Component-I

Component-IA (Personal Details)

Role Name Affiliation Principal Investigator Dr. C.P. Mishra Professor

Department of Community Medicine Institute of Medical Sciences Banaras Hindu University Varanasi, Uttar Pradesh, India

Paper Coordinator Dr. V.M. Gupta Ex-Professor & Head Department of Community Medicine Institute of Medical Sciences Banaras Hindu University Varanasi, Uttar Pradesh, India

Content Writer/ Author Dr. Madhutandra Sarkar Assistant Professor Department of Community Medicine Chettinad Hospital & Research Institute Chennai, Tamil Nadu, India

Content Reviewer Dr. C.P. Mishra Professor Department of Community Medicine Institute of Medical Sciences Banaras Hindu University Varanasi, Uttar Pradesh, India

Component-IB (Description of Module)

Items Description of Module

Subject Name Social Medicine and Community Health Paper Name Environmental Health

Module Name/Title Ozone Depletion and Green House Effect Module Id SMCH/EH/27

Pre-requisites Knowledge on ozone, ozone layer, greenhouse, greenhouse gases Objectives To study about ozone depletion and greenhouse effect

Key words Ozone, Depletion, Greenhouse gases, Greenhouse effect

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Component-II (e-Module)

Quadrant-I (e-Text)

1. Introduction

The ozone layer is a gaseous layer containing ozone (O3) molecules in the Earth’s upper atmosphere called stratosphere, between about 15-40 km above the Earth’s surface. The thickness of this layer varies according to season and geographical location. The ozone layer is essential to the survival of all living things. This layer protects life on Earth by filtering out most of the potentially harmful shortwave ultraviolet (UV) radiation from the sun. While both oxygen and ozone together absorb 95 to 99.9% of the sun’s UV radiation, only ozone effectively absorbs the most energetic types of UV light (UV-C and UV-B), which cause biological damage

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Each natural reduction in ozone levels has been followed by a recovery. A steady state concentration of ozone has been maintained for millions of years in the absence of anthropogenic influences. However, beginning in the 1970s, scientific evidence showed that the ozone shield was being depleted well beyond the changes due to natural processes

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Measurements from satellites, aircraft, ground-based sensors and other instruments indicate that the total integrated column levels of ozone, i.e. the number of ozone molecules occurring per square metre in sampled columns of air, decreased globally by roughly 5% between 1970 and the mid-1990s, with little change afterward

3. Human activities have had a significant effect

on the global concentration and distribution of stratospheric ozone since the early 1970s. The global decrease in stratospheric ozone is well correlated with the rising levels of chlorine and bromine in the stratosphere from the manufacture and release of chlorofluorocarbons (CFCs) and other halocarbons by human activities. Chlorine and bromine released from halocarbons react with and destroy ozone in the stratosphere. The largest decreases in ozone took place in the high latitudes (toward the poles), especially over Antarctica. The smallest decreases occurred in the lower latitudes (the tropics). In the mid-latitude (for example, over Australia), ozone layer is thinned. Ozone depletion is a major environmental problem, because it increases the amount of UV radiation that reaches the Earth’s surface. This could have serious impacts on human beings, animals and plants. Stopping ozone layer depletion is one of the major challenges facing the world today. The Montreal Protocol, ratified in 1987, was the first of several comprehensive international agreements to protect the stratospheric ozone layer by phasing out the production and consumption of ozone-depleting substances. As a result of continued international cooperation on this issue, the ozone layer is expected to recover over time. A greenhouse is a building made of glass that allows sunlight to enter but traps heat inside, so the building stays warm even when it is cold outside. Certain gases in the Earth’s atmosphere also let in sunlight but trap heat (like the glass walls of a greenhouse). This phenomenon is called the greenhouse effect. Jean Baptiste Joseph Fourier, a French mathematician, first discovered the greenhouse effect in 1824. The greenhouse effect works somewhat differently from an actual greenhouse, but the name has stuck anyway. The greenhouse gases, mainly water vapor (H2O), carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), act as effective global insulators, insulating the surface from the cold of space

4. These greenhouse gas molecules are responsible for the fact that the earth enjoys

temperatures suitable for our active and complex biosphere.

