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: since 1750, the CO2 in the air has risen by about half, from about 280 ppm to about 427 ppm in 2025.
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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: in New Zealand, farming is the largest source of greenhouse gases, mostly methane from the digestion of about 10 million cattle and 24 million sheep.
Gas plus source: Link each gas to a human activity: 'methane from the digestion of cattle', not just 'methane'.
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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: New Zealand makes most of its electricity from hydropower, so in 2021 agriculture (43.0 million tonnes CO2-eq) was its largest sector, ahead of energy (33.6).
Answer the year asked: If the question names a year, compare the bars for that year only, and give the sector's name.
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: the World Meteorological Organization found that 2024 was the first calendar year more than 1.5 °C warmer than before the Industrial Revolution.
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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: satellites have measured the Sun's energy output since 1978. It has gone up and down with the 11-year cycle but has not risen, while temperatures have climbed.
Do not say 'the Sun is getting hotter': Solar output has not risen since the 1950s, so it cannot explain the recent warming.
How this comes up: Paper 2, Section A: 'Outline how CO2 emissions can cause a change in the global climate' [2]: a chain from the gas to the climate.
Many countries are still increasing their use of coal for electricity.
Outline how carbon dioxide emissions can cause a change in the global climate.
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