Greenhouse Effect
This page covers the mechanism of the greenhouse effect, the major greenhouse gases and the measures to control global warming, from the REET exam's point of view. REET asks questions from this topic on correctly identifying gases and choosing the most effective control measure.
- 2 questions in REET 2024
- Class 6-8 level
- 9 min read
How this topic is asked in REET
REET 2024 asked two questions on this topic — both conceptual, with no calculation involved. The first was on the best measure for controlling global warming (answer: reducing greenhouse gas emissions), and the second was on identifying which gas is NOT among the greenhouse gases (answer: nitrogen). This makes it clear that REET checks whether students can tell real greenhouse gases (CO₂, methane, nitrous oxide, and so on) apart from other common gases present in the atmosphere (such as nitrogen, oxygen), and whether they can correctly identify control measures. This topic also connects to environmental studies (EVS) and social science topics like climate change, so understanding it thoroughly helps with several kinds of questions in REET.
What the Greenhouse Effect Is and How It Works
Short-wavelength radiation from the sun passes through the atmosphere, reaches the Earth's surface, and warms it. In return, the Earth's surface radiates heat back towards space as long-wavelength infrared radiation. Certain gases present in the atmosphere — called greenhouse gases — absorb a large part of this infrared radiation and send it back towards the Earth, keeping the atmosphere and the Earth's surface warm. This whole natural process is called the greenhouse effect, since it behaves in a way similar to a glass greenhouse (a glass house built for plants).
Major Greenhouse Gases
- Carbon dioxide (CO₂)
- Produced mainly by burning fossil fuels (coal, petrol, diesel) and deforestation; the most talked-about greenhouse gas[Source 1]
- Methane (CH₄)
- Produced from paddy fields, livestock farming (cattle digestion), landfills and wetlands[Source 1]
- Nitrous oxide (N₂O)
- Produced from excessive use of fertilisers in agriculture[Source 1]
- Water vapour
- A natural feedback gas — as temperature rises, evaporation from oceans and water bodies increases, which raises the amount of water vapour in the atmosphere and increases warming further[Source 1]
- CFCs (chlorofluorocarbons)
- Human-made gases used in refrigerators, air conditioners and aerosol sprays
- Nitrogen (N₂) — the exception
- Despite being the most abundant gas in the atmosphere (~78%), it does not absorb infrared radiation, so it is not considered a greenhouse gas
Effects of Global Warming
When the amount of greenhouse gases in the atmosphere increases beyond balance, the normal greenhouse effect becomes 'enhanced' — this is called global warming. Its major effects include the melting of polar ice caps and glaciers, a rise in sea level, irregularities in the normal weather cycle (rainfall, temperature), an increase in extreme events like floods and droughts, and adverse effects on biodiversity and crop production. It's worth noting that the greenhouse effect itself is not harmful — without it, the Earth's average temperature would be so low that life would become difficult — the problem is its excessive, human-caused increase.
Major Effects of Global Warming
- Glaciers and ice caps
- Polar ice sheets and mountain glaciers melting much faster than normal
- Sea level
- A gradual rise in sea level from melted ice and the thermal expansion of warming seawater, which increases the risk of flooding for coastal areas and islands
- Weather cycle
- Irregular rainfall and more intense, more frequent extreme weather events like floods, droughts and cyclones
- Agriculture and biodiversity
- Changes and uncertainty in traditional crop-production patterns, and a threat to the natural habitats of many species, with a risk of extinction
Measures to Control Global Warming
| Measure | Description |
|---|---|
| Reducing greenhouse gas emissions | The most effective measure — cutting dependence on fossil fuels |
| Afforestation and forest protection | Trees absorb atmospheric CO₂ through photosynthesis |
| Adopting renewable energy | Adopting clean sources like solar, wind and hydro power reduces both dependence on fossil fuels and emissions |
| Improving energy efficiency | Using low-energy appliances, LED bulbs and public transport more |
How to Teach This in Class
Activity
Fill one of two plastic bottles with ordinary air and the other with CO₂ generated from a baking soda-vinegar reaction, place both in sunlight, and have students compare the internal temperature after a while — this simple classroom experiment makes the heat-absorbing property of a greenhouse gas clear.
Teaching aid
Draw a simple diagram (sun → Earth's surface → infrared radiation → reflection back by the atmosphere) and explain the mechanism step by step.
