Greenhouse Gases and Climate Change

Greenhouse gases are gases in the atmosphere that absorb heat radiation leaving the Earth and help keep the planet warm. Without them the Earth would be far too cold to support life, so the greenhouse effect is a natural and necessary process.

For the exam you must know three greenhouse gases:

Greenhouse gas	Formula
Carbon dioxide	CO₂
Methane	CH₄
Water vapour	H₂O(g)

These gases are present only in small proportions compared with nitrogen and oxygen, yet they have a large effect on temperature because of the way they interact with radiation. The problem is not the greenhouse effect itself, but the extra warming caused when human activity increases the amounts of these gases.

The three greenhouse gases you must name are carbon dioxide, methane and water vapour. They keep the Earth warm enough to support life.

The specification asks you to describe the greenhouse effect in terms of short and long wavelength radiation interacting with matter. Work through it as a sequence.

1. The Sun radiates energy that reaches the Earth as mostly short wavelength radiation.
2. This short wavelength radiation passes through the atmosphere and is absorbed by the Earth's surface, warming it.
3. The warmed Earth re-emits energy, but now as long wavelength (infrared) radiation.
4. Greenhouse gases absorb this outgoing long wavelength radiation and re-emit some of it back towards the Earth's surface.
5. This trapping of energy keeps the surface warmer than it would otherwise be.

The key idea is that greenhouse gases interact mainly with the long wavelength radiation going out, not the shorter wavelength radiation coming in.

Short wavelength radiation from the Sun is absorbed by the Earth; the Earth re-emits long wavelength (infrared) radiation; greenhouse gases absorb that outgoing radiation and warm the atmosphere.

Some human activities raise the levels of carbon dioxide and methane. You must be able to recall two activities for each gas.

Greenhouse gas	Human activities that increase it
Carbon dioxide (CO₂)	Burning fossil fuels (in power stations, vehicles and industry); deforestation, which removes trees that would absorb CO₂
Methane (CH₄)	Livestock farming, especially cattle; decomposition of waste in landfill sites; rice paddy fields

For carbon dioxide, burning fossil fuels releases carbon that had been locked away for millions of years, while cutting down forests removes trees that would otherwise take in carbon dioxide by photosynthesis.

For methane, cattle produce methane as part of digestion, and organic waste rotting without oxygen in landfill sites and flooded rice fields also releases it.

Two CO₂ activities: burning fossil fuels and deforestation. Two methane activities: livestock (cattle) farming and decomposition of waste in landfill.

Based on peer-reviewed evidence, many scientists believe that human activity is causing the Earth's average surface temperature to rise, driving global climate change. Peer review means other scientists in the field check the work before it is published, which helps ensure conclusions are reliable.

The climate is a very complex system, which creates genuine difficulties:

• Models have to be simplified, so predictions carry uncertainty.
• This uncertainty leaves room for speculation and for biased or oversimplified media reports.
• It is important that results are communicated clearly to a range of audiences, including the public and policymakers.

When you are asked to evaluate a report on climate change, look at where the evidence came from, whether it was peer reviewed, whether the sample or data set was large enough, and whether the source might be biased.

The climate is complex and hard to model, so there is uncertainty; peer review and clear communication help judge how reliable a claim is.

An increase in the average global temperature is a major cause of climate change. You should be able to describe briefly four potential effects and comment on their scale and risk. Because exact future figures are uncertain, describe the type of effect rather than quoting precise numbers.

Effect	What happens
Rising sea levels	Melting ice and expansion of warming seawater raise sea levels, causing flooding and coastal erosion
More extreme weather	More frequent or severe storms, heatwaves, droughts and flooding
Changes to rainfall patterns	Shifts in the amount, timing and distribution of rainfall, affecting farming and water supplies
Changes to ecosystems and species	Changing temperatures alter habitats, threatening the distribution and survival of wild species

These effects vary in scale and risk: some regions are far more vulnerable than others, and the environmental and human costs can be severe. Any current figures for temperature rise or sea level should be treated as approximate, because they are estimates from models that are regularly updated.

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. "Full life cycle" means every stage, from obtaining raw materials and manufacturing through to use and disposal.

A carbon footprint can be reduced by cutting emissions of CO₂ and methane. Practical actions include:

• Using renewable energy or nuclear power instead of fossil fuels.
• Increasing energy efficiency and conserving energy.
• Introducing carbon capture and storage.
• Using carbon taxes, licences or carbon-neutral schemes (such as carbon offsetting).

You must also explain why actions may be limited. Common reasons are:

• The high cost of switching to new technology.
• A lack of international or political agreement on targets.
• Concerns about slowing economic growth or affecting people's lifestyles.
• Incomplete education and information so individuals do not act.

A carbon footprint covers the whole life cycle of a product, service or event. It can be cut by reducing CO₂ and methane emissions, but cost and lack of political agreement often limit how far actions go.

1. Saying greenhouse gases absorb incoming sunlight

Greenhouse gases mainly absorb the long wavelength (infrared) radiation re-emitted by the Earth, not the short wavelength radiation coming in from the Sun. Get the direction and wavelength right.

2. Confusing the greenhouse effect with the ozone layer

The greenhouse effect is about trapping heat and involves CO₂, methane and water vapour. It is not the same as the ozone layer, which is a separate topic and not what this question is testing.

3. Giving vague human activities

Name specific activities: burning fossil fuels and deforestation for CO₂; cattle farming and landfill waste for methane. "Pollution" or "factories" on their own are too vague to score.

4. Treating climate predictions as certain facts

Because the climate is complex and models are simplified, predictions carry uncertainty. Frame current figures as approximate and mention peer review when evaluating evidence.

5. Defining carbon footprint as only carbon dioxide from one stage

A carbon footprint is the total of CO₂ and other greenhouse gases across the full life cycle, not just the CO₂ released while a product is being used.