Environmental Issues Important Questions for CBSE Class 12 Biology Greenhouse Effect, Ozone Depletion and Deforestation

1.Greenhouse effect is a naturally occurring phenomenon that is responsible for heating of earth’s surface and atmosphere due to the presence of certain gases in the atmosphere.

(i) In the absence of greenhouse effect, the average temperature of earth would have been a chilly -18 °C rather than the present average of 15 °C.

(ii) Clouds and gases reflect about one-fourth of the incoming solar radiation and absorb some of it. But, almost half of incoming solar radiation falls on earth’s surface heating it, while a small proportion is reflected back.

(iii) Earth’s surface re-emits heat in the form of infrared radiation but part of this does not escape into space and atmospheric gases (e.g. carbon dioxide, methane, etc.) absorb a major fraction of it.

(iv) These gases radiate heat energy and a major part of which again comes to earth’s surface, thus heating it up once again. This cycle is repeated again and again.

(v) The above mentioned gases-carbon dioxide and methane are commonly known as greenhouse gases because they are responsible for the greenhouse effect.

2.  Global warming is the gradual and continuous increase in average temperature of surface of the earth as a result of increase in concentration of greenhouse gases.

(i) Cause of Global Warming

High levels of greenhouse gases (C02, CFCs, CH4, etc.) in the atmosphere. All these gases allow the heat waves to reach earth but prevent their escape and the earth becomes warm.

(ii) Effects of Global Warming

  • Earth’s temperature has increased by 0.6 °C in last three decades. This causes change in precipitation patterns.
  • The rise in temperature leads to harmful effects in environment leading to odd climatic changes, e.g. El Nino effect.
  • The high temperature will result in melting of polar ice caps, which will lead to rise in sea level and many coastal areas will be submerged.
  • The high levels of temperature lead to increased weed growth, eruption of diseases and pests. Thus, the crop productivity will decrease.


(iii) Global Warming can be Controlled by

  • Cutting down the use of fossil fuels.
  • Improving efficiency of energy usage.
  • Reducing deforestation.
  • Planting trees, slowing down the growth of human population.
  • Taking international initiatives to reduce the emission of greenhouse gases into the atmosphere.

3. Ozone Depletion in the Stratosphere

(i) Ozone can be

(a) Bad ozone It formed in troposphere and is harmful to plants and animals.

(b) Good ozone It present in stratosphere and acts as a shield, absorbing harmful UV     radiation from the sun.

(ii) The thickness of the ozone is measured in Dobson Units (DU).

(iii)  Ozone gas is continuously formed by the action of UV rays on molecular oxygen.


There should be a balance between production and degradation of ozone in the     stratosphere.

(iv) This balance gets disrupted due to enhancement of ozone degradation by   Chlorofluorocarbons (CFCs). CFCs are used as refrigerants.

(v) Ozone degradation When CFCs are discharged into lower part of the atmosphere, they move upward and reach the stratosphere.

  • In stratosphere, UV rays act on them releasing Cl atoms.
  • Cl degrades ozone releasing molecular oxygen, with these atoms acting merely as    catalysts.
  •  Cl atoms are not consumed in the reaction.
  • Hence, whatever CFCs are added to the stratosphere, they have permanent and continuous effect on ozone levels.
  • The ozone depletion is particularly marked over the Antarctic region. This led to formation of a large area of thinned ozone layer, called as ozone hole.


(vii) Harmful Effects of Ozone Depletion

  • UV-B damages DNA, causing mutation.
  • Ageing of skin, damage to skin cells and various types of skin cancer.
  • High dose of UV-B causes inflammation of cornea. This is called snow-blindness, cataract, etc. Such exposure may permanently damage the cornea.

(viii) Control of Ozone Depletion

  • An international treaty, known as the Montreal Protocol was signed at Montreal (Canada) in 1987 (effective in 1989) to control the emission of ozone depleting substances.
  • More protocols have been laid down for controlling emission of CFCs and other ozone depleting chemicals, separately for developed and developing countries.