Common misconception
Many students think the greenhouse effect and ozone layer depletion are the same event — clarify that the greenhouse effect is about trapping heat, while ozone depletion is a separate problem linked to reduced protection from ultraviolet radiation. Also clarify that the greenhouse effect itself is natural and necessary — only its excessive, human-caused increase is the problem, not the whole process.
Assessment
Give students a matching exercise linking a gas, its source and its effect, such as 'methane — paddy fields — global warming,' and have them list 2-3 activities they see at home or in their neighbourhood that increase greenhouse gas emissions, and 2-3 activities that help reduce them, followed by a short class discussion on these lists.
Memory trick / exam tip
REET repeatedly asks which gas is NOT a greenhouse gas — remember that the two most abundant gases in the atmosphere, nitrogen (N₂) and oxygen (O₂), are both not greenhouse gases, while CO₂, methane, nitrous oxide and water vapour, present in much smaller amounts, are the main greenhouse gases.
Questions Asked in REET 2024 (With Solutions)
Question 149
The best way to control global warming is
- Aजीवाश्म ईंधन के प्रयोग को बढ़ाकर
- Bग्रीनहाउस गैसों के उत्सर्जन को कम करके
- Cवनोन्मूलन को बढ़ाकर
- Dवृक्षारोपण को कम करके
Explanation
विश्वव्यापी उष्णता (global warming) का मुख्य कारण वायुमंडल में कार्बन डाइऑक्साइड, मेथेन जैसी ग्रीनहाउस गैसों की बढ़ती मात्रा है। इसलिए इनके उत्सर्जन को कम करना इसे नियंत्रित करने का सबसे अच्छा उपाय है। जीवाश्म ईंधन का अधिक प्रयोग, वनोन्मूलन और वृक्षारोपण में कमी – ये सभी उष्णता को और बढ़ाते हैं।
Question 130
Which of the following greenhouse gas does not contribute in global warming ?
- Aकार्बनडाइऑक्साइड
- Bमीथेन
- Cनाइट्रसऑक्साइड
- Dनाइट्रोजन
Explanation
कार्बन डाइऑक्साइड, मीथेन और नाइट्रस ऑक्साइड ग्रीनहाउस गैसें हैं, जो अवरक्त (ऊष्मा) विकिरण को रोककर वैश्विक ऊष्मीकरण बढ़ाती हैं। नाइट्रोजन (N₂) वायुमंडल में सबसे अधिक होने पर भी अवरक्त विकिरण को अवशोषित नहीं करती, इसलिए यह वैश्विक ऊष्मीकरण में योगदान नहीं देती।
Practice Questions
Question 1
Which is the most prominent gas mainly responsible for the greenhouse effect?
- Aनाइट्रोजन
- Bआर्गन
- Cहाइड्रोजन
- Dकार्बन डाइऑक्साइड
Explanation
Carbon dioxide (CO₂) is the most significant gas responsible for the greenhouse effect, released in large amounts by burning fossil fuels.
Question 2
Which gas is present in the largest amount in the atmosphere, yet is not considered a greenhouse gas?
- Aकार्बन डाइऑक्साइड
- Bमीथेन
- Cनाइट्रोजन
- Dजलवाष्प
Explanation
Nitrogen (N₂) makes up about 78% of the atmosphere, but it does not absorb infrared radiation, so it is not considered a greenhouse gas.
Question 3
A major source of methane gas is:
- Aसौर ऊर्जा संयंत्र
- Bधान के खेत व पशुपालन
- Cपवन चक्की
- Dजल विद्युत संयंत्र
Explanation
Methane gas is produced mainly by microbial activity in waterlogged paddy fields and by livestock farming (the digestive process of cattle).
Question 4
Which of the following gases is NOT a greenhouse gas?
- Aमीथेन
- Bनाइट्रस ऑक्साइड
- Cऑक्सीजन
- Dकार्बन डाइऑक्साइड
Explanation
Methane, nitrous oxide and carbon dioxide are major greenhouse gases that absorb infrared radiation, while oxygen (like nitrogen) does not absorb infrared radiation, so it is not a greenhouse gas.
Question 5
How does deforestation affect global warming?
- Aइससे वायुमंडलीय CO₂ कम हो जाती है
- Bइससे वायुमंडलीय CO₂ बढ़ती है, क्योंकि CO₂ सोखने वाले पेड़ कम हो जाते हैं
- Cइसका ग्रीनहाउस गैसों से कोई संबंध नहीं है
- Dइससे मीथेन गैस कम हो जाती है
Explanation
Trees absorb atmospheric CO₂ through photosynthesis. Deforestation reduces the number of trees, which increases the amount of CO₂ in the atmosphere and contributes to global warming.
