OZONE LAYER DEPLETION AND GLOBAL WARMING.
The ozone layer acts as a giant sunshade, protecting plants and animals from much of the sun’s harmful ultraviolet radiations.
Ozone forms a layer in the stratosphere, 15-40 km above the earth surface. If the ozone in the atmosphere from the ground level to a height of 60 km could be assembled at the earth’s surface, it would comprise a layer of gas only about 3mm thick.
Global stratospheric ozone levels have declined, which means that the ozone layer is changing. Stratospheric ozone has large natural temporal and spatial variations, up to 30 % variation may be regarded as normal. However, we now have evidence of significant thinning of the ozone layer during spring and summer. This is observed in both the Northern and the Southern hemispheres at middle and high latitudes. During the last 10-15 years, the ozone layer above the Northern hemisphere has been reduced by 5-6 % in spring per decade. The latest tests (January- March 1995) have shown very large reductions, with maximum of more than 30 % reduction compared to normal.
A depletion of the ozone layer will increase the UV- radiations at ground level. Increasing doses of UV-B may cause skin cancer, eye cataracts, damage to the immune system in animals as well as human beings, and have an adverse impact on plant growth.
The UV index developed by Environment Canada. It runs on a scale from 0 to 10 being a typical mid summer, sunny day in the tropics. A relative scale ranging from low to extreme is also applied: In extreme conditions ( UV Index higher than 9) light, sensitive and un tanned skin may burn in less than 15 minutes.
UV radiations will affect human health through for example sunburn, snow blindness, other eye damage, early aging of the skin and rising rates of skin cancer. It may also cause suppression of the immune system. It will like wise affect the productivity of the aquatic and terrestrial eco-systems. Single celled algae, chlorophyll and plant hormones are especially sensitive to UV radiation.
As the ozone layer is reduced the earth’s surface is exposed to more of the shorter UV wavelengths of sun’s radiation that damage living things. For each 10 % depletion of the ozone layer, we can expect 20 % more radiations in these damaging wavelengths.
OZONE DEPLETION GASES:
The Ozone depletion Potential, ODP , is described as a potential relative to that of CFC-11.The various ozone depleting substances vary in the degree to which they contribute to the reduction in the ozone layer. Halogens, for example, are more efficient than the CFCs in depleting ozone, and there fore have a higher ODP. Choro fluoro carbons CFCs, halogens, methyl chloroform, methyl bromide, carbon tetra chloride and several other chemicals are ozone depleting substances.
When CFCs and halogens are released into the atmosphere, they rise slowly, taking up to seven years to reach the stratosphere. But once they are there, under the influence of the sun’s ultraviolet light, chlorine is released and react with ozone layer as a consequence. This allows the harmful solar UV radiation to pass through to the earth’s surface. Because it takes so long for the CFCs and halogens to reach the stratosphere , any reduction in their use on the earth does not have an immediate effect on the concentration in the stratosphere. Some of the ozone depleting substances are persistent, remaining active in the atmosphere for upto 50 years. Most of the CFCs imported to Norway are eventually emitted into the atmosphere. A minor part is degraded and some is recycled. Imports of CFCs, halogens and carbon tetra chloride to Norway have been stopped. CFCs only allowed use is in certain refrigerating systems.
Halogens are only allowed in existing fire extinction plants, but refillment of halons is prohibited.
Impact on the...
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