What's in the air now?

For about 200 million years the proportions of gases in the atmosphere have been roughly the same as today:

Exact figures: AQA says "about four-fifths (~80%) nitrogen" and "about one-fifth (~20%) oxygen." The fuller numbers ~78% N₂ and ~21% O₂ are also accepted — the remaining ~1% is argon, carbon dioxide and water vapour.

Reading the air

Where the air came from

Evidence for the early atmosphere is limited because of the time scale of 4.6 billion years, so theories have changed and developed over time. One leading theory:

• During Earth's first billion years, intense volcanic activity released gases that formed the early atmosphere.
• It was likely mainly carbon dioxide, with little or no oxygen — much like Mars and Venus today.
• Volcanoes also released water vapour, which condensed to form the oceans, plus nitrogen (which built up), and small proportions of methane and ammonia.

How the air changed

Oxygen increased. About 2.7 billion years ago, algae first produced oxygen by photosynthesis; later plants evolved and the oxygen level gradually rose enough for animals to evolve:

Carbon dioxide decreased in several ways:

• Dissolving in the oceans (then locked away as carbonate sediments).
• Photosynthesis by algae and plants.
• Carbon being locked into sedimentary rocks (e.g. limestone) and into fossil fuels — coal, crude oil and natural gas.

Order the events

Tap the event that came next in the story of Earth's atmosphere.

Which process?

The greenhouse effect

Greenhouse gases keep Earth warm enough to support life. The three you must know are:

• Carbon dioxide (CO₂)
• Methane (CH₄)
• Water vapour (H₂O)

Common misconception: the greenhouse effect itself is natural and essential — without it Earth would be too cold for life. The concern is the enhanced effect from extra greenhouse gases, which raises average temperatures.

How it traps heat

Adding more greenhouse gases

Some human activities increase the amount of greenhouse gases in the air:

• Burning fossil fuels → more carbon dioxide.
• Deforestation → fewer trees to remove CO₂ by photosynthesis, so more CO₂ stays in the air.
• Agriculture / livestock (e.g. cattle and rice paddy fields) → more methane.
• Landfill → decomposing waste releases methane.

Exam tip: AQA wants two human activities for each gas. CO₂: burning fossil fuels & deforestation. Methane: livestock farming & landfill (also rice growing).

Based on peer-reviewed evidence, many scientists believe these extra gases will raise the temperature of the Earth's atmosphere at the surface, causing global climate change.

Activity → gas released

Tap an activity on the left, then tap the greenhouse gas it mainly increases.

How sure are scientists?

Global climate is a hugely complex system with many variables, so scientists use simplified models. This means parts of the evidence can be uncertain, and media reports may be simplified, speculative or biased if they use only part of the evidence.

• Peer review — other scientists check results before publication, which improves reliability.
• You should be able to evaluate the quality of evidence in a climate report and describe the uncertainties in it.
• Results should be communicated to a wide range of audiences.

Balanced view: there is a correlation between rising greenhouse gases and rising average global temperature, and most scientists link the two. Models simplify reality, so exact predictions carry uncertainty — that's normal in science, not a reason to dismiss the evidence.

Potential effects

An increase in average global temperature is a major cause of climate change. You should be able to describe four potential effects and discuss their scale, risk and environmental implications:

• Sea levels rise (melting ice and expanding water) → coastal flooding.
• More frequent or severe extreme weather (storms, droughts, floods).
• Changes to where species can live and to migration patterns (loss of habitats/biodiversity).
• Changes to rainfall patterns affecting food production and water supply.

Stay measured: these are potential effects whose scale and timing carry uncertainty. The exam wants you to discuss risk and implications factually, not to exaggerate.

Carbon footprint & reducing it

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 can be reduced by cutting emissions of carbon dioxide and methane, for example:

• Using renewable energy or more efficient processes instead of fossil fuels.
• Carbon capture and storage; planting trees.
• Government taxes / caps on emissions; reducing waste sent to landfill.

Why actions may be limited: cost, the need for global agreement, lack of public/political will, incomplete technology, and the difficulty of changing lifestyles can all hold back reductions.

Lowering the footprint

What burning fuels releases

Most fuels (including coal) contain carbon and/or hydrogen, and may contain some sulfur. Burning them is a major source of atmospheric pollutants:

• Complete combustion (plenty of oxygen) → carbon dioxide + water vapour.
• Incomplete combustion (not enough oxygen) → carbon monoxide (CO), soot (carbon particles) and unburned hydrocarbons.
• The sulfur in the fuel burns to form sulfur dioxide (SO₂).
• At high temperatures, nitrogen and oxygen in the air react to form oxides of nitrogen (NOₓ).

Misconception watch: complete combustion gives CO₂; incomplete combustion (too little oxygen) is what produces the toxic CO and soot.

Why each pollutant matters

• Carbon monoxide (CO) — a toxic gas; colourless and odourless, so not easily detected.
• Soot / particulates — cause global dimming and respiratory (health) problems.
• Sulfur dioxide (SO₂) and oxides of nitrogen (NOₓ) — cause respiratory problems and acid rain.

Complete or incomplete?

Pollutant → effect

Tap a pollutant on the left, then tap its main effect on the right.

The facts to know

You've covered all of AQA 4.9 — composition & evolution, greenhouse gases & climate change, and atmospheric pollutants. Press Finish to see your score.