4.12.1Theme D — Continuity and change
Causes of Climate Change — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 15 of 18 Unit quiz
Section 1 of 5The Greenhouse Effect
The natural greenhouse effect keeps the Earth warm enough for life.
Incoming radiation
- Short-wave radiation from the Sun passes through the atmosphere and warms the Earth's surface.
Re-radiation
- The warm surface emits longer-wavelength infrared radiation back toward space.
Greenhouse gases
- Gases such as carbon dioxide, methane and water vapour absorb infrared radiation and re-emit it, warming the lower atmosphere.
Enhanced effect
- Human activity raises greenhouse gas levels, so more heat is retained and average temperatures rise.
| Gas | Main human source |
|---|---|
| Carbon dioxide | Burning fossil fuels, deforestation, cement |
| Methane | Livestock, rice paddies, landfill, gas leaks |
| Nitrous oxide | Fertilisers and some industrial processes |
| Water vapour | Evaporation, which increases as air warms |
Section 2 of 5From Emissions to Warming
1
Burning coal, oil and gas releases carbon that was stored for millions of years, and adds carbon dioxide to the air.
2
Deforestation removes trees that take up carbon dioxide and often involves burning, which releases more.
3
Higher greenhouse gas concentrations trap more infrared radiation and the average global temperature rises.
4
Warming melts ice and permafrost, lowering reflectivity and releasing methane, which causes further warming through positive feedback.
Section 3 of 5Evidence and Complexity
Evidence and Complexity
Atmospheric records
- Direct measurements show carbon dioxide above 420 ppm compared with about 280 ppm before industrialisation.
Ice cores
- Air bubbles trapped in ice show past concentrations that rise and fall with temperature.
Methane
- Per molecule, methane traps more heat than carbon dioxide but it stays in the air for a shorter time.
Positive feedback
- Some changes, such as melting ice, amplify the original warming and make it harder to control.
Other contributors
- Cement production, burning of peat and some industrial gases add to emissions,
- and methane from melting permafrost is a growing concern.
Section 4 of 5Worked Example
Question
- Atmospheric carbon dioxide rose from about 280 ppm before industrialisation to about 420 ppm.
- Calculate the percentage increase.
- Increase = 420 − 280 = 140 ppm.
- Percentage increase = 140 ÷ 280 × 100 = 50%.
Answer A 50% increase
Section 5 of 5Exam Checklist & Tips
4.12.1 – Key Points for the Exam
- Greenhouse gas A gas that absorbs infrared radiation and warms the atmosphere.
- Enhanced greenhouse effect Increased warming caused by additional human-produced greenhouse gases.
- Positive feedback A process in which a change leads to further change in the same direction.
- Fossil fuel Fuel formed from ancient organisms, such as coal, oil and natural gas.
- Anthropogenic Caused by human activity.
Common Mistakes
- Confusing the greenhouse effect, which is natural, with the enhanced greenhouse effect.
- Blaming the ozone layer for global warming.
- Saying that radiation from the Sun is trapped, when it is the infrared radiation from Earth that is trapped.
- Forgetting that deforestation reduces carbon uptake as well as releasing carbon.
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