Question 6
The most effective measure to control global warming is:
- Aजीवाश्म ईंधन का प्रयोग बढ़ाना
- Bवनोन्मूलन को बढ़ाना
- Cवृक्षारोपण को कम करना
- Dग्रीनहाउस गैसों के उत्सर्जन को कम करना
Explanation
Reducing the emission of greenhouse gases is the most direct and effective measure for controlling global warming, since that is the root of the problem — this is also confirmed by REET 2024's official answer.
Question 7
Why is water vapour called a 'feedback gas'?
- Aक्योंकि यह कभी वायुमंडल में नहीं रहती
- Bक्योंकि तापमान बढ़ने पर वाष्पीकरण बढ़ता है, जिससे तापन और बढ़ जाता है
- Cक्योंकि यह ग्रीनहाउस गैस नहीं है
- Dक्योंकि यह केवल ध्रुवों पर पाई जाती है
Explanation
Water vapour is both a greenhouse gas and a feedback mechanism — as temperature rises, evaporation increases, which raises the amount of water vapour in the atmosphere, and this increases warming even further.
Question 8
CFCs (chlorofluorocarbons), which are also greenhouse gases, were mainly used in:
- Aरेफ्रिजरेटर व एयर कंडीशनर में शीतलक तथा एरोसोल स्प्रे में
- Bकेवल खाद्य पदार्थों में
- Cकेवल सौर पैनलों में
- Dकेवल कृषि उर्वरकों में
Explanation
CFC gases were mainly used as a coolant in refrigerators and air conditioners, and as a propellant in aerosol sprays — besides being a greenhouse gas, they also damage the ozone layer.
Question 9
A simple and safe classroom experiment to explain the greenhouse effect to class 7 students is:
- Aदो बंद बोतलों (सामान्य हवा व CO₂) को धूप में रखकर तापमान की तुलना करना
- Bकेवल पाठ्यपुस्तक पढ़ना
- Cप्रयोगशाला में सांद्र अम्ल का उपयोग करना
- Dकेवल परिभाषा रटवाना
Explanation
At the class 6-8 level, a simple, safe classroom experiment — placing two closed bottles, one with ordinary air and one with CO₂, in sunlight and comparing temperature after some time — lets students see for themselves the heat-trapping property of a greenhouse gas.
Question 10
If the greenhouse effect did not exist at all, how would Earth's average surface temperature compare to the current value (about 15°C)?
- Aवर्तमान से लगभग 33°C कम, अत्यंत ठंडा
- Bवर्तमान के बराबर ही रहता
- Cवर्तमान से अधिक गर्म हो जाता
- Dकेवल ध्रुवों पर कोई अंतर पड़ता
Explanation
Scientific estimates suggest that without the greenhouse effect, the Earth's average surface temperature would be about 33°C lower than the current level (~15°C), making the Earth extremely cold and difficult for most life to survive — this shows that the natural greenhouse effect is essential for life.
Frequently Asked Questions
Is the greenhouse effect natural or harmful?+
The greenhouse effect itself is a natural and necessary process that keeps the Earth at a temperature suitable for life. The problem arises when human activity increases greenhouse gases excessively, causing global warming.
Which is the most significant greenhouse gas?+
Carbon dioxide (CO₂) is the most significant and most talked-about greenhouse gas, produced mainly by burning fossil fuels.
What is the best way to stop global warming?+
According to REET 2024's official answer, reducing the emission of greenhouse gases is the most effective measure for controlling global warming.
What is the difference between ozone layer depletion and the greenhouse effect?+
The greenhouse effect is about trapping heat radiated back from the Earth, while ozone layer depletion is a separate problem linked to the weakening of the shield that protects against harmful ultraviolet (UV) rays from the sun — the two are different events, though both are connected to atmospheric pollution.
Why isn't nitrogen gas considered a greenhouse gas, and how does afforestation help reduce it?+
Nitrogen (N₂) is the most abundant gas in the atmosphere (~78%), but it does not absorb infrared radiation, so it is not considered a greenhouse gas. In contrast, trees and plants absorb atmospheric CO₂ through photosynthesis and convert it into organic matter, which is why afforestation helps slow down the pace of global warming.
Read Next
Official Sources
Change Log
- 17 September 2026 — English page published — mirrors the fact-checked Hindi page, independently bilingual-accuracy checked