Ozone Depletion, Greenhouse Effect, and Global Warming

Attempt the quiz above first, then review every question with its correct answer and a short explanation below.

1. Q1.Without the naturally occurring greenhouse effect, what would be the average temperature of the Earth's surface instead of the current comfortable 15°C?

   • A.0°C
   • B.-18°C✓ Correct Answer
   • C.-50°C
   • D.5°C

   Explanation: NCERT Class 12 Biology Chapter 16 states: 'The greenhouse effect is a naturally occurring phenomenon... Without greenhouse effect the average temperature at surface of Earth would have been a chilly -18°C rather than the present average of 15°C.'

2. Q2.According to NCERT Class 12 Biology, what are the relative percentage contributions of different greenhouse gases to total global warming?

   • A.CO2: 60%, CH4: 20%, CFCs: 14%, N2O: 6%✓ Correct Answer
   • B.CO2: 40%, CH4: 40%, CFCs: 10%, N2O: 10%
   • C.CO2: 80%, CH4: 10%, CFCs: 5%, N2O: 5%
   • D.CO2: 50%, CH4: 25%, CFCs: 15%, N2O: 10%

   Explanation: NCERT Chapter 16 provides a pie chart showing the relative contribution of greenhouse gases: Carbon dioxide (CO2) contributes 60%, Methane (CH4) contributes 20%, Chlorofluorocarbons (CFCs) contribute 14%, and Nitrous oxide (N2O) contributes 6%.

3. Q3.Where is the 'good' ozone found in the atmosphere, which acts as a shield absorbing harmful ultraviolet (UV) radiation from the sun?

   • A.Troposphere
   • B.Stratosphere✓ Correct Answer
   • C.Mesosphere
   • D.Thermosphere

   Explanation: NCERT documents: 'Ozone gas is continuously formed by the action of UV rays on molecular oxygen... "Bad" ozone is formed in the lower atmosphere (troposphere) that harms plants and animals. There is "good" ozone also, this is found in the upper part of the atmosphere called the stratosphere, and it acts as a shield absorbing ultraviolet radiation from the sun.'

4. Q4.The thickness of the ozone layer in a column of air from the ground to the top of the atmosphere is measured in terms of which unit?

   • A.Decibel Units (dB)
   • B.Dobson Units (DU)✓ Correct Answer
   • C.Parts per million (ppm)
   • D.Nanometers (nm)

   Explanation: NCERT Class 12 Biology Chapter 16 states: 'The thickness of the ozone in a column of air from the ground to the top of the atmosphere is measured in terms of Dobson units (DU).'

5. Q5.In the stratosphere, chlorofluorocarbons (CFCs) are broken down by UV rays to release which highly reactive atoms, acting as catalysts to degrade ozone?

   • A.Carbon atoms
   • B.Chlorine atoms✓ Correct Answer
   • C.Fluorine atoms
   • D.Oxygen free radicals

   Explanation: NCERT Chapter 16 explains: 'UV rays act on CFCs releasing Cl [chlorine] atoms. Cl degradation of ozone releases molecular oxygen... Cl atoms are not consumed in the reaction; they act merely as catalysts. Therefore, whatever CFCs are added to the stratosphere, they have permanent and continuing effects on Ozone levels.'

6. Q6.Although ozone depletion is occurring widely, the depletion is particularly marked over which geographic region, seasonally creating an 'Ozone Hole'?

   • A.The Equator
   • B.Antarctic region✓ Correct Answer
   • C.Arctic region only
   • D.Greenland

   Explanation: NCERT Class 12 Biology states: 'Although ozone depletion is occurring widely in the stratosphere, the depletion is particularly marked over the Antarctic region. This has resulted in formation of a large area of thinned ozone layer, commonly called as the ozone hole.'

