13.3–13.4 Greenhouse Gases and Global Climate Change

A free GCSE Chemistry lesson for AQA (8462) from OBA School: the greenhouse effect, how human activities add greenhouse gases, the effects of climate change, and carbon footprints. 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

• Name the main greenhouse gases and explain the greenhouse effect in terms of radiation
• Describe human activities that increase carbon dioxide and methane in the atmosphere
• Evaluate evidence for human-caused climate change, including uncertainty in models
• Describe the possible effects of global climate change
• Define carbon footprint, describe ways to reduce it and explain the problems in doing so

Key words

• Greenhouse gas: A gas that absorbs long-wavelength (infrared) radiation, e.g. carbon dioxide, methane and water vapour.
• Greenhouse effect: Greenhouse gases keep Earth warm enough for life by trapping some of the heat radiated from Earth’s surface.
• Global climate change: Long-term changes in Earth’s weather patterns, linked to the rise in average global temperature.
• Carbon footprint: The total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event.
• Carbon neutral: Adding no extra carbon dioxide overall: any emissions are balanced by removing the same amount.
• Peer review: Checking of scientific work by other experts before it is published.

The greenhouse effect

The main greenhouse gases are water vapour, carbon dioxide and methane. They let short-wavelength radiation from the Sun pass through the atmosphere. Earth’s surface absorbs it and warms up, then gives out long-wavelength infrared radiation. Greenhouse gases absorb some of this and re-emit it in all directions, including back towards Earth.

• About 30% of the Sun’s radiation is reflected straight back into space by clouds, ice and the surface. The rest is absorbed and warms the planet.
• When a greenhouse gas molecule absorbs infrared radiation, its bonds vibrate (bend and stretch) more. The energy is passed on to other molecules, so the air warms up.
• The more greenhouse gas there is, the more infrared is absorbed, and the warmer the lower atmosphere becomes.

Key fact: The greenhouse effect is natural and necessary. Without it, Earth’s average surface temperature would be about −18 °C instead of about 15 °C. The worry is the enhanced greenhouse effect caused by extra greenhouse gases from human activities.

Human activities that add greenhouse gases

• Carbon dioxide; Human activities that increase it burning fossil fuels (power stations, vehicles, heating); deforestation (fewer trees to absorb CO₂ by photosynthesis, and burning or rotting the wood releases more); making cement
• Methane; Human activities that increase it cattle and sheep farming (produced during digestion); rice paddy fields; decomposing waste in landfill sites

A growing world population needs more energy, more food (more cattle and rice) and makes more waste, so emissions keep rising. At the same time, natural carbon sinks are weakened: forests are cleared, and as the oceans warm, carbon dioxide becomes less soluble in sea water, so the oceans absorb it less well.

The concentration of carbon dioxide has risen from about 280 parts per million (ppm) before the Industrial Revolution to over 400 ppm today. Over the same time, the average global temperature has risen by about 1 °C.

Worked example: percentage increase in CO₂

1960: 317 ppm. 2020: 414 ppm.
increase = 414 − 317 = 97 ppm
percentage increase = 97 ÷ 317 × 100 = 30.599… %

Answer: 30.6% (3 s.f.)

Evaluating the evidence

• Most climate scientists agree that human activities are very likely to be causing the rise in temperature. Their work is checked by peer review.
• Predictions come from climate models. These are simplified versions of a very complex system, so there is uncertainty in how big the changes will be.
• A correlation between carbon dioxide and temperature does not prove cause on its own. The evidence is strong because scientists also understand the mechanism (absorption of infrared radiation).
• The media may report only part of the evidence, or present speculation as fact, which can confuse the public.

Ice cores give evidence from the past. Snow that fell on Antarctica and Greenland trapped tiny bubbles of air as it turned to ice. Drilling out long cores and analysing the bubbles shows how CO₂ and temperature changed over hundreds of thousands of years. Today’s CO₂ level is higher than at any time in that record. A small minority of scientists argue that recent warming is mostly natural variation, but the peer-reviewed evidence overwhelmingly links it to human activity.

Zooming in on monthly data from a northern-hemisphere site (graph above) shows a yearly rise and fall on top of the rising trend:

• In spring and summer, plants grow fast and photosynthesise more, taking in more CO₂ than respiration and decay release, so CO₂ falls.
• In autumn and winter, leaves fall and decay, photosynthesis slows and more fuel is burned for heating, so CO₂ rises.
• Each year’s peak is higher than the last: the overall trend is upwards.

