Ozone Depletion

The process of gradual “thinning” of the stratospheric ozone layer, as a result of the large scale release of compounds, such as, Chlorofluorocarbons into the earth's atmosphere, is known as ozone depletion. Ozone is a highly reactive molecule, comprising of three oxygen atoms, undergoing a natural cycle of continuous creation and destruction, with overall amount of ozone molecules remaining fundamentally stable. However, Chlorofluorocarbons released into the Earth's atmosphere in the form of anthropogenic pollutants, containing halogen radicals, such as, Chlorine and Bromine, destroy this fundamental balance of the ozone layer by reacting with the ozone molecules and sequestering them faster than they are naturally created, eventually depleting the ozone layer. The ozone layer acts as a protective shield to the earth's atmosphere by filtering out the harmful ultraviolet B solar radiation. Therefore, depletion of the ozone layer would mean penetration of higher levels of ultraviolet B radiation into the earth's surface, adversely affecting all life-forms on earth, with detrimental effects on the functioning of the whole ecosystem.


The Significance of the Ozone Layer

The release and gradual accumulation of oxygen through photosynthesis, associated with the evolution of single-celled organisms, resulted in the formation of the ozone layer, over the course of millions of years. The ozone molecule was first identified by the Swiss chemist Christian Schonbein, in 1893, who named it “ozone” for the Greek word ozien, meaning “to smell”. However, the discovery of the ozone layer is attributed to the French scientists, Charles Fabry and Henry Buisson in 1913. The ozone layer consists of ozone molecules, located primarily in the lower stratosphere, from approximately 12-19 miles above the earth's surface. It plays a vital role towards protecting all life forms on earth - human beings, animals and plants - by absorbing the harmful ultraviolet rays emitted by the sun. It has been observed by scientists that this “protective shield” has the capacity to obstruct solar radiations of wavelengths less than 290 nanometers, from hitting the surface of the earth. In effect, the ozone layer absorbs 97-99 % of the dangerous ultraviolet radiation emitted by the sun. The ozone layer is an indispensable element to the existence and progression of life on earth. In the 1970's, however, researchers announced the hypothesis that there was an average decrease in annual ozone concentrations, on account of modern lifestyles and industrialization, and large-scale release of harmful chemicals into the atmosphere, associated with it. The scientists further established that human beings had been damaging the ozone layer - their “planetary sunscreen” - through the excessive use of these man made chemicals, known as ozone depleting substances, exposing living beings to the hazardous effects of the ultraviolet B solar radiations.

The Causes of Ozone Depletion

Ozone depletion takes place when the rate of destruction of ozone molecules in the upper atmosphere far outweighs the rate of its natural creation, decreasing its concentration, over a period of time. The destruction of ozone molecules occur primarily due to the release of man-made chemicals, such as Chlorofluorocarbons, into the atmosphere, as an ingredient used in modern day amenities and products, such as air-conditioners, refrigerators, hair sprays, and also in hazardous industrial waste. Chlorofluorocarbons and other chemical compounds, such as methyl bromide, methyl chloroform, carbon tetrachloride and halons are termed as ozone depleting substances. They are volatile and insoluble in water, thereby easily permeating through to the upper atmosphere. In the stratosphere, these ozone depleting substances react with the ultraviolet radiation of the sun to release chlorine and Bromine. Chlorine and Bromine, in turn, react with the ozone molecule, stripping it of one of its oxygen atom, thereby, breaking the ozone molecule. This process could eventually lead to the depletion of ozone layer. Whereas, one chlorine atom can destroy around100,000 ozone molecules, Bromine atoms are 40 times more capable of destroying the ozone molecules.

Daily observations of the destructive effects of Bromine on the ozone layer.

It has been estimated that the effusion of ozone depleting substances accounts for 90% of depletion of ozone layer in the stratosphere. Chlorofluorocarbons are manufactured in the industries for various purposes, such as refrigerants in refrigerators and air conditioners, solvents for dry cleaning, and propellants for aerosol cans. Practical measurements of stratospheric ozone concentrations through satellites and sensors have substantiated the theoretical findings, regarding the depletion of the ozone layer. The depletion of ozone layer was noted to be more conspicuous in the Polar Regions, than in the tropics. Some natural causes like sun-spots, stratospheric winds and volcanic eruptions are also potential factors for ozone depletion. However, their impact is negligible and temporary.