7. Q7.What is the season during which the Antarctic 'Ozone Hole' becomes most prominent and is monitored annually?

   • A.Late August to early October✓ Correct Answer
   • B.December to February
   • C.April to June
   • D.No seasonal pattern exists

   Explanation: NCERT Class 12 Biology specifies that the ozone hole occurs over Antarctica seasonally, developing every year between late August and early October.

8. Q8.In the human eye, high doses of UV-B radiation cause inflammation of the cornea, commonly known as:

   • A.Glaucoma
   • B.Snow-blindness✓ Correct Answer
   • C.Presbyopia
   • D.Myopia

   Explanation: NCERT Class 12 Biology Chapter 16 lists UV-B effects: 'In human eye, cornea absorbs UV-B radiation, and a high dose of UV-B causes inflammation of cornea, called snow-blindness cataract, etc. Such exposure may permanently damage the cornea.'

9. Q9.Which historic international treaty was signed in 1987 (and became effective in 1989) to control the emission of ozone-depleting substances?

   • A.Kyoto Protocol
   • B.Montreal Protocol✓ Correct Answer
   • C.Vienna Convention
   • D.Paris Agreement

   Explanation: NCERT documents: 'Recognising the deleterious effects 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.'

10. Q10.The Kyoto Protocol (1997) is an international agreement linked to which UN framework convention, committing parties to reduce greenhouse gas emissions?

    • A.UNCED (Earth Summit)
    • B.UNFCCC (UN Framework Convention on Climate Change)✓ Correct Answer
    • C.UNEP
    • D.CBD

    Explanation: The Kyoto Protocol was adopted under the United Nations Framework Convention on Climate Change (UNFCCC). It committed industrialized countries and economies in transition to limit and reduce greenhouse gases (GHGs) emissions in accordance with agreed individual targets.

11. Q11.In the Paris Agreement (2015), nations pledged to limit global warming to well below how many degrees Celsius above pre-industrial levels?

    • A.Well below 2°C, with efforts to limit it to 1.5°C✓ Correct Answer
    • B.Exactly 5°C
    • C.Zero growth limits
    • D.Below 10°C

    Explanation: The Paris Agreement is a legally binding international treaty on climate change. It was adopted by 196 Parties at COP 21 in Paris in December 2015. Its goal is to limit global warming to well below 2 degrees Celsius, preferably to 1.5 degrees Celsius, compared to pre-industrial levels.

12. Q12.Greenhouse gases warm the Earth by absorbing and re-emitting which type of radiation reflected from the Earth's surface?

    • A.Incoming solar ultraviolet radiation
    • B.Outgoing longwave infrared (heat) radiation✓ Correct Answer
    • C.Incoming visible light
    • D.Cosmic rays

    Explanation: NCERT Chapter 16 explains: 'Earth's surface re-emits heat in the form of infrared radiation but part of this does not escape into space as atmospheric gases (e.g., carbon dioxide, methane, etc.) absorb a major fraction of it. The molecules of these gases radiate heat energy... and keep the Earth warm.'

13. Q13.During which season is the stratospheric temperature over Antarctica cold enough to form polar stratospheric clouds, which facilitate seasonal ozone depletion?

    • A.Antarctic Winter (June to August)✓ Correct Answer
    • B.Antarctic Summer (December to February)
    • C.Equinox months only
    • D.No temperature variations occur

    Explanation: During the dark Antarctic winter (June to August), temperatures drop below -78°C, forming Polar Stratospheric Clouds (PSCs). These clouds provide surface areas where inactive reservoir chlorine compounds are converted into highly reactive forms, which are activated by spring sunlight in late August to destroy ozone.

14. Q14.Which of the following describes the difference between tropospheric ozone and stratospheric ozone?

    • A.Tropospheric ozone is a protective shield, while stratospheric ozone is a pollutant.
    • B.Tropospheric ozone is a harmful pollutant ('bad' ozone), while stratospheric ozone is a protective UV shield ('good' ozone).✓ Correct Answer
    • C.Both are harmful pollutants in equal measures.
    • D.Both are protective shields in equal measures.

