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Human Influences on the EnvironmentEdexcel IGCSE Biology: Revision notes

Section 1

What are the biological consequences of air pollution?

Burning fossil fuels releases pollutant gases that damage living organisms.

  • Sulfur dioxide (SO2) — released when fossil fuels containing sulfur impurities are burned; dissolves in atmospheric moisture to form acid rain, which damages leaves, lowers soil and water pH (harming aquatic life), and irritates the human respiratory system
  • Carbon monoxide (CO) — released by incomplete combustion of fuels (e.g. in car engines); binds permanently to haemoglobin in red blood cells in place of oxygen, reducing the blood's oxygen-carrying capacity and causing tiredness, headaches or, at high concentrations, death
Key termssulfur dioxidecarbon monoxideacid rain
Common mistake

Carbon monoxide is not the same as carbon dioxide — CO poisons the blood by binding to haemoglobin, whereas CO2 is a greenhouse gas.

Section 2

Which gases contribute to the greenhouse effect, and where do they come from?

Greenhouse gases trap heat energy in the atmosphere. The main ones are water vapour, carbon dioxide, nitrous oxide, methane and CFCs (chlorofluorocarbons).

Human activities increase levels of these gases:

  • Burning fossil fuels (power stations, transport) releases carbon dioxide
  • Livestock farming and rice paddies release methane
  • Use of nitrogen fertilisers releases nitrous oxide
  • Historic use of aerosols and refrigerants released CFCs
  • Deforestation reduces the removal of CO2 from the atmosphere by photosynthesis
Key termsgreenhouse gases

Section 3

What is the enhanced greenhouse effect, and what are its consequences?

The natural greenhouse effect keeps Earth warm enough to support life. Human activities have increased concentrations of greenhouse gases, causing an enhanced greenhouse effect — more heat is trapped than before, leading to global warming.

Consequences include:

  • Rising average global temperatures
  • Melting of polar ice and glaciers, causing sea levels to rise and coastal flooding
  • Changing rainfall patterns, causing droughts in some regions and flooding in others
  • Disruption of habitats and shifts in species distribution, threatening biodiversity
Key termsenhanced greenhouse effectglobal warming

Section 4

What are the biological consequences of water pollution?

Sewage pollution: untreated sewage entering waterways provides food for decomposing bacteria, whose populations grow rapidly and use up dissolved oxygen through respiration, killing fish and other aquatic organisms that need oxygen.

Eutrophication: fertiliser minerals (especially nitrates and phosphates) leach from farmland into waterways, causing:

  1. Rapid growth of algae (an algal bloom), blocking light from reaching plants below the surface
  2. Death of submerged plants due to lack of light
  3. Bacteria decompose the dead plant and algal matter, using up dissolved oxygen through respiration
  4. Oxygen levels fall so low that fish and other aquatic animals die
Key termseutrophicationalgal bloom
Exam tip

In eutrophication questions, always explain the full chain: excess nutrients → algal bloom → light blocked → plants die → decomposers respire → oxygen falls → animals die. Marks are awarded for each linked step.

Section 5

What are the effects of deforestation?

Large-scale removal of trees has several biological consequences:

  • Leaching: without tree roots to hold soil nutrients, minerals are washed (leached) out of the soil more easily by rain
  • Soil erosion: tree roots no longer bind soil particles together, so topsoil is more easily removed by wind and rain
  • Disturbed evapotranspiration: fewer trees means less water is drawn up and released as vapour, changing local humidity and rainfall patterns
  • Disruption of the carbon cycle: fewer trees means less photosynthesis to remove CO2, and burning cleared vegetation releases stored carbon
  • Changes to atmospheric gas balance: reduced photosynthesis and increased burning together raise atmospheric carbon dioxide levels
Key termsdeforestationleachingsoil erosion

Must Know

  • Sulfur dioxide causes acid rain and respiratory irritation; carbon monoxide binds to haemoglobin, reducing oxygen transport
  • Greenhouse gases (water vapour, CO2, nitrous oxide, methane, CFCs) trap heat; human activity increases their levels
  • The enhanced greenhouse effect causes global warming: rising temperatures, melting ice, sea level rise, changing rainfall
  • Sewage pollution and eutrophication both cause oxygen depletion in water, killing aquatic life, but eutrophication is driven by excess fertiliser minerals causing an algal bloom first
  • Deforestation causes leaching, soil erosion, disrupted evapotranspiration and disturbs the carbon cycle
  • Always link cause → mechanism → biological consequence when explaining pollution effects

That's the notes covered.

Carry on to the next subtopic.