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2. Learning Outcomes

After completing this module, the students will be able to:

1. define ozone depletion and state the causes of ozone depletion 2. understand the impacts or effects of ozone depletion 3. explain the measures to control ozone depletion 4. define greenhouse effect and describe the greenhouse gases 5. understand the impacts of greenhouse effect and explain the measures to reduce

greenhouse effect

3. Ozone Depletion: Details

3.1. Definition

Ozone depletion is gradual thinning of Earth’s ozone layer in the upper atmosphere caused by the release of chemical compounds from industry and other human activities that contain gaseous chlorine and bromine.

Fig. 1: Ozone Layer Depletion

Source: http://www.celsias.com/article/how-guide-save-our-ozone-layer/

One example of ozone depletion is the annual ozone "hole" over Antarctica that has occurred during the Antarctic spring (September-November) since the early 1980s. This is not really a hole through the ozone layer, but rather a large area of the stratosphere with extremely low amounts of ozone (220 Dobson Units or lower).

Fig. 2: The Antarctic Ozone Hole (on October 4, 2004)

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Source: http://ozonewatch.gsfc.nasa.gov/facts/hole_SH.html 3.2. Causes of Ozone Depletion

3.2.1. Natural causes

Natural phenomena can cause temporary ozone loss. Natural phenomena, such as sunspots and stratospheric winds, decrease stratospheric ozone levels, but typically not by more than 1-2%. The major volcanic eruptions (mainly El Chichón in 1982 and and Mt. Pinatubo in 1991) have also contributed towards ozone depletion. 3.2.2. Man-made causes

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The ozone­depleting substances (ODS) are mainly responsible for man-made ozone depletion. Scientific evidence indicates that stratospheric ozone is being destroyed by the ozone­depleting substances, which are a group of manufactured chemicals containing chlorine and/or bromine. These chemicals are very stable, nontoxic and environmentally safe in the lower atmosphere. However, their very stability allows them to float up, intact, to the stratosphere. Then they are broken apart by the intense UV light, releasing chlorine (e.g. from chlorofluorocarbons) and bromine (e.g. from halons) in the stratosphere. Chlorine and bromine demolish ozone at an alarming rate, by stripping an atom from the ozone molecule. It is estimated that a single molecule of chlorine can break apart thousands of molecules of ozone. The ODS have a long lifetime (up to several centuries) in the atmosphere. Most of the ODS released by human activities over the last 80 years are still making their way to the stratosphere, adding to the ozone destruction.

Source: https://www.e-education.psu.edu/egee102/node/1973

3.2.2.1. Main ozone­depleting substances (ODS)5

Chlorofluorocarbons (CFCs): CFCs are the most widely used ODS, accounting for over 80% of total stratospheric ozone depletion. They are used as coolants in refrigerators, freezers and air conditioners in buildings and cars. They are found in industrial solvents, dry­cleaning agents and hospital sterilants. They are also used in foam products, such as soft­foam padding (e.g. cushions and mattresses) and rigid foam (e.g. home insulation).

Fig. 3: Sources of ODS

OOODSDepleting

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Halons: Halons (brominated fluorocarbons) also play a large role in the ozone depletion. They are used in specialized fire extinguishers, in cases where materials and equipment would be destroyed by water or other fire extinguisher chemicals. The problem with halons is that they can destroy up to 10 times as much ozone as CFCs can. However, their application is quite limited.

Methyl chloroform: This is used mainly in industry for vapour degreasing, cold cleaning, aerosols, adhesives and chemical processing.

Carbon tetrachloride: This is used in solvents and some fire extinguishers.

