Environmental Issues - Notes | Class 12 | Part 4: Greenhouse Effects, Global Warming, Ozone Depletion



Greenhouse is a small glass house used for growing plants during winter. The glass panel lets the light in, but does not allow heat to escape. Thus the greenhouse warms up. 

Greenhouse effect is a natural phenomenon that causes heating of Earth’s surface and atmosphere. It maintains the present average temperature (15° C). 

Without greenhouse effect, the average temperature at Earth surface would have been at –18° C.

Clouds & gases reflect 1/4th of the incoming solar radiation and absorb some of it. But half of it falls on Earth’s surface heating it, while a small amount is reflected back. Earth’s surface re-emits heat as infrared radiation (long wave). But a part of it is absorbed by atmospheric gases (CO2, CH4 etc.) and so cannot escape into space. These gases (greenhouse gases) radiate heat energy. It comes to Earth’s surface, heating it up again. It causes the greenhouse effect.

Overheating of Earth due to increased level of greenhouse Gases, is called global warming.

During the past century, the temperature of Earth has increased by 0.6° C, most of it during the last 3 decades.

Contribution of greenhouse gases to total global warming: CO2 (60%), CH4 (20%), CFCs (14%) & N2O (6%).

Impacts of global warming:
  • Climatic changes (e.g. El Nino effect).
  • Melting of polar ice caps, Himalayan snow caps etc.
  • Future impact: Rise in sea level submerging coastal areas.
Control of global warming:
  • Reduce the use of fossil fuel.
  • Improve efficiency of energy usage.
  • Reduce deforestation and plant trees.
  • Slowing down the growth of human population.
  • International initiatives to reduce greenhouse gases.


‘Bad’ ozone is formed in troposphere (lower atmosphere). It harms plants and animals.

‘Good’ ozone is found in the stratosphere. It acts as a shield absorbing ultraviolet radiation from the sun.

UV rays are highly injurious since they cause mutation.

The thickness of the ozone (O3) in a column of air from the ground to the top of the atmosphere is measured in terms of Dobson units (DU).

In stratosphere, UV rays act on molecular oxygen (O2) to produce ozone. UV rays also cause the degradation of ozone to O2. These processes are balanced.

But this balance is disrupted due to ozone degradation by chlorofluorocarbons (CFCs- used as refrigerant).

CFCs move upward and reach stratosphere. UV rays act on them releasing Cl atoms. In presence of Cl (catalyst), ozone degrades to O2. This causes ozone depletion. It has formed Ozone hole over the Antarctic region.

UV radiation of wavelengths shorter than UV-B, are almost completely absorbed by Earth’s atmosphere. But, UV-B causes DNA mutation. It causes aging of skin, damage to skin cells and skin cancers. A high dose of UV-B causes inflammation of cornea (snow-blindness), cataract etc. It permanently damages the cornea.

The Montreal Protocol: An international treaty (Canada, 1987) to control emission of ozone depleting substances.
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