History of the Atmosphere

This lesson introduces the history of the Earth's atmosphere, covering how its composition has changed over approximately 4.6 billion years, as required by the AQA GCSE Chemistry specification (5.9.1). Understanding how the atmosphere evolved from its original state to the mixture of gases we breathe today is essential for this topic. You need to know the key phases of atmospheric change, the gases involved, and the processes that drove these changes.

The Age of the Earth

The Earth is approximately 4.6 billion years old. Scientists believe that for the first billion years or so, there was intense volcanic activity on the surface. During this time, the atmosphere was very different from today. The evidence for the composition of the early atmosphere comes from analysis of volcanic gases, ancient rocks, and comparison with the atmospheres of other planets such as Mars and Venus.

Because the Earth is so old, there is limited direct evidence about the exact composition of the early atmosphere. Scientists use a range of indirect evidence, and there is some uncertainty about precise proportions. However, the general pattern of change is well established.

Exam Tip: When discussing the early atmosphere, always acknowledge that scientists are not certain about the exact composition — this is a common exam point. Use phrases like "scientists believe" or "evidence suggests."

Phases of Atmospheric Change

The history of the atmosphere can be divided into three broad phases:

Phase	Time Period	Key Features
Phase 1	4.6 billion – 2.7 billion years ago	Intense volcanic activity released gases including carbon dioxide, water vapour, nitrogen, methane, and ammonia. Very little or no oxygen.
Phase 2	2.7 billion – 200 million years ago	First photosynthetic organisms appeared. Oxygen levels gradually increased. Carbon dioxide levels decreased as it was absorbed by oceans, photosynthetic organisms, and locked into sedimentary rocks and fossil fuels.
Phase 3	200 million years ago – present	Atmosphere reached approximately its current composition. Nitrogen ~78%, oxygen ~21%, other gases ~1% (including 0.04% carbon dioxide and small amounts of noble gases).

Exam Tip: You do not need to memorise exact dates for each phase, but you must know the general sequence of changes and the processes that caused them.

Visualising Atmospheric Change

The following diagram summarises the key phases of atmospheric evolution:

flowchart LR
    A["Phase 1: Early Atmosphere"] --> B["Phase 2: Oxygen Begins to Rise"]
    B --> C["Phase 3: Modern Atmosphere"]
    A --- D["Volcanic activity<br/>High CO2<br/>Water vapour<br/>No oxygen"]
    B --- E["Photosynthesis begins<br/>O2 increases<br/>CO2 decreases"]
    C --- F["N2 78%<br/>O2 21%<br/>CO2 0.04%<br/>Noble gases"]

    style A fill:#d35400,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#27ae60,color:#fff

The Current Composition of the Atmosphere

The atmosphere today has remained relatively stable for about 200 million years. Its composition is:

Gas	Approximate Percentage
Nitrogen (N₂)	78%
Oxygen (O₂)	21%
Argon (Ar)	0.93%
Carbon dioxide (CO₂)	0.04%
Water vapour	Variable (0–4%)
Other noble gases	Trace amounts

Key Points

Nitrogen is the most abundant gas in the atmosphere. It is relatively unreactive and was released by volcanic activity and from the reaction of ammonia with oxygen.
Oxygen makes up about one-fifth of the atmosphere. Almost all of it has been produced by photosynthesis.
Carbon dioxide is present in only a very small proportion, but it plays a critical role as a greenhouse gas.
Water vapour content varies with temperature and location — it is not usually included in fixed composition tables.
The noble gases (argon, neon, helium, krypton, xenon) are chemically inert and present in trace amounts.

Exam Tip: Learn the three key percentages: nitrogen 78%, oxygen 21%, and "about 1% other gases." Carbon dioxide is approximately 0.04%. These figures appear regularly in exam questions.

Evidence for Atmospheric Change

Scientists use several lines of evidence to reconstruct the history of the atmosphere:

Volcanic gases — Modern volcanoes emit carbon dioxide, water vapour, nitrogen, sulfur dioxide, and other gases. Scientists assume ancient volcanoes produced similar mixtures.
Ancient rocks — Iron-rich rocks (banded iron formations) show evidence of oxidation, indicating when oxygen first appeared in significant quantities.
Fossils — Fossilised cyanobacteria (stromatolites) provide evidence of early photosynthetic organisms.
Ice cores — Air bubbles trapped in ancient ice reveal the composition of the atmosphere at different times, though these only go back about 800,000 years.
Sedimentary rocks — Limestone and chalk contain carbon that was once atmospheric carbon dioxide, locked away by marine organisms.

How Scientists Study the Past Atmosphere

Ice Cores

Ice cores drilled from glaciers in Antarctica and Greenland contain tiny bubbles of trapped air. By analysing the gas composition within these bubbles, scientists can determine the levels of carbon dioxide, methane, and other gases going back hundreds of thousands of years. This provides direct evidence of how the atmosphere has changed in relatively recent geological time.

Rock Analysis

The presence of banded iron formations (BIF) in rocks dating from about 2.7 to 1.8 billion years ago provides key evidence. Iron dissolved in ancient oceans reacted with the increasing oxygen produced by photosynthetic organisms, forming layers of iron oxide. Once all the dissolved iron had been oxidised, free oxygen began to accumulate in the atmosphere.

Comparison with Other Planets

The atmospheres of Mars and Venus are dominated by carbon dioxide (about 95–96%). Scientists believe the early Earth had a similarly CO₂-rich atmosphere. The key difference is that Earth had liquid water and developed photosynthetic life, which transformed its atmosphere.

Timeline of Key Events

Approximate Time	Event
4.6 billion years ago	Earth forms; intense volcanic activity begins
4.0 billion years ago	Oceans begin to form as water vapour condenses
3.4 billion years ago	First photosynthetic organisms (cyanobacteria) appear
2.7 billion years ago	Significant oxygen production begins ("Great Oxidation Event")
2.0 billion years ago	Oxygen levels reach approximately 1–2%
500 million years ago	Oxygen levels approach modern values
200 million years ago	Atmosphere reaches approximately its current composition

Exam Tip: You may be asked to describe how one specific gas changed over time. Practise writing about how oxygen increased (from nearly zero to 21%) and how carbon dioxide decreased (from a high proportion to 0.04%). Always link changes to specific processes such as photosynthesis or formation of sedimentary rocks.

Summary

The Earth is approximately 4.6 billion years old and its atmosphere has changed dramatically over time.
The early atmosphere was rich in carbon dioxide and water vapour, with little or no oxygen, similar to the atmospheres of Mars and Venus today.
Volcanic activity was the main source of gases in the early atmosphere.
Photosynthetic organisms (algae and plants) gradually increased the oxygen level and decreased the carbon dioxide level.
Carbon dioxide was also removed by dissolving in the oceans, being locked into sedimentary rocks (limestone), and forming fossil fuels.
The atmosphere has been approximately 78% nitrogen, 21% oxygen, and about 1% other gases for around 200 million years.
Scientists use ice cores, ancient rocks, fossils, and comparison with other planets as evidence for atmospheric change.

Exam Tip: A 6-mark question on this topic will expect you to cover at least three phases of change, name specific gases, and describe the processes that caused the changes. Always write in a logical, chronological order — start with the early atmosphere and work forwards to the present day.