The lower most atmospheric layer is troposphere extending upto 8 to 10 km at poles and 16 to 18 km at equator.
The next layer is stratosphere extending from troposphere upto a height of 50 to 55 km. Ozone (O3) is present in stratosphere and, therefore, stratosphere is also called as ozonosphere.
It is rich layer of ozone that absorbs 9990 harmful ultraviolet radiations from the sun. (Fig. 9.6)
Thickness of ozone is measured in Dobson units (D.U.). The concentration of ozone is 0.3 ppm or 300 D.U. in stratosphere. Protection against UV radiation is proportional to thickness of ozone layer. In stratosphere ozone is formed and dissociated reversibly.
Depletion of ozone refers to thinning of ozone shield or depletion in concentration of ozone. Drastic depletion in concentration of ozone over a restricted area is called ozone hole (example-over antarctica). Under such circumstances, more and more UV radiation would reach earth-surface. This radiation has mutagenic property.
It causes dimer formation by pairing thymine with thymine during DNA replication. It adversely affects human health with increasing severe sunburns, skin cancer and catract of the eyes. The earth would become uninhabitable for most of flora and fauna. UV radiations have three forms, viz., UV-A (wave length 320 to 400 nm); UV-B (280-320 nm) and UV-C (200-280 nm).
Lesser the wave length, greater is the lethal effect. Thus UV-C is most dangerous which is completely absorbed by ozone umbrella. UV-B radiation is likely to cause immune system suppression in animals, lower crop yield a decline in productivity of forests and surface dwelling planktons.
Causes of Ozone Depletion:
Chlorofluorocarbons (CFCs), methane and nitrogen oxides are responsible for ozone layer depletion in stratosphere. Excessive use of fossil fuels, burning of biomass, use of biogas plants, paddy cultivation, use of air conditioners and refrigerators using CFCs are the important man made sources. Volcanoes, lightning and thunder and anaerobic decomposition of organic matter are the natural causes of ozone depletion.
International Effect to Reduce Ozone Depletion:
United Nations Environment Programme has taken steps to find solution for ozone depletion.
1. Montreal Protocol (Kith Sept. 1987) 27 industrialised countries agreed to limit production of chlorofluorocarbons to half the level of 1986.
2. Helsinki Declaration (May ’89) 82 nations pledged to phase out CFCs by 2000 A.D.
3. In June 1990, 93 nations agreed to phase out CFCs and other ozone depleting compounds by the end of 20th century. Till now 175 nations have signed it.
Degradation by Improper Resource Utilisation and Maintenance:
Nature and environment around us provide variety of materials which are collectively called as natural resources. These are air, water, soil, minerals along with climate and solar energy that are abiotic factors and the other part is biotic components such as plants, animals and microorganisms. Pollution is not the only cause of degradation of natural resources. Improper utilization practices can also lead to degradation of natural resources.
The following two misutilization of soil resources have been studied:
1. Soil Erosion and Desertification:
Soil is the upper most layer of earth crust ranging in thickness from a few cm to several metres. Fertile top soil takes hundreds of years to develop. But, faulty utilization practices can destroy it within a few years and may be converted to an arid patch. This happens due to human activities like deforestation, overgrazing, over-cultivation leaving borren soil without seedling and improper irrigation. Soil without vegetation cover is eroded by both wind and water being converted into a sandy patch.
2. Waterlogging and Soil Salinity:
Excessive irrigation, ‘Kutcha’ irrigation channels, impermeable underground soil patches and poor drainage system result in water logging of soil. Waterlogged soil is unfit for plant growth because such soil has poor aeration.
Evaporation of water from surface causes salt deposition on the soil. A crust of salt on the surface of soil makes it saline and unfit for growth of crops.