Hydrochlorofluorocarbons (HCFCs): HCFCs have become major, transitional substitutes for CFCs. They are much less harmful to stratospheric ozone than CFCs are. But they still cause some ozone destruction and are potent greenhouse gases.

3.2.2.2. Substitutes for ozone-depleting substances

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Hydrofluorocarbons (HFCs): These chemicals were developed as a replacement for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). CFCs and HCFCs are being phased out under the Montreal Protocol. HFCs do not deplete the stratospheric ozone layer. Unfortunately, they are potent greenhouse gases with long atmospheric lifetimes and high global warming potentials (GWPs). They are used as refrigerants, aerosol propellants, solvents and fire retardants. The source of major emissions of these compounds is their use as refrigerants (for example, in air conditioning systems in both vehicles and buildings). They are released into the atmosphere through leaks, servicing and disposal of equipment in which they are used. CFCs are even more powerful contributors to global climate change, though. HFCs are still the better option until even safer substitutes are discovered.

Perfluorocarbons (PFCs) and sulphur hexafluoride (SF6): These are other substitutes for ozone-depleting substances.

3.3. Impacts or Effects of Ozone Depletion

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3.3.1. Effects on human health

Ozone layer depletion increases the amount of UV-B that reaches the Earth’s surface. It causes non-melanoma skin cancer and plays a major role in the development of malignant melanoma. It can cause sunburns and premature aging of the skin. In addition, UV-B has been linked to the development of cataracts, a major cause of blindness in the world. UV radiation can cause genetic damage and immunosuppression.

3.3.2. Effects on plants

UV-B radiation affects the physiological and developmental processes of plants. Despite mechanisms to reduce or repair these effects and an ability to adapt to increased levels of UV-B, plant growth can be directly affected by UV-B radiation. Several of the world's major crop species are particularly vulnerable to increased UV, resulting in reduced growth, photosynthesis and flowering. These species include wheat, rice, barley, oats, corn, soya beans, peas, tomatoes, cucumbers, cauliflower, broccoli and carrots. Indirect changes caused by UV-B (such as changes in plant form, how nutrients are distributed within the plant, timing of developmental phases and secondary metabolism) may be equally

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or sometimes more important than damaging effects of UV-B. These changes can have important implications for competitive balance between plant species, herbivory, plant diseases and biogeochemical cycles. 3.3.3. Effects on marine ecosystems

Phytoplankton, also known as microalgae, form the foundation of aquatic food webs. In a balanced ecosystem, they provide food for a wide range of sea creatures including whales, shrimp, snails and jellyfish. Phytoplankton productivity is limited to the euphotic zone, the upper layer of the water column in which there is sufficient sunlight to support net productivity. Exposure to solar UV-B radiation has been shown to affect both orientation and motility in phytoplankton, resulting in reduced survival rates for these organisms. Scientists have demonstrated a direct reduction in phytoplankton production due to ozone depletion-related increases in UV-B. UV-B radiation has been found to cause damage to early developmental stages of fish, shrimp, crab, amphibians and other marine animals. The most severe effects are decreased reproductive capacity and impaired larval development. Small increases in UV-B exposure could result in population reductions for small marine organisms with implications for the whole marine food chain. 3.3.4. Effects on animals

In domestic animals, UV overexposure may cause eye and skin cancers similar to those observed in humans. 3.3.5. Effects on biogeochemical cycles

Increases in UV-B radiation could affect terrestrial and aquatic biogeochemical cycles, thus altering both sources and sinks of greenhouse and chemically important trace gases (e.g. carbon dioxide, carbon monoxide, carbonyl sulfide, ozone and possibly other gases). These potential changes would contribute to biosphere-atmosphere feedbacks that mitigate or amplify the atmospheric concentrations of these gases. 3.3.6. Effects on materials

Synthetic polymers (plastic, rubber), naturally occurring biopolymers (wood), fabrics as well as some other materials of commercial interest are adversely affected by UV-B radiation. In recent years, materials are somewhat protected from UV-B by special additives. However, increases in UV-B levels will accelerate their breakdown, limiting the length of time for which they are useful outdoors. The socio-economic impacts of replacing and/or protecting materials could be significant in developing countries including those in near-equator regions, where plastic and wood are popular building materials. 3.4. Measures to Control Ozone Depletion

3.4.1. Measures to help protect the ozone layer at the individual level

Make sure that old refrigerators and air conditioners are disposed of safely by giving them to a recycling yard. Take care not to damage the cooling circuit which contains the ODS.

