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B17.5 Global Warming

GCSE Combined Science · AQA · Chapter B17: Biodiversity and Ecosystems

A free GCSE Combined Science: Trilogy lesson for AQA (8464) 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, including effects on biodiversity
  • Evaluate evidence for global warming, including correlation and uncertainty
  • 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 energy given out by its surface.
  • Global warming: The gradual increase in the Earth’s mean surface temperature (land and oceans) caused by rising levels of greenhouse gases.
  • Distribution: Where a species is found. It can change when temperature or rainfall changes.
  • Correlation: A link between two variables: when one changes, so does the other. A correlation alone does not prove that one causes the other.

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 energy is trapped and the mean temperature rises. This is global warming. The Earth’s climate has changed naturally in the past (ice ages and warm periods), but scientists now agree that the present warming is caused by human activity.

Why greenhouse gases are increasing

  • Carbon dioxide; Human activities that increase it Burning fossil fuels (coal, oil, natural gas) 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 361 ppm in 1995 and 425 ppm in 2025.
Increase = 425 − 361 = 64 ppm in 30 years
Mean rate = 64 ÷ 30 = 2.1 ppm per year (2 s.f.)
Percentage increase from 1965 (320 ppm) to 2025 = (425 − 320) ÷ 320 × 100 = 32.8%

Answer: about 2.1 ppm per year; about 33% higher than in 1965

Biological consequences of global warming

  • Loss of habitat; Effect on living things Melting ice and expanding warmer water raise sea level, flooding low-lying coasts and islands. Arctic sea ice shrinks, so polar bears find it harder to hunt.
  • Changes in distribution; Effect on living things Species move to areas that suit them (towards the poles or uphill). Insects such as malaria-carrying mosquitoes could spread into countries that are now too cold for them.
  • Changes to migration patterns; Effect on living things Migrating birds may arrive earlier, leave later or change route. Their timing may no longer match their food supply.
  • Reduced biodiversity; Effect on living things Species that cannot move or adapt fast enough may become extinct. Coral reefs are damaged by warmer seas.
  • Threat to food security; Effect on living things Droughts, floods and heat waves damage crops and livestock.

Exam tip: “Climate change” on its own is not a biological consequence. Name a specific effect on organisms and link it 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

A graph of mean temperature against CO₂ concentration shows a strong positive correlation. A correlation alone does not prove cause, but scientists also understand the mechanism (greenhouse gases absorb infrared), and confidence is high because:

  • results come from many independent studies using different methods (thermometers, ice cores, satellites, sea level), and they agree
  • work is checked by peer review before it is published
  • measurements have been made over long periods, so trends are clear

There is still uncertainty in predictions, for example exactly how fast temperatures will rise or how each species will respond, because the climate system is very complex. Some people still doubt the cause, e.g. because the climate has changed naturally before.

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 acid rain or with damage to the ozone layer. These are different problems with different causes.
  • Writing vague effects such as “animals will die” or “climate change”. Give a specific consequence (habitat loss, distribution change, migration change) and link it to 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 rising because of human activities? (Choose from: Oxygen and nitrogen, Carbon dioxide and methane, Sulfur dioxide and argon, Nitrogen and helium.)

Answer: Carbon dioxide and methane

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

Question 2. The diagram shows the greenhouse effect. Which arrow shows infrared radiation being sent back towards the Earth by greenhouse gases? (Choose from: 1, 2, 3, 4.)

Answer: 3

Working: Arrow 3 goes from the greenhouse-gas layer back to the Earth. Arrow 1 is sunlight arriving, 2 is infrared given out by the warm Earth and 4 is infrared escaping to space.

Question 3. True or false? Without any greenhouse effect, the Earth would be too cold for most life.

Answer: True

Working: True. The natural greenhouse effect keeps the Earth warm; human activities are making it stronger.

Related lessons

Continue in the full course

This lesson is section B17.5 (AGCSC1705) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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