Global warming and the greenhouse effect

A blanket you cannot see

Carbon dioxide is only 0.04% of the air. How can such a tiny amount change the temperature of a whole planet?

Sunlight arrives as short-wave radiation that passes through the air and heats the ground. The warm ground then gives out long-wave radiation. Greenhouse gases such as carbon dioxide, methane and water vapour absorb most of it, so the heat cannot escape to space and Earth cools more slowly.

This warming by a blanket of gases is the greenhouse effect. The more greenhouse gas, the more heat is absorbed and the warmer Earth gets. Burning coal, oil and gas has raised these gases for 300 years; cutting forests removes the trees that take up CO₂; rotting vegetation, animal digestion and rice fields add methane.

Effects: warmer seas expand and ice from Antarctica and Greenland melts, so sea levels rise and low coasts such as Bangladesh are threatened. Some regions get far more rain and floods. Crops that suit each region change, and winters become shorter and warmer while summers get longer and hotter.

Greenhouse gases let sunlight in but absorb the heat Earth gives out, so more of them means a warmer planet.

Key terms

Greenhouse effect

Explaining global warming.

Greenhouse gas

Trapping heat.

Long-wave radiation

What the gases absorb.

Sea-level rise

A major effect.

Short questions with model answers

1. Q1. CO₂ is only 0.04% of air. How does it become harmful for the ecosystem? (Constructed response ii, p. 144)

   • What it does
   • More means more
   • Result

   Even a small share traps heat; as it rises, Earth warms and climates change.

2. Q2. Which gas do rice fields and rotting garbage give out? (Exercises 2 and 3, p. 138)

   • Source
   • Gas

   Methane, CH₄.

Common mistakes

• ✗ Greenhouse gases warm Earth by blocking incoming sunlight.

  ✓ Sunlight passes through; they absorb the long-wave heat going out (p. 136).

• ✗ Carbon dioxide is the only greenhouse gas.

  ✓ Methane and water vapour are greenhouse gases too; water vapour is the most abundant (p. 137).

• ✗ Global warming and acid rain are the same problem.

  ✓ Different gases, different effects: CO₂/CH₄ warm; SOx/NOx acidify.

MCQs

1. 1. Which gases are responsible for the greenhouse effect? (Exercise i, p. 142)

   1. (a) SO₂, NO₂
   2. (b) NO₂, CO
   3. (c) CO₂, CH₄
   4. (d) O₂, N₂
   Show answer

   (c) p. 136.

2. 2. Earth's most abundant greenhouse gas is:

   1. (a) Carbon dioxide
   2. (b) Methane
   3. (c) Water vapour
   4. (d) Nitrogen
   Show answer

   (c) About half the effect (p. 137).

3. 3. Greenhouse gases mainly absorb:

   1. (a) Short-wave sunlight
   2. (b) Long-wave radiation from the ground
   3. (c) Sound
   4. (d) Rain
   Show answer

   (b) p. 136.

4. 4. Why does cutting forests add to global warming?

   1. (a) Trees give out CO₂
   2. (b) Fewer trees remove less CO₂
   3. (c) Wood blocks sunlight
   4. (d) Trees make methane
   Show answer

   (b) p. 137.

5. 5. Which effect of global warming threatens Bangladesh most, according to the book?

   1. (a) Sea-level rise
   2. (b) Colder winters
   3. (c) Less rainfall everywhere
   4. (d) More ozone
   Show answer

   (a) Low-lying coasts (p. 138).

Quick revision

• Greenhouse gases (CO₂, CH₄, water vapour) absorb the long-wave heat Earth gives out.
• Fossil fuels, deforestation, rice fields and livestock have raised these gases.
• Effects: rising seas, floods, changing crops and hotter summers.