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Ensure technicians repairing refrigerator or air conditioner recover and recycle the old ODS, so they are not released into the atmosphere.

Conduct regular inspection and maintenance of air-conditioning and refrigeration appliances to prevent and minimize refrigerant leakage.

Buy air-conditioning and refrigeration equipment that do not use HCFCs as refrigerant.

Buy aerosol products that do not use HCFCs or CFCs as propellants.

When motor vehicle air-conditioners need servicing, make sure that the refrigerants are properly recovered and recycled instead of being vented to the atmosphere.

When renovating house, make sure that old insulation foams containing ODS are

disposed of as environmentally hazardous waste.

Plant trees.

Increase awareness of the problem and initiate local action. 3.4.2. International effort

In September 1987, an international treaty aimed at saving the Earth's ozone layer, known as the Montreal Protocol on Substances that Deplete the Ozone Layer, was signed in Montreal, Canada. The Protocol requires the phasing out of the ODS in accordance with agreed schedules. Atmospheric levels of the ODS rapidly increased before the implementation of the Montreal Protocol and its subsequent revisions and amendments. However, the atmospheric levels of nearly all these substances have declined substantially in the past two decades. Continued declines in the ODS emissions are expected to result in a near complete recovery of the ozone layer near the middle of the 21st century. The long time scale for this recovery is due to the slow rate at which the ODS are removed from the atmosphere by natural processes.

4. Greenhouse Effect: Details 4.1. Definition

The greenhouse effect is a natural process that warms the Earth’s surface. The sun powers the Earth’s climate, radiating energy at very short wavelengths, predominately in the visible or near-visible (e.g. ultraviolet) part of the spectrum. Roughly, one-third of the solar energy that reaches the top of the Earth’s atmosphere is reflected directly back to space. The remaining two-thirds is absorbed by the surface and, to a lesser extent, by the atmosphere. To balance the absorbed incoming energy, the Earth must, on average, radiate the same amount of energy back to space. Because the Earth is much colder than the sun, it radiates at much longer wavelengths, primarily in the infrared part of the spectrum. Much of this thermal radiation emitted by the land and ocean is absorbed by the atmosphere (including clouds) and reradiated back to the Earth

8. The atmosphere surrounding the Earth contains the greenhouse gases and

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acts like a blanket, trapping more heat and turning the Earth into a greenhouse. This is called the greenhouse effect. This process maintains the Earth’s temperature at around 33 degrees C warmer than it would otherwise be, allowing life on the Earth to exist. If there were no greenhouse effect, the Earth’s average surface temperature would be a very chilly -18 degrees C (0 degree F) instead of the comfortable 15 degrees C (59 degrees F) that it is today

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The greenhouse effect is somewhat similar to the process that goes on in a real greenhouse. The glass walls in a greenhouse reduce airflow and increase the temperature of the air inside. Analogously, but through a different physical process, the Earth’s greenhouse effect warms the surface of the planet. Without the natural greenhouse effect, the average temperature at the Earth’s surface would be below the freezing point of water. The natural greenhouse effect is beneficial for life on Earth. The most important greenhouse gases are water vapour and carbon dioxide. Clouds exert a blanketing effect similar to that of the greenhouse gases. However, this effect is offset by their reflectivity and overall, clouds tend to have a cooling effect on the Earth’s climate

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4.1.1. Enhanced greenhouse effect

Human activities, particularly the burning of fossil fue ls (coal, oil and natural gas) and clearing of forests, are increasing the concentrations of greenhouse gases and thus are greatly intensifying the natural greenhouse effect, causing global warming. This is the enhanced greenhouse effect.

