PhysicsICSE Class 9 · Heat and Energy

Greenhouse Effect, Global Warming and Energy Degradation: Causes, Effects and Efficiency (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

The greenhouse effect is the natural warming of the Earth because gases such as carbon dioxide, methane, water vapour and nitrous oxide absorb and re-emit the infrared radiation from the warm surface. Global warming is the extra rise in the average temperature caused by human activity adding more of these gases. In every energy change some energy degrades into less useful heat.

Notice that short-wave sunlight enters freely, but part of the surface's infrared heat is absorbed by greenhouse gases and sent back down.

Illustration drawn for QuestionBuddy.

Understand Greenhouse Effect, Global Warming and Energy Degradation

Greenhouse effect. Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth’s surface. The warm surface emits long-wavelength (infrared) radiation. Greenhouse gases absorb part of this and re-emit it, some of it back to the surface, so the surface stays warmer than it would otherwise be. This natural effect keeps the Earth warm enough for life.

Greenhouse gases: carbon dioxide, methane, water vapour, nitrous oxide, chlorofluorocarbons (CFCs).

Global warming. The increase in the average temperature of the Earth’s atmosphere caused by an enhanced greenhouse effect due to human activity.

*Causes:* burning of fossil fuels (coal, petroleum, natural gas) releasing carbon dioxide; cutting of forests (fewer trees to absorb CO₂); agriculture and livestock (methane); industrial gases.

*Effects:* melting of glaciers and polar ice; rise in sea level and flooding of coastal areas; more extreme weather (heatwaves, droughts, floods); harm to coral reefs and ecosystems; effects on crops and health.

*Remedies:* use cleaner and renewable energy, plant trees, use energy efficiently, reduce waste and burning of fuels.

Burning carbon and CO₂. A fuel that is carbon burns as $C + O_{2} \to C O_{2}$ . Taking atomic masses C = 12 and O = 16, 12 kg of carbon gives 44 kg of carbon dioxide, so each kg of carbon burnt produces $\frac{44}{12} \approx 3.67$ kg of $C O_{2}$ .

Energy degradation. In any energy conversion the total energy is conserved, but a part always changes into low-temperature heat spread into the surroundings, which cannot be fully used again. The energy is *degraded*: its usefulness decreases.

Examples: electrical energy in a bulb gives light plus much more heat; a car engine wastes most of the chemical energy of the fuel as heat; friction in machines turns mechanical energy into heat.

Efficiency. $efficiency = \frac{useful energy output}{total energy input}$ (as a fraction or percentage). Degraded energy $=$ input $-$ useful output. For a chain of conversions, multiply the individual efficiencies.

Heat and temperature units (for conversions). $1 cal = 4.186$ J; $1$ kcal $= 1000$ cal; $T (K) = t (° C) + 273$ . Heat is energy (joule) and depends on the mass; temperature (kelvin) tells the direction in which heat flows: from higher to lower temperature.

Greenhouse Effect, Global Warming and Energy Degradation: formulas and key facts

Efficiency: $η = \frac{useful output}{input} \times 100%$; Degraded = input − useful.
Chain of conversions: $η_{t o t a l} = η_{1} \times η_{2} \times η_{3}$; Multiply as fractions.
Units: $1 cal = 4.186 J; T (K) = t (° C) + 273$; 1 kcal = 1000 cal.
Carbon burning: $C + O_{2} \to C O_{2} : 12 kg C \to 44 kg CO_{2}$; C = 12, O = 16.

How to solve Greenhouse Effect, Global Warming and Energy Degradation questions

Separate the science explanation from the numerical part.

1For energy chains convert each stage to a fraction and multiply.
2Degraded energy = total input − useful output at each stage.
3For unit questions use 1 cal = 4.186 J and T(K) = t + 273.
4For CO₂ questions balance the equation and use the atomic masses given.
5In explanations name the gases, the radiation (infrared) and the result.

Greenhouse Effect, Global Warming and Energy Degradation in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Why a bulb gets hot

Suppose an old filament bulb takes 60 J of electrical energy every second and gives out only 6 J as light.

Find its efficiency and the degraded energy per second.

