AQA Chemistry (8462) 4.9.2.1–4.9.2.4 · Model
Greenhouse gases and climate change
The same few gases keep Earth warm enough for life, and are now warming it further. How do they hold on to heat, and why is cutting them so hard?
Triple Science · Higher tier
Start here
Take away three gases and Earth would average about −18 °C.
Today the Earth’s surface averages about 15 °C. Same Sun, same distance, same ground. The difference is made by water vapour, carbon dioxide and methane in the air.
If you had to guess, do those gases keep the surface warm by reflecting sunlight back down, or by soaking up heat given off by the ground?
The Sun is very hot, and most of its radiation is short wavelength: visible light, with some ultraviolet. It reaches the Earth and warms the surface. The warmed surface then gives out radiation of its own, but at long wavelengths: infrared. What happens to that infrared on its way out decides how warm the surface stays.
Radiation bench
No greenhouse gases
Sunlight warms the ground, and the ground gives out infrared. Where does the infrared go?
Human activities add more of two greenhouse gases. Carbon dioxide comes from burning any fuel that contains carbon, and from deforestation: fewer trees take it in, and felled trees release it when burned or left to rot. Methane is given off when some living things digest food or rot with little air, and natural gas is mainly methane.
Sort the sources
Which gas does it add?
Each card is a human activity. Decide which greenhouse gas it mainly adds to the air.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
An increase in average global temperature is a major cause of climate change. Its effects reach the sea, the weather, food and wildlife. Scientists weigh each effect by its scale, its risk and what it does to the environment.
Exam trap
DescribeA student lists four effects of global climate change. Which one would not earn a mark as an effect of climate change?
Based on peer-reviewed evidence, many scientists believe human activities will raise the temperature of the atmosphere at the Earth's surface, causing global climate change. Peer review means other experts check the work before it is published. But the climate is a complex system and hard to model, so models are simplified and predictions are uncertain. Media reports may use only part of the evidence, and may be biased. That is why results must be communicated clearly, to everyone.
Judge the evidence · two practice reports
Which report is better evidence about climate?
Report A · blog post
“Warming is natural, not caused by us”
- Paid for by
- A company that sells coal
- Data
- 12 weather stations in one country, over 5 years
- Checked before publishing
- No
Report B · science journal
“Global surface temperature since 1900”
- Paid for by
- A university research fund
- Data
- Thousands of land stations, ships and buoys, plus carbon dioxide records, over 120 years
- Checked before publishing
- Yes, peer reviewed
Which report’s data better represents the whole planet’s climate?
Which report has a possible source of bias?
Report B was peer reviewed. What does that tell you?
Report B shows that carbon dioxide and temperature have both risen since 1900. What turns that correlation into evidence of a cause?
The 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: renewable energy, saving energy with insulation and efficient vehicles, capturing and storing carbon dioxide, carbon taxes, offsetting such as tree planting, and less waste sent to landfill. Actions may be limited: they cost money, some technology is not ready, people are reluctant to change their lifestyles, not everyone is informed, some doubt the evidence, and countries find it hard to agree.
Your call
SuggestA town replaces its diesel buses with electric ones to cut its carbon footprint. Which reason could limit the cut?
Command words in this lesson
- Give
- A short answer, no reason: two activities for each gas.
- Describe
- Four effects means four different ones, not four ways of saying hotter.
- Explain
- The radiation chain in order: short in, long out, absorbed, re-emitted.
- Evaluate
- Use the data and the quality of the source, then judge the claim.
Key note · AQA 4.9.2.1–4.9.2.4 (8462)
Greenhouse gases and climate change
- Greenhouse gases (water vapour, carbon dioxide and methane) keep the Earth warm enough to support life.
- Short-wavelength radiation from the Sun passes through the atmosphere and warms the surface; the surface emits long-wavelength (infrared) radiation; greenhouse gases absorb it and re-emit it in all directions, including back to Earth.
- Human activities: carbon dioxide from burning fossil fuels and deforestation; methane from cattle farming and landfill.
- Effects of climate change: rising sea levels, more severe storms, changed rainfall, changes to food production and to where species live.
- Carbon footprint: the total carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event.
- Cutting emissions may be limited by cost, technology, lifestyle, lack of information and countries failing to agree.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Not sure why the gases absorb the infrared but not the sunlight?
The bench draws a few rays and molecules. Real radiation is a continuous flow, some sunlight is reflected or absorbed on the way in, and the temperatures are global averages. The two reports and the T-shirt data are made up for practice.