Fig. 4: Greenhouse Effect

Source: http://www.environment.gov.au/climate-change/climate-science/greenhouse-effect

The greenhouse effect (natural as well as enhanced) can be simplified in the following steps as depicted in the figure above:

Step 1: Solar radiation reaches the Earth's atmosphere. Some of this is reflected back into space.

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Step 2: The rest of the sun's energy is absorbed by the land and the oceans, heating the Earth.

Step 3: Heat radiates from the Earth towards space.

Step 4: Some of this heat is trapped by the greenhouse gases in the atmosphere, keeping the Earth warm enough to sustain life (natural greenhouse effect).

Step 5: Human activities such as burning fossil fuels, agriculture and land clearing are increasing the amount of greenhouse gases released into the atmosphere.

Step 6: This is trapping extra heat and causing the Earth's temperature to rise (enhanced greenhouse effect).

4.2. Greenhouse

A greenhouse is a house where the walls and roof are made of glass. An important property of glass is that it transmits infrared radiation of very short wavelength (coming from extremely hot bodies like the sun), while it reflects infrared radiations of long wavelengths (emitted by less hot objects like those inside a greenhouse). This property of glass is used to keep the plants warm and safe in a greenhouse during the winter. The glass of a greenhouse allows the visible light and infrared radiations of short wavelength to pass through. The radiations from the sun are, therefore, able to penetrate the glass and enter into the greenhouse. These radiations are then absorbed by the plants and other objects inside the greenhouse. As a result, they become warm. The warm objects emit infrared radiations of longer wavelengths. These longer wavelength radiations are reflected by the glass and are retained inside. The heat remains inside the greenhouse and keeps it warm.

Fig. 5: A Greenhouse

Source: http://www.conserve-energy-future.com/GreenhouseEffectCauses.php

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4.3. Greenhouse Gases

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The gases that trap heat in the atmosphere are called the greenhouse gases. They have the property of absorbing infrared radiation (net heat energy) emitted from the Earth’s surface and reradiating it back to the Earth’s surface, thus contributing to the greenhouse effect. Many greenhouse gases, including carbon dioxide, methane, nitrous oxide, ozone and water vapour, are naturally present in the atmosphere. Other greenhouse gases are synthetic chemicals that are emitted only as a result of human activity. Human activities, especially combustion of fossil fuels since the Industrial Revolution, are responsible for steady increases in atmospheric concentrations of various greenhouse gases, especially carbon dioxide, methane, nitrous oxide and fluorinated gases. Human activities are increasing the concentrations of greenhouse gases faster than these can be offset by natural sinks.

4.3.1. Carbon dioxide (CO2) CO2 is the most significant greenhouse gas. Natural sources of atmospheric CO2 include outgassing from volcanoes, the combustion and natural decay of organic matter, and respiration by aerobic organisms. These sources are balanced, on average, by a set of physical, chemical or biological processes, called “sinks” that tend to remove CO2 from the atmosphere. Significant natural sinks include terrestrial vegetation, which takes up CO2 during the process of photosynthesis. A number of oceanic processes also act as carbon sinks.

Carbon dioxide enters the atmosphere through the burning of fossil fuels (for use in transportation, heating and the production of electricity), solid waste, trees and wood products, and also as a result of certain chemical reactions (e.g. manufacture of cement).