Try it first, then show the solutionSolution

$η = \frac{6}{60} = 0.10 = 10%$ .
Degraded energy $= 60 - 6 = 54$ J every second, mostly as heat.

Answer: 10%; 54 J s⁻¹ becomes heat.

Memory hook: The warm bulb is the degraded energy.

Tricks to remember Greenhouse Effect, Global Warming and Energy Degradation

Short memory aids. Say each one aloud once, then use it on a question the same day.

Sun in short, Earth out long
Short-wave in, long-wave (infrared) out, greenhouse gases trap the infrared.
Use it when: Explaining the effect.
Efficiency chain
Multiply the fractions: 0.8 × 0.4 × 0.1 = 0.032.
Use it when: Multi-step energy.
12 to 44
12 kg of carbon makes 44 kg of CO₂.
Use it when: Burning-fuel numericals.

Greenhouse Effect, Global Warming and Energy Degradation: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: A greenhouse gas · 1 mark · MCQBoard level

Which of the following is a greenhouse gas?
(a) nitrogen
(b) oxygen
(c) methane
(d) argon

Try it first, then show the solutionSolution

Methane absorbs infrared radiation; the others listed do not.

Answer: (c) methane

Example 2: Joules to calories · 2 marks · Short answerBoard level

A heater supplies 8372 J of heat. Express this in calories and in kilocalories ( $1$ cal $= 4.186$ J).

Try it first, then show the solutionSolution

Calories $= \frac{8372}{4.186} = 2000$ cal. (1)
Kilocalories $= \frac{2000}{1000} = 2$ kcal. (1)

Answer: 2000 cal; 2 kcal

Example 3: Coal to light · 3 marks · Short answerHarder board level

A power station converts 2000 MJ of coal energy into 700 MJ of electrical energy. Transmission loses 8% of this. A lamp at a house converts 5% of the delivered electrical energy into light. Find (a) the electrical energy delivered, (b) the light energy produced, (c) the percentage of the coal energy that ends up as light.

Try it first, then show the solutionSolution

(a) $700 \times 0.92 = 644$ MJ. (1)
(b) $0.05 \times 644 = 32.2$ MJ. (1)
(c) $\frac{32.2}{2000} \times 100 = 1.61%$ . (1)

Answer: (a) 644 MJ (b) 32.2 MJ (c) 1.61%

Example 4: Bucket and mug · 3 marks · Short answerHarder board level

A bucket holds 10 kg of water at 40 °C and a mug holds 0.2 kg of water at 90 °C. (a) Which has more heat energy? (b) Express both temperatures in kelvin. (c) If the mug is dipped in the bucket (without mixing), in which direction does heat flow? Give reasons.

Try it first, then show the solutionSolution

(a) The bucket: heat depends on the mass as well as the temperature (many more molecules). (1)
(b) 313 K and 363 K. (1)
(c) From the mug to the bucket, because heat flows from the higher temperature to the lower temperature, whatever the amounts. (1)

Answer: (a) bucket (b) 313 K; 363 K (c) mug to bucket

Example 5: A fuel-burning plant · 4 marks · Case-basedHOTS / competency

Suppose a town’s plant burns 6000 kg of pure carbon every hour (assume pure carbon; $C = 12$ , $O = 16$ ).
(a) Write the balanced equation and find the mass of CO₂ produced per hour.
(b) Find the mass per day.
(c) Suppose one tree absorbs 22 kg of CO₂ in a year (assumed). How many trees would be needed to absorb one day’s emission in a year?
(d) Give two ways to reduce this problem.

Try it first, then show the solutionSolution

(a) $C + O_{2} \to C O_{2}$ ; $\frac{44}{12} \times 6000 = 22000$ kg per hour. (1½)
(b) $22000 \times 24 = 528000$ kg. (½)
(c) $\frac{528 000}{22} = 24000$ trees. (1)
(d) Use renewable or cleaner energy; plant more trees; improve efficiency to burn less fuel. (1)

Answer: (a) 22 000 kg per hour (b) 528 000 kg (c) 24 000 trees (d) cleaner energy, more trees

Where students lose marks in Greenhouse Effect, Global Warming and Energy Degradation

The slips examiners see most often, and what to write instead.