4.Degradation by Improper Resource Utilisation and Maintenance

(i) The degradation of natural resources can occur not just by the action of pollutants but also by improper resource utilisation practices.

(ii) Results of improper resource utilisation are:

(a) Soil erosion and desertification

  • The formation of fertile topsoil takes centuries.
  • Due to human activities like over-cultivation, unrestricted grazing, deforestation and poor irrigation practices, fertile topsoil is removed, resulting into arid patches of land.
  • These barren patches when left for long time, form deserts.
  • Desertification is a major problem now-a-days due to increased urbanisation.

(b) Water logging and soil salinity

  • Irrigation without proper drainage of water leads to water logging in the soil.
  • This leads to more salt deposition on the land surface or collection at the roots of the plants.
  • This salt content inhibits the growth of crops and damages agriculture.

5.Deforestation is the conversion of forested areas to non-forested areas.

  • Almost 40% forests have been lost in the tropics, compared to only 1% in the temperate region.
  • In India, in the beginning of the 20thth century, forests covered about 30% of the land, whereas by the end of the century, it reduced to 19.4%.
  • The National Forest Policy (1988) has recommended 33% forest cover for the plains and 67% for the hills.
  • Reasons of Deforestation

(a) Urbanisation                                                                   (b) Overgrazing by animals

(c) Forest fires (d) Demand of wood and other forest products

(e) Slash and burn agriculture or Jhum cultivation In this, farmers cut down the trees and burn the plant remains. The ash is used as a fertiliser and the land is then used for farming or cattle grazing. After cultivation, the area is left for several years to allow its recovery. In earlier times, enough time gap was given so that the land recovered from the effect of cultivation With increasing population and repeated cultivation, this recovery phase is not available leading to deforestation.

  • Effects of Deforestation

(a) Increased levels of C02 concentration in the atmosphere.

(b) Loss of biodiversity due to habitat destruction.

(c) Disturbs hydrologic cycle.

(d) Leads to soil erosion.

(e) Desertification also occurs in extreme cases.

  • Conservation of Forests

Reforestation is the process of restoring a forest that once existed but was removed at some point of time in the past. Reforestation may :

(a) occur naturally in a deforested area.

(b) can be speed up by planting trees.

A Case Study—People’s Participation in Conservation of Forests

  • In 1731, a Bishnoi woman Amrita Devi (Jodhpur) showed, exemplary courage by hugging a tree and prevented it from cutting. Her three daughters and hundreds of other Bishnois, followed her and lost their lives, while saving trees.
  • The Government of India has recently instituted the Amrita Devi Bishnoi Protection Award for individuals or communities from rural areas that have shown extraordinary courage and dedication in protecting wildlife.
  • Chipko movement in Garhwal Himalayas started in 1974. Local women showed enormous courage in protecting trees from the axe of contractors by hugging them.
  • The Government of India in 1980s has introduced the concept of Joint Forest Management (JFM), after realising the value of participation of local communities in forest conservation. In return for their services to the forest,5 the communities get benefit of various forest products, thus sustainable development of forest can occur.


Previous Years Questions  

1 Mark Questions

1.Why is Gambusia introduced into  drains and ponds?                         [All India 2014]

Ans. Gambusia is introduced into drains and ponds to control breeding and propagation of mosquitoes as they feed on mosquito larva. Thus, controls mosquito population.

2.Name the greenhouse gases that contribute to global warming.  [Delhi 2014]

Ans. The greenhouse gases that contribute to  global warming are C02, CH4, N20 and CFCs.

3. State the purpose of signing the  Montreal Protocol.                       [Foreign 2014]

Ans. The purpose of signing the Montreal  Protocol is to control the emission of ozone depleting substances.

4. Where is good ozone present? Why is  it called SO?           [All India 2014C]  

Ans. Good ozone is present in stratosphere, the  upper part of atmosphere.

It is called ‘good ozone’ because it acts as a shield absorbing ultraviolet radiations from sun.

5. Name the two gases contributing maximum to the greenhouse effect.  [Delhi 2014C]

Ans. The two gases contributing maximum to greenhouse effect are C02 and CH4.