The Impact of Ozone Depletion

The unrestrained use of ozone depleting substances is likely to produce catastrophic effects on the ozone layer in the upper atmosphere, through a chain of destructive chemical reactions. The impact is believed to be more prominent in the Polar Regions, where concentrations of stratospheric ozone are estimated to fall below 200 Dobson units, especially during springtime. The observation of such drastic reduction in ozone concentrations over Antarctica, during spring, has popularized the term “ozone hole” to refer to such severe ozone losses. The special meteorological conditions and low air temperatures in Antarctica acts as a catalyst in accelerating ozone depletion in this zone. Lowers levels of ozone concentrations in the upper atmosphere would facilitate increased penetration of the harmful Ultraviolet B solar radiation into the earth's surface, with devastating consequences on the overall health of the “blue planet”. Such increased levels of ultraviolet B radiation would be perilous for human health, leading to, skin cancers, cataracts and various infectious diseases. Laboratory experiments have demonstrated that chronic exposure to ultraviolet B rays could decrease the immune response to cancers, infectious diseases and other antigens. The increased levels of ultraviolet B radiation could also act as an impediment to physiological and developmental processes of plants, adversely affecting their growth and productivity. The repercussions of ozone depletion in the plant world could be profound, in that, the use of more ultraviolet B-tolerant cultivator in agriculture could result in changes in species composition, ultimately changing the bio-diversity of plant ecosystems. Since marine ecosystems contribute to around 30% of world's protein consumption, there is a widespread concern about the detrimental effects of increased ultraviolet- B penetration, on the productivity of the marine ecosystems.


A cross section of the ozone layer.

Higher levels of ultraviolet B solar radiation could have deleterious effects on the survival rates of the marine phytoplanktons, on account of their sensitivity to environmental changes. It could have a lethal impact on the whole aquatic eco-system, for, the marine planktons serve as the base for marine food chains and provide oxygen to the marine world. Studies have indicated a 6-12% decrease in their production around the marginal ice zones, due to increased levels of penetration of ultraviolet B radiation. The harmful effects of Ultra violet B radiation is also projected to have a negative impact on the development of animals , especially in the early stages of their life cycle, such as the eggs of frogs in shallow water. The impaired larval development and the decreased reproductive capacity of animals such as, crabs, shrimp, fish, amphibians, implies a significant shift in the domain of food webs. The loss of biodiversity could have debilitating consequences on the ecosystem, leaving it more susceptible to other environmental stresses, such as, the climate change induced by the Greenhouse effect. Further, increased levels of Ultraviolet B radiation, by reducing the plant cover, may decrease the pool of carbon dioxide gas in the planet, translating into profound climatic shifts. The Ozone molecules could be a boon or a bane, depending on its location. Whereas, the stratospheric ozone layer is comprised of the “good ozone” that protects life on earth, by absorbing and filtering out the harmful ultraviolet radiation emitted by the sun, ozone present in the troposphere, is the “bad ozone “in the form of harmful pollutants, causing breathing- related disorders in human beings and damaging the plants.

Protecting the Ozone Layer

In 1974, F. Sherwood Rowland and Mario Molina identified chlorofluorocarbons as destroyers of the ozone layer. Subsequent research and findings have substantiated their claims. Invented in 1928, the consumption of chlorofluorocarbons shot up to millions of tons by the 1980's, with large-scale use in various appliances and products. While developed countries were the main consumers, the developing countries were trailing with their huge population and growing economies. The challenge of the policy makers lied in devising and implementing plans for reduction and eventual elimination of man-made ozone depleting substances - the potential threats to ozone layer. To this end, the first international deliberations took place in 1977, under the auspices of United Nations Environmental Program and World Meteorological Organization, resulting in a coordinating committee of the ozone layer, for regular assessment and further research on ozone layer. Negotiations for formulating a treaty to restrict the ozone depleting substances led to the Vienna convention in 1985 and ultimately culminated in the Montreal protocol on substances that deplete the ozone layer, in 1987. The Montreal Protocol put limits on production and consumption of ozone depleting substances by various nations. The protocol also made provisions for an assessment of these control measures every four years. The original protocol aimed at cutting five chlorofluorocarbons by 50% , and three halons were to be frozen. The 1990 London amendments to the Montreal Protocol required complete elimination of chlorofluorocarbons, methyl chloroform and Halons. The 1992 Copenhagen amendments provided for controls on and eventual phase-outs of hydro chlorofluorocarbons by the end of 1995. The developing countries were given a grace period of 10 years under Article 5. Also, a multilateral fund was created to assist the developing countries in implementing their commitments towards the Montreal Protocol. The Montreal protocol marks a historic step towards ozone layer recovery through strict control measures, periodic assessments, ongoing research and strict compliance measures with stiff penalties for non-compliance.


Ozone depletion, as a human-induced environmental catastrophe, resulting from the large-scale production and use of chlorofluorocarbons plus other ozone depleting substances, with the consequent ozone-destroying chemical reactions, was estimated to have devastating effects on human, animal, plant and marine ecosystems. “The Montreal Protocol on Substances that Deplete the Ozone Layer”, signed in 1987 is an environmental landmark towards pre-empting ozone depletion by phasing out the ozone depleting substances through restrictions on production, consumption and trade in such harmful substances. Fortunately, it has been easy to develop environment friendly substitutes for ozone depleting substances. With due compliance, the Montreal protocol and the subsequent amendments are likely to eliminate the emission of major ozone depleting substances into the atmosphere. Strict control measures and ongoing vigilance for uniform compliance across nations, is required for its success. Since, chlorine and Bromine atoms do not remain in the upper atmosphere, forever, elimination of ozone depleting substances from the stratosphere, may provide the ozone layer a golden opportunity to repair itself, paving the way for ozone layer recovery.


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