4.3.2. Methane (CH4)

CH4 is the second most important greenhouse gas. CH4 is more potent than CO2. However, CH4 exists in far lower concentrations than CO2 in the atmosphere. CH4 also has a considerably shorter residence time in the atmosphere than CO2. The residence time for CH4 is roughly 10 years, compared with hundreds of years for CO2. Natural sources of CH4 are tropical and northern wetlands, termites, volcanoes, oceans, etc. The primary natural sink for methane is the atmosphere itself, as methane reacts readily with the hydroxyl radical (OH-) within the troposphere to form CO2 and water vapour (H2O). When CH4 reaches the stratosphere, it is destroyed. Another natural sink is soil, where methane is oxidized by bacteria. Globally, over 60% of total CH4 emissions come from human activities. Methane is emitted during the production and transport of fossil fuels. Methane emissions also result from livestock and other agricultural practices, and by the decay of organic waste in municipal solid waste landfills. 4.3.3. Nitrous oxide (N2O) Nitrous oxides have small background concentrations due to natural biological reactions in soil and water. Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste.

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4.3.4. Fluorinated gases

The fluorinated gases owe their existence almost entirely to industrial sources. Hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6) and nitrogen trifluoride (NF3) are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes. Fluorinated gases are sometimes used as substitutes for stratospheric ozone-depleting substances (e.g. chlorofluorocarbons, hydrochlorofluorocarbons and halons). These gases are typically emitted in smaller quantities. However, they are potent greenhouse gases and they are sometimes referred to as high global warming potential gases. 4.3.5. Surface-level ozone (O3)

Surface or low-level ozone (O3) is a result of air pollution. It must be distinguished from naturally occurring stratospheric O3, which has a very different role in the planetary radiation balance. The primary natural source of surface O3 is the subsidence of stratospheric O3 from the upper atmosphere. The primary anthropogenic source of surface O3 is photochemical reactions involving the atmospheric pollutant carbon monoxide (CO). 4.3.6. Water vapour (H2O)

It is the most potent of the greenhouse gases in the Earth’s atmosphere. However, its behaviour is fundamentally different from that of the other greenhouse gases. The primary role of water vapour is not as a direct agent of radiative forcing but rather as a climate feedback, i.e. as a response within the climate system that influences the system’s continued activity. This distinction arises from the fact that the amount of water vapour in the atmosphere cannot, in general, be directly modified by human behavior. Instead it is set by air temperatures. The warmer the surface, the greater is the evaporation rate of water from the surface. As a result, increased evaporation leads to a greater concentration of water vapour in the lower atmosphere capable of absorbing infrared radiation and emitting it downward. 4.4. Impacts of Greenhouse Effect

Global warming: The main cause of the current global warming trend is the enhanced greenhouse effect, i.e. warming that results when the atmosphere traps heat radiating from the Earth toward space. Global warming is the main effect of increases in atmospheric greenhouse gas concentrations. It could have disastrous consequences (vide Module SMCH/EH/28 for details).

Albedo effect: Albedo is the ratio of the outgoing solar radiation reflected by an object to the incoming solar radiation incident upon it. Lighter coloured surfaces (such as snow) reflect more light and heat back into space than the dark coloured surfaces (that of a full forest and tree canopy). Sometimes the impacts of the greenhouse effect are stated in terms of the albedo of the earth, the overall average reflection coefficient. For example, the albedo of the Earth is 0.39 (Kaufmann) and this affects the equilibrium temperature of the Earth. The greenhouse effect, by trapping infrared radiation, can lower the albedo of the earth and cause global warming.

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4.5. Measures to Reduce Greenhouse Effect12

The following steps are effective in reducing greenhouse gas emissions:

Reduce, reuse, recycle. It helps conserve energy and reduces pollution and greenhouse gas emissions from resource extraction, manufacturing and disposal.

Use water efficiently. It takes lots of energy to pump, treat and heat water. Saving water reduces greenhouse gas emissions.

Use energy-efficient light bulbs at home. Use energy-efficient products and appliances.

Save energy and reduce greenhouse gas emissions by setting computer, monitor and

any other equipment to power down when not in use.

Use green power. This is environmentally friendly electricity generated from renewable energy sources such as wind and the sun.

Switching to public transportation, carpooling, biking, or telecommuting can save energy and reduce greenhouse gas emissions.

