20.3 Pollution

2026 Syllabus Objectives

  1. Describe the effects of untreated sewage and excess fertiliser on aquatic ecosystems
  2. Describe the effects of non-biodegradable plastics, in both aquatic and terrestrial ecosystems
  3. Describe the sources and effects of pollution of the air by methane and carbon dioxide, limited to: the enhanced greenhouse effect and climate change
  4. Explain the process of eutrophication of water, limited to:
    • increased availability of nitrate and other ions
    • increased growth of producers
    • increased decomposition after death of producers
    • increased aerobic respiration by decomposers
    • reduction in dissolved oxygen
    • death of organisms requiring dissolved oxygen in water

Greenhouse Gases and Climate Change 🌍

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What is Pollution?

Pollution means the addition to the environment of something that harms it. One of the most significant forms of pollution today is the release of greenhouse gases into the atmosphere.

Greenhouse Gases 🔑

The Earth's atmosphere contains several gases that act like a blanket, keeping the Earth warm. These are called greenhouse gases. The two most important greenhouse gases are:

  • Carbon dioxide (CO2\text{CO}_2)
  • Methane (CH4\text{CH}_4)

Key Definition: Greenhouse gases are gases such as carbon dioxide and methane that trap heat within the atmosphere.

The Greenhouse Effect

How it Works ⚡

Carbon dioxide allows short-wave radiation from the Sun to pass through it. Here's the process:

  1. Sunlight passes freely through the atmosphere and reaches the ground
  2. The ground is warmed by the radiation
  3. The ground emits longer wavelength infrared radiation
  4. Carbon dioxide does not let all of this infrared radiation pass through - much of it is kept in the atmosphere
  5. The atmosphere becomes warmer

This is called the greenhouse effect because it is similar to the effect which keeps an unheated greenhouse warmer than the air outside. The glass around the greenhouse behaves like the carbon dioxide in the atmosphere - it lets short-wave radiation in, but does not let out the long-wave radiation. The long-wave radiation is trapped inside the greenhouse, making the air inside it warmer.

Key Definition: The greenhouse effect is the heating effect on the Earth of the trapping of heat by greenhouse gases. Note that this is a natural and useful effect - without it the Earth would be too cold to support life.

Why We Need the Greenhouse Effect

We need the greenhouse effect. If it did not happen, then the Earth would be frozen and lifeless. The average temperature on Earth would be about 33 ∘C33\,^{\circ}\text{C} lower than it is now.

The Enhanced Greenhouse Effect 📌

However, the amount of carbon dioxide and other greenhouse gases in the atmosphere is increasing. This may trap more infrared radiation and make the atmosphere warmer.

Key Definition: The enhanced greenhouse effect is the increased heating effect caused by an increase of greenhouse gases in the atmosphere.

Evidence of Increasing CO₂ Levels

Data shows that carbon dioxide concentration in the atmosphere has increased dramatically since 1750:

  • Before 1750: Relatively stable levels around 280 ppm280\,\text{ppm}
  • From 1900 onwards: Sharp exponential increase
  • Current levels: Over 400 ppm400\,\text{ppm}

This sharp increase coincides with the Industrial Revolution and increased burning of fossil fuels.

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