    Explanation: Tropospheric ozone is formed by photochemical reactions of pollutants (like NOx and VOCs) near the ground, causing respiratory issues and crop damage ('bad' ozone). Stratospheric ozone is formed naturally by UV photolysis of oxygen and shields the planet from UV radiation ('good' ozone) (NCERT).

15. Q15.Which of the following is a primary control measure to mitigate the greenhouse effect and reduce global warming?

    • A.Increasing coal power plant constructions
    • B.Improving energy efficiency, reducing fossil fuel use, and massive afforestation✓ Correct Answer
    • C.Speeding up deforestation globally
    • D.Using more chlorofluorocarbons

    Explanation: NCERT Class 12 Biology lists global warming control measures: 'The measures include cutting down use of fossil fuel, improving efficiency of energy usage, reducing deforestation, promoting afforestation and slowing down the growth of human population.'

16. Q16.The chemical compound nitrous oxide (N2O), which contributes 6 percent to global warming, is released primarily from which human activity?

    • A.Deforestation only
    • B.Agricultural fertilizers use and industrial chemical processes✓ Correct Answer
    • C.Coal burning in power plants only
    • D.Refrigerator leakages

    Explanation: Nitrous oxide (N2O) is released from agricultural soil management (primarily fertilizer application), combustion of fossil fuels, and industrial chemical processes. It is a powerful greenhouse gas with high warming potential.

17. Q17.Why does UV-B radiation damage living organisms so severely, causing mutations and cancers?

    • A.It boils the water inside cells.
    • B.Because DNA and proteins in living cells preferentially absorb UV-B radiation, which breaks chemical bonds and damages DNA structures.✓ Correct Answer
    • C.It turns cells into carbon.
    • D.It has no physical energy.

    Explanation: NCERT Class 12 Biology explains: 'UV radiation of wavelength shorter than UV-B are almost completely absorbed by Earth's atmosphere... But UV-B damages DNA and mutation may occur. It causes aging of skin, damage to skin cells and various types of skin cancers.' This is because biological macromolecules (DNA, proteins) absorb UV wavelengths, leading to photochemical bond disruptions.

18. Q18.Which of the following compounds, widely used in aerosol sprays and air conditioners, is the primary source of stratospheric chlorine that depletes ozone?

    • A.Carbon dioxide
    • B.Chlorofluorocarbons (CFCs)✓ Correct Answer
    • C.Methane
    • D.Nitrous oxide

    Explanation: Chlorofluorocarbons (CFCs) are stable, non-toxic chemicals widely used as refrigerants and propellants. Because they are stable, they do not decompose in the troposphere but diffuse into the stratosphere, where UV light breaks them down to release chlorine catalysts.

19. Q19.The 'El Nino' effect, which causes unusual warming of sea surface temperatures in the Pacific Ocean and alters global weather patterns, is increasingly linked to:

    • A.Ozone depletion
    • B.Global Warming (Greenhouse gas rises)✓ Correct Answer
    • C.Acid rain
    • D.Soil erosion

    Explanation: NCERT Class 12 Biology states: 'During the past century, the temperature of Earth has increased by 0.6°C, most of it during the last three decades... This rise in temperature is leading to deleterious changes in the environment and resulting in odd climatic changes (e.g. El Nino effect)...'

20. Q20.What is the primary natural source of methane (CH4) gas, which contributes 20 percent to global warming?

    • A.Volcanic dust
    • B.Anaerobic decomposition in wetlands, rice paddies, and digestive tracts of ruminant livestock✓ Correct Answer
    • C.Aerosol sprays
    • D.Photosynthesis by trees

    Explanation: Methane is produced by methanogenic archaebacteria under anaerobic (oxygen-free) conditions. The main sources are waterlogged rice paddies, wetlands, swamps, landfills, and enteric fermentation in the gut of cattle (which release gas during digestion).