☀ Step 1: In
Short wavelength radiation from the Sun passes through the atmosphere and reaches the Earth's surface.
Greenhouse Gases » Greenhouse Gases and Human Activity
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.9.2.1, 4.9.2.2
- How greenhouse gases keep the Earth warm
- Two human activities that add carbon dioxide and two that add methane
- Why scientists link human activity to climate change
- How to judge the quality of evidence in a report
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Unlock This CourseWhy the Earth is warm enough for life
Morning mist and a warm green Earth: greenhouse gases like carbon dioxide and methane keep our planet warm enough for life
The atmosphere does not just give us air to breathe. It also keeps the Earth warm. Without it, the surface would be far too cold for most life. The gases that do this job are called greenhouse gases. They keep temperatures on Earth high enough to support life.
Three greenhouse gases matter for your exam: water vapour, carbon dioxide and methane.
Key terms:
- Greenhouse gas: a gas in the atmosphere that absorbs long wavelength radiation and helps to keep the Earth warm.
- Greenhouse effect: the warming of the Earth because greenhouse gases absorb and re-emit radiation given out by the Earth's surface.
- Radiation: energy that travels as waves. Short wavelength and long wavelength radiation behave differently when they meet matter.
How the greenhouse effect works
You need to describe the greenhouse effect in terms of short and long wavelength radiation interacting with matter. Think of it as three steps.
🌍 Step 2: Out
The surface absorbs this radiation and warms up. It then gives out radiation of a longer wavelength.
☁ Step 3: Trapped
Greenhouse gases absorb some of this long wavelength radiation and re-emit it in all directions. Some goes back towards the Earth, so the surface stays warmer.
Notice that the atmosphere lets the short wavelength radiation in, but greenhouse gases absorb the long wavelength radiation on the way out. That is why they keep us warm.
Nitrogen and oxygen make up almost all of the air, but they do not absorb long wavelength radiation in this way. The greenhouse gases are only small amounts of the atmosphere, yet they have a big effect.
Common mistakes
Do not say the greenhouse effect is a hole in the atmosphere. It is not. Also, do not say the Sun gives out the long wavelength radiation that is trapped. The Earth's warm surface gives it out.
Human activities that add greenhouse gases
Cows burp methane and traffic burns fossil fuels, two big ways humans add greenhouse gases to the air
Some human activities increase the amounts of carbon dioxide and methane in the atmosphere. You must be able to recall two activities for each gas.
🏭 Carbon dioxide
Burning fossil fuels in power stations, homes, factories and vehicles releases carbon dioxide.
Deforestation (cutting down forests) means fewer trees take in carbon dioxide by photosynthesis. Burning or rotting the felled trees also releases it.
🌾 Methane
Farming animals, such as cattle, because they release methane as they digest their food.
Rice paddy fields and rubbish in landfill sites, because rotting material gives off methane.
For the exam, pick two from each list and write them clearly. For example: carbon dioxide from burning fossil fuels and from deforestation; methane from cattle farming and from landfill sites.
Worked example
Question: Give one human activity that increases the amount of methane in the atmosphere.
Answer: Keeping cattle (or growing rice, or burying waste in landfill sites). One clear activity gets the mark. Writing just 'pollution' or 'factories' is too vague.
Global climate change and the evidence
Based on peer-reviewed evidence, many scientists believe that human activities will cause the temperature of the Earth's atmosphere to increase at the surface. They believe this will result in global climate change.
Key terms:
- Global climate change: a change in the Earth's climate over the whole planet.
- Peer review: other scientists in the same field check a piece of research before it is published, looking for mistakes or weak methods.
Peer review matters because it helps to make sure results are trustworthy. Scientists must also communicate their results to a wide range of audiences, not only to other scientists, so that everyone can understand them.
Why it is hard to be certain
Global climate is a very complex system. It is difficult to model it. This leads to simplified models, which leave some things out, and so there are uncertainties in the evidence base.
It also leads to speculation and opinions in the media. Some of these may be based on only parts of the evidence, and some may be biased. That is why you must be able to judge how good the evidence in a report is.
How to evaluate a report
Ask these questions:
1. Is the evidence peer reviewed, or just one person's opinion?
2. Does it use the whole of the evidence, or only the parts that suit one side?
3. Who wrote it? Could they be biased, for example because of who pays them?
4. Is the data from careful measurements, and are the uncertainties described?
5. Is it written for the right audience in a clear, honest way?
Common mistakes
Do not say that because a model is simplified it must be wrong. Models are useful, but they have limits, and these should be described. Also do not say that a report is good just because it is on TV or the internet. Look at the quality of the evidence behind it.
Exam-style question
A newspaper article says that burning fossil fuels has no effect on climate. It uses data from one town over three years and was not checked by other scientists.
(a) Name two greenhouse gases. (b) Give one human activity that increases carbon dioxide. (c) Give two reasons why the evidence in the article is poor quality.
Model answer
(a) Any two from: water vapour, carbon dioxide, methane.
(b) Burning fossil fuels (or deforestation).
(c) It uses data from only one town over a short time, so it is only part of the evidence. It has not been peer reviewed, so nobody has checked it for mistakes or bias.
Exam tip
When you describe the greenhouse effect, use the ideas: short wavelength radiation comes in, long wavelength radiation goes out, and greenhouse gases absorb the long wavelength radiation.