Purchase a fuel-efficient, low-greenhouse gas vehicle. Drive smart to improve fuel economy and reduce greenhouse gas emissions. Maintain car regularly. A well-maintained car is more fuel-efficient and produces fewer greenhouse gas emissions.

5. Summary

Ozone depletion is a major environmental problem, because it increases the amount of UV radiation that reaches the Earth’s surface. Ozone depletion is caused by the release of certain chemical compounds, containing gaseous chlorine and/or bromine, from industry and other human activities. These chemicals are called ozone­depleting substances (ODS). The impacts of ozone depletion include increasing the rate of skin cancer, eye cataracts, and genetic and immune system damage. It can cause damage to crops, harm to marine life. It can affect certain materials such as plastic, rubber, wood. Apart from the efforts at the individual level, the international effort is also necessary to save the Earth's ozone layer. The Montreal Protocol on Substances that Deplete the Ozone Layer is a successful international effort in this regard. The greenhouse effect is a natural process that warms the Earth’s surface. Without the natural greenhouse effect, the average temperature at the Earth’s surface would be below the freezing point of water. The natural greenhouse effect is beneficial for life on Earth. Human activities, particularly the burning of fossil fuels (coal, oil and natural gas) and clearing of forests, are increasing the concentrations of greenhouse gases and thus are greatly intensifying the natural greenhouse effect, causing global warming. This is the enhanced greenhouse effect. The greenhouse gas emissions can be reduced by following certain simple steps like reduce, reuse and recycle, use of energy-efficient light bulbs, other energy-efficient products and appliances, use of public transportation, etc.

6. References 1. University Corporation for Atmospheric Research (UCAR). Introduction to ozone.

Available at: https://www.ucar.edu/learn/1_5_1.htm. Accessed March 17, 2016.

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2. United States Environmental Protection Agency. Basic ozone layer science. Available at:

https://www.epa.gov/ozone-layer-protection/basic-ozone-layer-science. Accessed March 17, 2016.

3. Encyclopaedia Britannica. Ozone depletion. Available at:

http://www.britannica.com/science/ozone-depletion. Accessed March 17, 2016. 4. University Corporation for Atmospheric Research (UCAR). The greenhouse effect.

Available at: https://www.ucar.edu/learn/1_3_1.htm. Accessed March 17, 2016. 5. British Columbia Air Quality. The causes of ozone depletion. Available at:

http://www.bcairquality.ca/101/ozone-depletion-causes.html. Accessed March 17, 2016. 6. United States Environmental Protection Agency. Emissions of fluorinated gases.

Available at: https://www3.epa.gov/climatechange/ghgemissions/gases/fgases.html. Accessed March 17, 2016.

7. United States Environmental Protection Agency. Health and environmental effects of

ozone layer depletion. Available at: https://www.epa.gov/ozone-layer-protection/health-and-environmental-effects-ozone-layer-depletion. Accessed March 17, 2016.

8. Le Treut, H., Somerville, R., Cubasch, U., Ding, Y., Mauritzen, C., Mokssit, A., Peterson,

T., Prather, M. Historical overview of climate change. In: Climate change 2007: the physical science basis. Contribution of working group I to the fourth assessment report of the Intergovernmental Panel on Climate Change [Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller, H.L. (eds.)]. Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press, 2007.

9. NASA Earth Observatory. Global warming. Available at: http://earthobservatory.nasa.gov/Features/GlobalWarming/page2.php. Accessed March 17, 2016.

10. United States Environmental Protection Agency. Overview of greenhouse gases.

Available at: https://www3.epa.gov/climatechange/ghgemissions/gases.html. Accessed March 17, 2016.

11. Encyclopaedia Britannica. Greenhouse gas. Available at:

http://www.britannica.com/science/greenhouse-gas. Accessed March 17, 2016. 12. United States Environmental Protection Agency. What you can do. Available at:

https://www3.epa.gov/climatechange/wycd/. Accessed March 17, 2016.

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