Mistake: Greenhouse effect described as visible light trapped
It is infrared radiation from the Earth that is absorbed.
Wrong working
Gases trap sunlight.
Right working
Gases absorb and re-emit infrared radiation from the Earth’s surface.
Do this: Say "infrared" and "from the surface".
Usually costs: 1 mark.
Mistake: Energy said to be destroyed in degradation
Energy is conserved; its usefulness falls.
Wrong working
Energy is lost.
Right working
Energy is converted to less useful heat spread into the surroundings.
Do this: Say "degraded", not "destroyed".
Usually costs: 1 mark.
Mistake: Efficiencies added instead of multiplied
Each stage acts on what the last stage delivered.
Wrong working
80% + 40% + 10%
Right working
0.8 × 0.4 × 0.1 = 3.2%
Do this: Convert to fractions and multiply.
Usually costs: The answer.

Exam tips

Name at least three greenhouse gases and the radiation involved.
Give two causes and two effects of global warming, in separate lines.
Use "degraded", "less useful" and "conserved" when discussing energy.
Show the equation with atomic masses for the CO₂ calculation.

Practice questions on Greenhouse Effect, Global Warming and Energy Degradation

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Global warming is mainly caused by an increase in the atmospheric concentration of:
(a) oxygen
(b) nitrogen
(c) carbon dioxide
(d) hydrogen
Hint
Fossil-fuel burning.
Answer
(c)
Q2 · 1 mark · Very short answerBoard level
Express 27 °C in kelvin.
Hint
Add 273.
Answer
300 K.
Q3 · 2 marks · Short answerHarder board level
Convert 3000 cal into joules and into kilojoules ( $1$ cal $= 4.186$ J).
Hint
Multiply.
Answer
$3000 \times 4.186 = 12558$ J $= 12.558$ kJ.
Q4 · 3 marks · Short answerHarder board level
A motor takes 5 kJ of electrical energy every second and delivers 3.5 kJ as mechanical work every second. Find its efficiency and the energy degraded as heat and sound in 10 minutes.
Hint
Per-second figures first.
Answer
$η = \frac{3.5}{5} = 70%$ . Degraded per second $= 1.5$ kJ; in 600 s: $1.5 \times 600 = 900$ kJ.
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): The greenhouse effect is completely harmful to life on the Earth.
Reason (R): The greenhouse effect keeps the Earth’s surface warm enough for life.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
The natural effect is useful.
Answer
(d). The natural greenhouse effect is essential; only the enhanced effect causes global warming.
Q6 · 3 marks · Short answerHOTS / competency
A fuel supplies 500 MJ. The boiler converts 80% of it into steam energy, the turbine converts 40% of that into electrical energy, and a lamp converts 10% of the electrical energy into light. Find the light energy and the degraded energy.
Hint
Multiply efficiencies.
Answer
Overall $= 0.8 \times 0.4 \times 0.1 = 0.032$ ; light $= 500 \times 0.032 = 16$ MJ; degraded $= 500 - 16 = 484$ MJ.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which radiation do greenhouse gases absorb?Show answer
Infrared radiation from the Earth’s surface.
2.Name three greenhouse gases.Show answer
Carbon dioxide, methane, water vapour (also nitrous oxide, CFCs).
3.What is energy degradation?Show answer
Conversion of energy into less useful forms, mostly low-temperature heat.
4.1 cal in joules?Show answer
4.186 J.
5.Efficiency formula?Show answer
Useful output ÷ input.

Greenhouse Effect, Global Warming and Energy Degradation: frequently asked questions

What is the greenhouse effect?

The warming of the Earth’s surface because greenhouse gases absorb and re-emit the infrared radiation emitted by the surface.

What are the causes of global warming?

Burning fossil fuels, deforestation, methane from agriculture and livestock, and industrial gases that increase the greenhouse gases in the atmosphere.

What is meant by energy degradation?

In every energy change some energy is converted into heat that spreads into the surroundings and cannot be used fully again; total energy is conserved but its usefulness decreases.