The enhanced greenhouse effect at Higher Level: The same ideas as SL, with different cases: methane's rise, Australia's coal-powered electricity, the Lytton heat and the IPCC's attribution of warming. At HL, practise the long answer: weighing natural against human causes with evidence on both sides.
Practise this as you read
- Chain every point: gas, heat kept in, warming, climate change.
- Weigh natural causes with numbers before you dismiss them.
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A natural blanket, made thicker by people: Greenhouse gases keep the Earth warm. Human emissions have added more of them, so they trap more heat.
The points to remember
- Short-wave radiation from the Sun passes through the atmosphere and warms the surface.
- The warm surface gives off long-wave (infrared) radiation.
- Greenhouse gases absorb this heat and re-emit some of it back down: the natural greenhouse effect.
- It keeps the Earth at about 15 °C on average instead of about -18 °C: life needs it.
- Human emissions add more greenhouse gas, so more heat is absorbed and re-emitted: the enhanced greenhouse effect.
Remember it as: Short-wave in, long-wave out, more gas keeps more in.
Real example: methane in the air is now more than 2.5 times its level in 1750: about 1,936 ppb in 2025, against about 729 ppb before the Industrial Revolution.
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The guide names three gases that people emit in large quantities, and smaller amounts of others. Learn each gas with its human sources.
The points to remember
- Carbon dioxide: burning fossil fuels, deforestation, cement. Up from about 280 to 427 ppm.
- Methane: cattle and sheep (digestion), rice paddies, landfill, gas leaks. More than 2.5 times its 1750 level.
- Nitrous oxide: nitrogen fertilizers, manure, burning fuel. About a quarter higher than in 1750.
- Smaller quantities of other GHGs: F-gases such as HFCs from fridges and air conditioners.
- GWP (global warming potential) compares a gas's warming with the same mass of CO2 over 100 years.
- CO2 has the biggest total effect because there is so much of it: about three-quarters of all emissions.
Real example: worldwide in 2019, CO2 made up about three-quarters of all greenhouse gas emissions (as CO2-equivalent), methane about 18%, nitrous oxide about 4% and F-gases about 2% (IPCC).
Gas plus source: Link each gas to a human activity: 'methane from the digestion of cattle', not just 'methane'.
Papers give a chart of a country's emissions by sector and ask which sector contributes most.
The points to remember
- Countries report emissions by sector: energy (including transport), agriculture, industry, waste.
- Read the key first, then compare bar heights for the year asked.
- Agriculture is the biggest sector where farming is big and electricity is clean (New Zealand).
- Energy is the biggest where coal or oil make the electricity (Australia).
- Emissions are given as CO2-equivalent: each gas converted to the CO2 that would cause the same warming.
Real example: Australia makes much of its electricity from coal, so energy (427.2 million tonnes CO2-eq in 2021) is by far its largest sector; agriculture (128.1) comes second.
Answer the year asked: If the question names a year, compare the bars for that year only, and give the sector's name.
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Warming first, then a changed climate: The enhanced greenhouse effect leads to global warming, and global warming leads to climate change.
The points to remember
- More GHG means more infrared absorbed and re-emitted, so the global average temperature rises: global warming.
- A warmer atmosphere changes the whole climate: that is climate change.
- More evaporation, so heavier rain in some places and drought in others.
- Winds and precipitation patterns shift; seasons change.
- More extreme weather: heatwaves, storms, hurricanes, floods.
- Warming also melts ice and raises sea level.
More CO2
- Emissions raise the concentration of a greenhouse gas.
More heat kept
- More infrared absorbed and re-emitted.
Global warming
- The average temperature rises.
Climate change
- Evaporation, winds, rain, droughts, storms change.
Real example: on 29 June 2021, the village of Lytton in Canada reached 49.6 °C, Canada's highest temperature ever, during a heatwave that scientists found would have been virtually impossible without human-caused warming.
The climate has always changed naturally. So how do we know the recent warming is human-made? Check each natural cause against what has happened since 1950.
The points to remember
- Milankovitch cycles: changes in the Earth's orbit and tilt drive ice ages over about 100,000 years: far too slow.
- Solar output varies by only about 0.1% over an 11-year cycle and has not risen since the 1950s.
- Volcanic eruptions cause short-term cooling (aerosols reflect sunlight) for a year or two.
- El Niño and La Niña move heat around for a year or so; they do not add heat over decades.
- So natural factors cannot explain the fast warming since 1950; the rise in greenhouse gases matches it.
Natural causes
- Orbit cycles: about 100,000 years
- Sun: flat since the 1950s
- Volcanoes: cool for 1-2 years
- El Niño: a year or so
Human causes
- Fossil fuels: CO2
- Deforestation: CO2
- Farming: methane, nitrous oxide
- Industry: F-gases, cement CO2
Real example: the IPCC (2021) concluded that it is 'unequivocal' that human influence has warmed the planet: about 1.07 °C of warming by 2010-2019, while natural causes changed the temperature by only -0.1 to +0.1 °C.
Do not say 'the Sun is getting hotter': Solar output has not risen since the 1950s, so it cannot explain the recent warming.
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How this comes up: Paper 2, Section B (c) [9]: weigh natural and human causes of recent warming, with evidence and a conclusion.
Some people argue that the climate has always changed and that today's warming is natural.
To what extent is the global warming observed since 1950 caused by human activity?
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.