9.2.1 The Earth's early atmosphere

Volcanoes released the gases of the early atmosphere

1. The young Earth was volcanically active on a scale unlike anything today.
2. Eruptions released large volumes of gas from inside the Earth.
3. Those gases collected around the planet and became its first atmosphere.
4. The process lasted for hundreds of millions of years.
5. Nothing was alive at this stage, so no gas came from living things.

What that atmosphere is thought to have contained

1. There was little or no oxygen, which is the largest difference from today.
2. There was a large amount of carbon dioxide.
3. There was water vapour, released as steam from the eruptions.
4. Small amounts of other gases were present, including ammonia and methane.
5. This composition resembles the atmospheres of Mars and Venus today.

The evidence, and why it is uncertain

1. Gases released by volcanoes today are used as a guide to what was released then.
2. Rocks that formed in the earliest period contain iron compounds that only form without oxygen.
3. Later rocks contain oxidised iron, which shows oxygen had appeared by then.
4. The evidence is indirect, because no sample of the early atmosphere survives.
5. Different lines of evidence can point to slightly different pictures, so theories are revised.

Reading a claim about the early atmosphere

1. A claim is judged on the evidence offered for it, not on how confidently it is stated.
2. Evidence from today's volcanoes assumes the Earth's interior has not changed much.
3. Evidence from rocks depends on dating them correctly.
4. Two pieces of evidence pointing the same way make a conclusion stronger.
5. An honest answer says what is known and what remains uncertain.

9.2.2 How the atmosphere evolved

Water vapour condensed to form the oceans

1. The early atmosphere held a great deal of water vapour.
2. As the Earth cooled, that vapour could no longer stay as a gas.
3. It condensed into liquid water, which fell as rain.
4. The water collected in hollows on the surface and formed the oceans.
5. Condensation removed most of the water vapour from the atmosphere itself.

Carbon dioxide dissolved in the new oceans

1. Carbon dioxide is soluble in water.
2. As the oceans formed, large amounts of it dissolved out of the atmosphere.
3. Once dissolved, it reacted to form carbonates.
4. Those carbonates were deposited as sediment on the sea floor.
5. The amount of carbon dioxide in the atmosphere therefore decreased.

Photosynthesis added oxygen

1. Primitive plants and algae appeared in the oceans.
2. They carried out photosynthesis: $$carbon\; dioxide+water\to glucose+oxygen$$
3. Photosynthesis used up carbon dioxide and released oxygen.
4. As these organisms spread, the amount of oxygen gradually increased.
5. The increase took a very long time, because it depended on how fast the organisms spread.

Two processes acting on carbon dioxide at once

1. Dissolving in the oceans removed carbon dioxide from the atmosphere.
2. Photosynthesis removed it as well, and added oxygen in its place.
3. The two together explain why carbon dioxide fell so far from its early level.
4. Only photosynthesis explains the rise in oxygen, since dissolving adds none.
5. An account of the change names both processes and says what each one did.

9.2.3 Chemical test for oxygen

A glowing splint relights in oxygen

1. A wooden splint is lit and then blown out, so that it is glowing but not burning.
2. The glowing splint is put into the mouth of the test tube.
3. In oxygen the splint relights, bursting back into flame.
4. The relighting is the positive result that identifies oxygen.
5. A splint that simply goes out shows the gas is not oxygen.

Why the test works

1. A glowing splint is burning slowly, held back by the small amount of oxygen in air.
2. Pure oxygen is about five times as concentrated as the oxygen in air.
3. More oxygen means the wood burns fast enough to produce a flame again.
4. Combustion needs oxygen, so the gas is taking part in the reaction rather than just being present.
5. The test is therefore a chemical test, not simply an observation.

Carrying out the test and reading it

1. The gas is collected so that it does not escape before the test is made.
2. The splint is lowered in promptly, because the gas mixes with air over time.
3. A clear relight is the only result that confirms oxygen.
4. A splint that stays glowing but does not flame is not a positive result.
5. Naming the gas and the observation is what makes an answer complete.

9.2.4 The greenhouse effect and climate change

Greenhouse gases keep the Earth warm

1. Energy arrives from the Sun and warms the Earth's surface.
2. The warmed surface radiates heat back outwards.
3. Some gases in the atmosphere absorb that radiated heat.
4. Those gases then release energy, some of it back towards the Earth.
5. The result is that the Earth stays warmer than it would otherwise be.

Which gases are involved

1. Carbon dioxide is the greenhouse gas discussed most often.
2. Methane is another, and it absorbs heat strongly for the amount present.
3. Water vapour is a greenhouse gas as well, and is the most abundant of them.
4. These gases are present in small proportions apart from water vapour.
5. Nitrogen and oxygen make up most of the air but are not greenhouse gases.

The composition of today's atmosphere

1. Nitrogen makes up about $78\%$ of the air by volume.
2. Oxygen makes up about $21\%$.
3. Together those two account for about $99\%$ of the atmosphere.
4. The remainder is mostly argon, with carbon dioxide at a small fraction of a percent.
5. Water vapour is also present, in an amount that varies with place and weather.

The evidence linking human activity to climate change

1. Atmospheric carbon dioxide has risen since large-scale burning of fossil fuels began.
2. The consumption of fossil fuels has risen over the same period.
3. Average global temperature has risen over that period too.
4. Three quantities rising together is a correlation, and a strong one.
5. A mechanism supports it, since carbon dioxide is known to absorb radiated heat.

The uncertainties in that evidence

1. Measurements taken at different places can give different readings.
2. A reading taken near a city may be affected by local conditions rather than global ones.
3. Historical records were made with less accurate instruments than today's.
4. Records from long ago are estimated from ice cores and tree rings rather than measured directly.
5. These uncertainties affect how precisely the trend is known, not whether it exists.

The potential effects of more carbon dioxide and methane

1. Human activity raises both gases: burning fossil fuels releases carbon dioxide, and livestock farming releases methane.
2. Rice paddies and landfill sites release methane as well.
3. More greenhouse gas absorbs more of the heat radiated from the surface, so average global temperature is expected to rise.
4. Warmer oceans expand and ice sheets melt, so sea levels rise and low-lying land is at risk of flooding.
5. Shifts in rainfall change which crops grow where, and extreme weather is expected to become more frequent.

Whether those effects can be mitigated

1. Mitigation means cutting the amount of greenhouse gas released, or removing it once it has been released.
2. Generating electricity from renewable sources in place of fossil fuels cuts carbon dioxide at source.
3. Carbon capture stores carbon dioxide underground instead of letting it reach the atmosphere.
4. Changing diets and farming reduces the methane that livestock produce.
5. Every measure is judged on its scale, risk and cost, since one that works in a trial may not work worldwide.