Ozone layer present in stratosphere plays a protective role and filters the sun's damaging UV radiations. Thus, preventing them from reaching the earth surface and saving life.
Concentration of ozone in the stratosphere is sufficient to remove more than 95% of radiant energy of UV-B range with wavelength between 290 and 320 nanometer. In fact, it is in this range that UV radiations are harmful to plants and animals.
A close look at the evolution of life shows that initially the atmosphere was devoid of oxygen but photosynthesis activities of blue green algae another anaerobic unicellular organism helped in building up oxygen level in atmosphere, which in turn helped in the formation of ozone layer in stratosphere and the deadly ultraviolet radiation of the sun was detained in the upper atmosphere. It is believed that before formation of ozone layer, organisms evolved in shallow water, because only visible light could penetrate here and not the dangerous ultraviolet light. However, after formation of ozone layer, the most complexly forms of life on earth have evolved.
The major cause of depletion of ozone layer or formation of holes are certain locations in ozone layer are Chlorofluorocarbons (CFCs) and halons, which are mostly used in air conditions, refrigerators, aerosol, electric and metal clearing, foam blowing and modern fire fighting. They serve as catalyst in destruction of ozone in stratosphere. In addition, oxides of nitrogen, released from the exhausts of large fleet of supersonic aircraft (SST) have also been held responsible for increasing the rate of ozone destruction.
As per estimates traveling time for CFCs and halons to reach the stratosphere, may rang from 20 to 40 years. Once they reach the stratosphere, the chlorine and bromine atoms present within these chemicals are released due to interaction with UV radiations. The chlorine and bromine atoms act as a catalyst to break down ozone into oxygen molecule and chlorine monoxide (CIO). CIO in turn interacts with another ozone molecule to give us two oxygen molecules. Thus, one atom of chlorine released by the action of UV radiations on CFCs break two molecules of ozone into three oxygen and again the same chlorine acts afresh on new ozone molecules.
The chlorine/bromine atoms remain active for 100 years and have capacity to break down several thousands of ozone molecules before the released chlorine gets converted into dilute hydrochloric acid and comes down as acid rain. It has been observed that this break down process becomes much more faster in presence of green house gases like CO2, CH4 etc.
The depletion of ozone layer has been found to be much more acute on polar regions, particularly Antarctica (South Pole) than in other parts of earth. This could be due to prevailing cold climatic conditions and other complex atmospheric parameters including air turbulence. The other reason put forward for acute depletion of ozone at polar region is the absent of Nitrous oxide in these areas. N2O destroys chlorine monoxide and check ozone depletions, whereas, in polar regions, N2O at subzero temperature freezes into droplets or clouds, the leaving CIO free to act on ozone molecules.
Mathematical models estimate that 16% reduction of ozone concentration at global scale would increase UV-B penetration to earth by 40% and 30% reduction in ozone would double the UV-B penetration. In general it has been estimated that intensity of UV radiations that reaches the earth's surface is expected to increase by 2% for every 1% decrease in ozone concentration.
UV-B radiation has potential to affect genetic material and photosynthetic chemicals because peak absorption spectra for DNA and photosynthetic chemicals (Plastoquinine and plastoquinol) lie in the range from 260 to less than 320nm. DNA disorders lead to mutation, genetic defects and cancer development. The most common consequence would increase in skin cancer cases, over other types of cancer or genetic disorder. Skin cancer could be fatal (melanoma) or chronic (on-melanoma). However, it is believed that chronic skin cancer cases would be much more prevalent than fatal ones. Estimates released by UNEP predict 26% rise in incidence of non-melanoma skin cancer worldwide, if overall ozone levels drop even by 10%.
Increased inflow of UV radiation will lower down the sea productivity thereby depleting the oxygen concentration of marine water, which will adversely affect the marine flora and fauna. The temperature of the earth surface would also increase resulting into melting of ice from high altitude and polar caps, which in turn would raise the level of sea resulting into many islands and low lying areas submerging into water. It will lower the food productivity, particularly in temperate region. And certainly the biodiversity of these regions would also get affected. It will in general, affect the health of living organisms and particularly there would be marked increases in eye and skin disease cases.
Efforts are being made at international level to decrease the thinning of ozone layer/widening of ozone holes. The Montreal Protocol is its result. Major reduction was achieved due to use of hydrocarbons as aerosol propellant and as blowing gent for flexible foams instead of CFCs and recycling of CFC refrigerants and increasing the use of HFCs (hydrofluoric carbons) by refrigeration and air-conditioning industry. HFCs are environment friendly as chlorine is not resent in them, which is the main culprit. Ammonia can also be used as alternative to CFCs as it has been used in refrigeration in the past, but there are some disadvantages in its use as refrigerant chemical.
US scientists have proposed that ozone loss could be arrested by injecting the alkenes, ethane's or propane into Antarctic atmosphere. The alkenes could rapidly react with chlorine atoms, making them ineffective. But the solution can have its own side effects not known yet. Therefore, the most wise move is the drastic reduction in use of chemicals responsible for ozone layer degradation and an earnest effort has been going on through Montreal Protocol to achieve it. However, the point to be booted is that till a complete phase-out is archived the world will continue to add ozone depleting substances to the atmosphere.
It could be concluded that over-indulgence with our life supporting systems had led us to a state where our own survival has become doubtful. We are now trying desperately to get rid of these self-created problems.