Environmental Issues - Notes | Class 12 | Part 4: Greenhouse Effects, Global Warming, Ozone Depletion
Greenhouse Effect and Global Warming
A greenhouse is a small glass house used for growing plants during winter. The glass panel lets light in but does not allow heat to escape, thus warming the greenhouse.
The greenhouse effect is a natural phenomenon that causes heating of Earth’s surface and atmosphere, maintaining the present average temperature of 15°C. Without the greenhouse effect, the average temperature at Earth’s surface would be –18°C.
Clouds and gases reflect one-fourth of the incoming solar radiation and absorb some of it. About half of the solar radiation 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). Part of this is absorbed by atmospheric gases (e.g., CO2, CH4), known as greenhouse gases, which prevent it from escaping into space. These gases radiate heat energy back to Earth’s surface, causing the greenhouse effect.
Global warming is the overheating of Earth due to increased levels of greenhouse gases. During the past century, Earth’s temperature has increased by 0.6°C, most of it in the last three decades.
Contribution of greenhouse gases to total global warming: CO2 (60%), CH4 (20%), CFCs (14%), and N2O (6%).
Impacts of Global Warming
Climatic changes (e.g., El Niño 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 fuels.
Improve efficiency of energy usage.
Reduce deforestation and plant trees.
Slow down the growth of the human population.
International initiatives to reduce greenhouse gases.
Ozone Depletion in the Stratosphere
‘Bad’ ozone is formed in the troposphere (lower atmosphere) and harms plants and animals.
‘Good’ ozone is found in the stratosphere and acts as a shield, absorbing ultraviolet radiation from the sun.
UV rays are highly injurious as they cause mutations. The thickness of the ozone (O3) layer in a column of air from the ground to the top of the atmosphere is measured in Dobson units (DU).
In the stratosphere, UV rays act on molecular oxygen (O2) to produce ozone, and UV rays also cause the degradation of ozone back to O2. These processes are normally balanced.
This balance is disrupted by ozone degradation caused by chlorofluorocarbons (CFCs), used as refrigerants. CFCs move upward to the stratosphere, where UV rays release chlorine atoms. In the presence of chlorine (a catalyst), ozone degrades to O2, causing ozone depletion and forming an ozone hole over the Antarctic region.
UV radiation with wavelengths shorter than UV-B is almost completely absorbed by Earth’s atmosphere. However, UV-B causes DNA mutations, leading to aging of skin, damage to skin cells, and skin cancers. High doses of UV-B cause inflammation of the cornea (snow-blindness), cataracts, and permanent corneal damage.
The Montreal Protocol, an international treaty signed in Canada in 1987, aims to control the emission of ozone-depleting substances.