Exam tip: When asked to evaluate data, quote figures from it, describe the trend, then say what the data can and cannot show (e.g. correlation is not proof; data from one place; models simplified).

Effects of global climate change

• Sea levels rise (ice melts, sea water expands); Why it matters flooding of low-lying land; more coastal erosion
• More frequent and severe storms; Why it matters damage to homes and infrastructure; loss of life
• Changes in amount, timing and distribution of rainfall; Why it matters droughts in some places, floods in others
• Temperature and water stress for people and wildlife; Why it matters some areas may become too hot or dry to live in
• Changes in food-producing capacity; Why it matters some crops may fail; others may grow in new places
• Changes in distribution of wildlife species; Why it matters some species move or become extinct

Carbon footprint and how to reduce it

A carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event. It is reduced by cutting emissions of carbon dioxide and methane.

• Alternative energy; Example wind, solar, hydroelectric or nuclear power instead of fossil fuels
• Energy conservation; Example insulating homes; using public transport; efficient appliances
• Carbon capture and storage; Example capturing CO₂ from power stations and pumping it deep underground into porous rocks, e.g. old oil and gas fields (adds to the cost of electricity)
• Cutting methane; Example eating less beef and dairy (fewer cattle); collecting methane from landfill sites to use as a fuel
• Hydrogen as a fuel; Example 2H₂ + O₂ → 2H₂O: only water is made. But much hydrogen is still made using fossil fuels, and it is hard to store
• Carbon taxes and licences; Example making polluters pay for, or limit, their emissions
• Carbon offsetting; Example planting trees to absorb the same mass of CO₂ that is emitted
• Carbon neutrality; Example using fuels such as biofuels where growing the crop absorbs the CO₂ released

Problems in reducing carbon footprints: scientists disagree about some causes and consequences; lack of public information and education; people do not want to change their lifestyle (e.g. less flying or driving); economic considerations (cost, and poorer countries need to develop); incomplete international cooperation (poorer countries need to develop their industries, and the biggest emitters must agree too). Scientists therefore need to communicate clearly to the public and to governments, not just to other scientists.

Worked example: carbon dioxide from burning natural gas (Higher tier)

CH₄ + 2O₂ → CO₂ + 2H₂O. Mr: CH₄ = 16, CO₂ = 44
A home burns 600 kg of methane in a year. 1 mol CH₄ gives 1 mol CO₂, so 16 kg gives 44 kg.
mass of CO₂ = 600 × 44 ÷ 16 = 1650 kg

Answer: 1650 kg (1.65 tonnes) of CO₂

Common mistakes

• Mixing up the greenhouse effect with the hole in the ozone layer. They are different problems.
• Saying greenhouse gases “trap the Sun’s rays”. They absorb the long-wavelength infrared radiation given out by Earth’s surface, not the short-wavelength radiation coming in.
• Saying the greenhouse effect is bad. It is natural and needed; the increase in it is the problem.
• Forgetting methane and water vapour. Carbon dioxide is not the only greenhouse gas.
• Defining carbon footprint as only carbon dioxide. It includes other greenhouse gases over the whole life cycle.

Revision questions

Try these, then check your answers below each one.

Question 1. Which list contains only greenhouse gases? (Choose from: carbon dioxide, methane, water vapour, carbon dioxide, nitrogen, oxygen, methane, argon, nitrogen, oxygen, water vapour, argon.)

Answer: carbon dioxide, methane, water vapour

Working: The main greenhouse gases are carbon dioxide, methane and water vapour.

Question 2. CO₂ at one monitoring site was 317 ppm in 1960 and 414 ppm in 2020. Calculate the percentage increase, to 3 significant figures.

Answer: 30.6 %

Working: Increase = 97 ppm. 97 ÷ 317 × 100 = 30.6%.

Question 3. (Higher tier) Methane burns: CH₄ + 2O₂ → CO₂ + 2H₂O. What mass of carbon dioxide, in kg, is made when 40 kg of methane burns completely? (Mr: CH₄ = 16, CO₂ = 44)

Answer: 110 kg

Working: 40 × 44/16 = 110 kg of CO₂.

Related lessons

• Previous lesson: 13.2 Our Evolving Atmosphere
• Next lesson: 13.5 Atmospheric Pollutants
• In this chapter: 13.1 History of Our Atmosphere

Continue in the full course

This lesson is section 13.3–13.4 (AGCCH1303) of our free GCSE Chemistry 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 Chemistry course.