible solar radiation from the sun is absorbed by plants, soil and other things inside the building.

The absorbed radiation gets accumulated and converted to heat energy (lower frequencies of infrared thermal radiation). Infrared radiation is absorbed by green house gasoes and water vapours. Some of the heat rays one reflected by the glass panes and again come back to the surface. Warming effect found in green house is due to accumulation of heat rays. Green house warm up is similar to the inside of a car parked in the sun for some time. (Fig. 9.4)

The gases which allow the solar radiations to pass through but retain the long wave heat radiations are called green house gases. The various green house gases are CO₂, CH₄, CFCs and N₂0 and others of minor significance are water vapours and ozone. They prevent a substantial part of long wave radiations emitted by earth to escape into space.

(i) Carbon dioxide:

It is considered as the most important green house gas. Its normal level in atmosphere is 0.03%. Its concentration was 280 ppm (parts per million) 1750, 368 ppm in 2000 and 380 in 2007. The rise of CO₂ is due to large scale deforestation and combustion of fossil fuels. The forest coverage was 33% in the beginning of 20th century but reduced to 19.4% by the end of the century. Excessive use of fossil fuel is adding more CO₂ to atmosphere. About 5 x 10⁹ tons of CO₂ is added every year through burning of fossil fuel. (Fig. 9.5)

(ii) Methane:

It traps 20 times as much heat as CO₂. Its concentration was 700 ppb (parts per billion) in pre-industrial times but 1750 ppb in the year 2000. CH₄ is produced by incomplete biomass combustion incomplete decomposition mostly by anaerotric methanogens, flooded paddy fields, marshes, enteric fermentation of cattle etc. In arctic regions, methane comes out of earth’s interior at many places called methane chimneys.

(iii) Chlorofluorocarbons (CFCs):

These are synthetic gaseous compounds of carbon and halogen which are frequently used as cooling gas in refrigerators and air conditioners (Freon-12). These are odourless, non-toxic, non-inflammable, chemically inert propellants used in aerosol cans and jet fuel, fire extinguishers, plastic foams and room fresheners. They are highly stable and non-degradable. All CFCs liberated into atmosphere slowly move to stratosphere and cause ozone depletion. They trap 1500 to 7000 times as much heat as per molecule of CO₂.

(iv) Nitrous Oxide (N₂O):

It is another green house gas released in atmosphere through burning of fossil fuels, lightning and thunder, microbial activities, livestock wastes, breakdown of nitrogen fertilizers in soil etc. Its lifespan in atmosphere is about 120 years. It traps about 2000 times as much heat per molecule as CO₂.

(v) Water vapour:

It also helps in heat trapping mechanism in atmosphere and its role as a green house gas cannot be ignored.

The green house effects are global warming depletion of ozone layer in stratosphere and CO₂ fertilization effect on plants.