6. Write the unit used for measuring  ozone thickness.                               [Delhi 2011]

Ans. Dobson Unit (DU) is used for measuring  ozone thickness.

7. How does jhum cultivation promotes  deforestation?                                     [All India 2011c]

Ans. In jhum cultivation, the farmers cut down  the trees of forest and burn the plant remains. The ash is used as fertiliser and land is used for farming. After cultivation the area is left free for several years for recovery.  The recovery  phase is   often  ignored leading to deforestation.

8. How is snow blindness caused in  humans?                                                       [All India 2010]

Ans. Snow blindness is caused by absorption of  UV-B radiation.

9. Name two greenhouse gases produced by anaerobic microbes.  [Foreign 2010]

Ans. Carbon dioxide and methane are the  green house gases produced by anaerobic microbes.

10. Mention the causes of thinning of  ozone layer.                                             [Delhi 2010c]

Ans. CFCs release chlorine atoms, when UV-rays act on them. Chlorine degrades the ozone into molecular oxygen. Thus, causing   thinning of     ozone         layer.

2 Marks Questions

11. Write, what was the percentage of  forest cover of India at the beginning and at the end of twentieth century? How different is it from the one recommended by National Forest Policy?         [Foreign 2014]

Ans. In the beginning of the twentieth century,  the forest cover was about 30% while towards its end is reduced to only 19.4%. The National Forest Policy of India recommends approximately 60% forest cover for hilly regions including Himalayas, while 33% for plains. However, the situation is contrastingly different as the production of forests in India is very slow.

12. (i) State the cause of depletion of  ozone layer.

(ii) Specify any two ill effects that it can cause in the human body.  [Foreign 2014]

Ans. (i) The main cause of depletion of ozone  layer is due to imbalance between production and degradation of ozone in the stratosphere. This balance is disrupted mainly due to increase in ozone degradation by GFCs.

(ii) The two ill-effects that ozone depletion can cause in human body are:

  • Allows UV-B radiation to enter atmosphere that increases mutation and damages DNA.
  • Causes ageing of skin and damage to skin cells and cancer.
  • Inflammation of cornea (snow  blindness), cataract, etc.

13. Refrigerants are considered to be a  necessity in modem living, but are said to be responsible for ozone holes detected in Antarctica. Justify.   [Foreign 2012]


Chlorofluorocarbons (CFCs) are widely used as refrigerants. Then why it is suggested to reduce its emission as far as possible? Explain.     [Delhi 2010]  

Ans. (i) Refrigerants contain CFCs which are released in lower part of the atmosphere. (/) They move upward and reach the  stratosphere.

(ii) In stratosphere, UV-rays act on them releasing Cl atoms. Cl degrade ozone and releases molecular oxygen.

(iii) Cl atoms act only as catalysts, the CFCs have a permanent damaging effect on ozone.

14. The figure given below shows the relative contribution of four greenhouse gases to global warming  

  • Identify the gases A and G.
  • Why are these four gases called greenhouse gases? [Foreign 2011]

Ans. (i) A- Carbon dioxide (C02)  C – Chlorofluorocarbons CFCs.

(ii) These for gases (C02, CH4, N20 and CFCs) are called greenhouse gases, as they absorb infrared radiations emitted by the earth’s surface.
15. (i) Name the greenhouse gases that caused global warming.

(ii) Which of them has caused ozone hole and how? [Foreign 2008]

Ans. Greenhouse gases that caused global  warming, i.e. carbon dioxide, methane, Chlorofluorocarbons (CFCs) and nitrous oxide.

(ii) CFCs cause ozone hole. These are degraded in the stratosphere in the presence of UV-radiation, releasing Cl atom. Cl atoms act as catalyst in the  degradation of ozone releasing molecular oxygen. Since, all Cl atoms are not used up in the reaction, they continue to affect the ozone.

