18.5 Global Warming

A free GCSE Biology lesson for AQA (8461) from OBA School: greenhouse gases, the causes of global warming and its biological consequences. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe how carbon dioxide and methane in the atmosphere cause global warming
• Explain which human activities increase these greenhouse gases
• Describe the biological consequences of global warming
• Evaluate evidence for global warming and understand why scientists agree about it
• Calculate percentage change and rate of change from climate data

Key words

• Greenhouse gas: A gas in the atmosphere, such as carbon dioxide or methane, that absorbs infrared radiation and keeps the Earth warm.
• Greenhouse effect: The natural warming of the Earth because greenhouse gases trap some of the heat given out by its surface.
• Global warming: The increase in the Earth’s mean surface temperature caused by rising levels of greenhouse gases.
• Climate change: Long-term changes in weather patterns, including temperature and rainfall, linked to global warming.
• Peer review: Checking of scientific work by other scientists in the same field before it is published.

The greenhouse effect

Without any greenhouse gases the Earth would be far too cold for most life. The problem is that human activities are increasing the levels of carbon dioxide and methane, so more heat is trapped and the mean temperature rises. This is global warming.

Why greenhouse gases are increasing

• Carbon dioxide; Human activities that increase it Burning fossil fuels in power stations, vehicles, homes and industry; deforestation (burning and decay of trees, less photosynthesis); destroying and burning peat
• Methane; Human activities that increase it Cattle and other farm animals (digestion); rice paddy fields; decay of waste in landfill sites

Worked example: rate of increase

Use the graph: CO₂ was about 370 ppm in 2000 and 414 ppm in 2020.
Increase = 414 − 370 = 44 ppm in 20 years
Mean rate = 44 ÷ 20 = 2.2 ppm per year
Before the Industrial Revolution CO₂ was about 280 ppm. Percentage increase to 414 ppm = (414 − 280) ÷ 280 × 100 = 47.9%

Answer: 2.2 ppm per year; about 48% higher than pre-industrial levels

Biological consequences of global warming

• Loss of habitat; Effect on living things Rising sea levels (from melting ice and expanding warmer water) flood low-lying coasts, marshes and islands. Ice habitats in the Arctic shrink.
• Changes in species distribution; Effect on living things Where temperature or rainfall changes, some species move to new areas (often towards the poles or uphill). Others cannot move and die out locally.
• Changes to migration patterns; Effect on living things Migrating birds may arrive earlier or leave later, or change their routes. Their timing may no longer match their food supply.
• Loss of biodiversity; Effect on living things Species that cannot adapt or move fast enough may become extinct. Coral reefs bleach in warmer seas.
• More extreme weather; Effect on living things More floods, droughts and storms damage habitats and crops.

Exam tip: In exam answers, link each effect to biodiversity: e.g. “sea level rises, so coastal habitats are flooded, so species that live there lose their habitat and biodiversity falls.”

Evaluating the evidence

Thousands of scientists around the world collect data on temperature, sea level, ice cover and greenhouse gases. The overwhelming majority agree that the Earth is warming and that human activities are the main cause. Confidence is high because:

• results come from many independent studies using different methods, and they agree
• work is checked by peer review before it is published
• measurements have been made over long periods of time, so trends are clear

There is still some uncertainty in predictions, for example exactly how fast temperatures will rise or how individual species will respond, because the climate system is very complex.

Common mistakes

• Saying that the greenhouse effect is bad. The natural greenhouse effect keeps the Earth warm enough for life; the problem is that it is being increased by human activities.
• Mixing up global warming with damage to the ozone layer or with acid rain. These are different problems.
• Writing vague effects such as “animals will die”. Give a specific consequence (habitat loss, distribution change, migration change) and explain the effect on biodiversity.
• Forgetting methane. Cattle, rice fields and landfill sites all release methane.

Revision questions

Try these, then check your answers below each one.

Question 1. Which pair are both greenhouse gases whose levels are being increased by human activities? (Choose from: Sulfur dioxide and oxygen, Nitrogen and argon, Carbon dioxide and methane, Oxygen and nitrogen.)

Answer: Carbon dioxide and methane

Working: Carbon dioxide and methane are the greenhouse gases named in the specification.

Question 2. True or false? Without any greenhouse effect, the Earth would be too cold for most life.

Answer: True

Working: True. The problem is that human activities are increasing the effect.

Question 3. The CO₂ concentration was about 330 ppm in 1975 and 420 ppm in 2025. Calculate the mean rate of increase per year.

Answer: 1.8 ppm per year

Working: 420 − 330 = 90 ppm. 90 ÷ 50 = 1.8 ppm per year.

Related lessons

• Previous lesson: 18.4 Deforestation and Peat Destruction
• Next lesson: 18.6 The Impact of Change
• In this chapter: 18.1 The Human Population Explosion
• In this chapter: 18.2 Land and Water Pollution
• In this chapter: 18.3 Air Pollution

Continue in the full course

This lesson is section 18.5 (AGCBH1805) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.