16. How does global warming pose a threat to the coastal areas of the earth? Explain. [All India 2008C]

Ans. Rise in global temperature is causing deleterious changes in the environment resulting in odd climatic changes (El Nino effect). It leads to melting of ice caps as well as of Himalayan snow caps. Over many years, this will result in a rise in sea leve! that may submerge many coastal areas. Hence, it is a threat to coastal areas

 3 Marks Questions  

17. How do human activities cause  desertification?         [Delhi 2009c]

Ans. The human activities like over-cultivation,  unrestricted grazing, poor irrigation practices, results in arid patches of land (deforestation). When these large barren patches extend and remain unattended for long, a desert is created. Because the formation of fertile top layer of soil takes millions of years, desertification easily takes over.

18. Explain greenhouse effect and depletion of ozone layer with reference to global warming.  [All India 2009C]

Ans. Greenhouse effect is the phenomenon  responsible for heating of the earth due to the presence of certain gases in the atmosphere. Greenhouse gases (C02 CH4 N20 and CFCs) allow the solar radiations to enter, but prevent the escape of heat radiations of longer wavelength. When these greenhouse gases increases4n concentration in the atmosphere, global warming occurs .

Depletion of ozone layer occur due to increased level of CFCs. CFCs react with UV-rays in stratosphere to release chlorine atoms. Cl degrades ozone and releases molecular oxygen. CFCs have permanent and continuous effect as Cl atoms are not consumed. This leads to thinning of ozone layer. As a result, harmful radiations enter the earth’s atmosphere, which causes rise in temperature and thus, global warming results.

19. How is ozone formed in the stratosphere? Why is it called good ozone? Explain CFCs contribute to ozone hole formation. [All India 2009c]

Ans. Formation of ozone in stratosphere

(i) Nascent oxygen combines with molecular oxygen (02) to form ozone by the action of UV-rays.

It is called good ozone as it absorbs harmful UV radiations from the sun.

CFCs react with UV in stratosphere to release Cl atoms. Cl atoms act as catalyst to degrade ozone and release molecular oxygen. CFCs have permanent and continuous effect as Cl atoms are not consumed. This leads to thinning of ozone layer. It has resulted in the formation of ozone hole as observed over the Antarctica region.

20. It has been recorded that the temperature of the earth’s atmosphere has increased by 0.6 °C.

(i) What has caused this increase? (ii) Explain its consequences.  [Delhi 2008]

Ans. Increase in the level of greenhouse gases  (C02 CFCs, etc.) in the atmosphere allow the heat waves to reach earth but prevent their escape and thus, the earth becomes warm.

(ii) Effects of increased temperature

  • It leads to deleterious changes in environment resulting into odd climatic changes called El Nino effect.
  • It results into increased melting of polar ice caps which will cause the rise in sea level and many coastal areas also be submerged.
  • It leads to increased weed growth, eruption of diseases and pests. Thus, the crop productivity also decreases.

5 Marks Questions  

21. Why is the ozone layer required  in the stratosphere? How does it get degraded? Explain.

(ii) Why is the ozone depletion a threat to mankind? [Delhi 2013c]

Ans. Ozone found in stratosphere is  required because it acts as a shield absorbing harmful ultraviolet radiations coming from the sun. (l)’ Ozone layer gets depleted by the ozone depleting compounds mainly Chlorofluorocarbons (CFCs), which travel to the stratosphere after being released from the refrigerants and industrial emissions. UV rays from the sun acts on CFCs releasing atoms which degrade ozone releasing molecular oxygen.

(ii) Ozone depletion is proved to be a threat to mankind in following ways

  • Causes damage to human skin cells.
  • Causes mutation and induce  breaks in chemical bonds of DNA molecules.

22. (i) Expand CFG.

(ii) CFGs are a part of greenhouse gases. Name the other gases.

(iii) Explain the major harms caused by these gases.

(iv) Mention the consequences of the degradation of O3 .  [All India 2009C]

Ans. (i) Chlorofluorocarbon.

(ii) C02, CH4, N2Oare other  greenhouse gases.

(iii) These gases absorb radiations, that comes to earth’s surface and heat it. This cycle is repeated many times, heating the earth.

(iv)  Ozone depletion is proved to be a threat to mankind in following ways

  • Causes damage to human skin cells.
  • Causes mutation and induce  breaks in chemical bonds of DNA molecules.

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