What is the atmosphere made of today, and how did it change over the Earth's history?
The composition of the present-day atmosphere and how the early atmosphere changed, including the roles of oceans, plants and the early volcanic atmosphere.
A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on the atmosphere, covering the composition of the present-day atmosphere and how it evolved from the early volcanic atmosphere through the role of the oceans and photosynthesis.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
WJEC Double Award Unit 2 wants you to state the composition of today's atmosphere and explain how the early atmosphere changed over the Earth's history.
The atmosphere today
These proportions have been fairly stable for about 200 million years.
The early atmosphere
The Earth's early atmosphere is thought to have come from intense volcanic activity, which released gases from inside the Earth. It was very different from today:
- mostly carbon dioxide,
- water vapour,
- small amounts of other gases such as nitrogen, methane and ammonia,
- little or no oxygen.
This is similar to the atmospheres of Mars and Venus today.
How the atmosphere changed
- The oceans formed: as the Earth cooled, water vapour condensed into liquid water, forming the oceans. Carbon dioxide dissolved into the oceans, reducing the amount in the air.
- Photosynthesis began: early plants and algae used carbon dioxide and released oxygen, slowly building up the oxygen we have today.
- Carbon was locked away: dissolved carbon dioxide formed carbonate rocks (such as limestone), and the remains of organisms formed fossil fuels, locking carbon out of the atmosphere.
Evidence and the link to life
The rise in oxygen allowed animals and more complex life to evolve, because they need oxygen for respiration. The drop in carbon dioxide also reduced the early greenhouse effect. This shows how the living world and the atmosphere have shaped each other over time.
How we know about the early atmosphere
Because no one was there to measure it, ideas about the early atmosphere are based on evidence rather than direct observation. Scientists study the gases released by volcanoes today, the atmospheres of other planets such as Mars and Venus (thought to be like the early Earth), and the rocks that formed at different times, which record the conditions then. Because the evidence is indirect and incomplete, scientists' ideas have changed over time and are still being refined. Knowing that this is a developing, evidence-based theory is itself an exam point.
The noble gases in the atmosphere
About 1% of the air is made of gases other than nitrogen and oxygen, and most of this is argon, a noble gas from Group 0. Small amounts of other noble gases such as neon and helium are also present. Because the noble gases are unreactive, they have stayed in the atmosphere rather than reacting away, and they are useful for jobs that need an inert gas, such as argon in light bulbs and helium in balloons. This links the composition of the atmosphere back to the chemistry of Group 0.
Try this
Q1. Name the process that added oxygen to the early atmosphere. [1 mark]
- Cue. Photosynthesis (by plants and algae).
Q2. What released the gases that formed the early atmosphere? [1 mark]
- Cue. Volcanoes (volcanic activity).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style3 marksState the approximate proportions of the two main gases in today's atmosphere and name one other gas present in small amounts.Show worked answer →
A Unit 2 recall question. Reward: about 78 percent nitrogen (1) and about 21 percent oxygen (1); plus a small amount of another gas such as carbon dioxide, argon (a noble gas) or water vapour (1). Markers credit the two main proportions and one minor gas. A common error is to swap the nitrogen and oxygen figures or to say carbon dioxide makes up a large fraction.
WJEC style4 marksExplain how the amount of oxygen in the atmosphere increased and the amount of carbon dioxide decreased over the Earth's history.Show worked answer →
A Unit 2 explain question worth 4 marks. Reward: the early atmosphere was rich in carbon dioxide from volcanoes with little oxygen (1); as the Earth cooled, water vapour condensed to form the oceans, which dissolved carbon dioxide (1); early plants and algae carried out photosynthesis, taking in carbon dioxide and releasing oxygen (1); carbon was also locked up in sedimentary rocks and fossil fuels, lowering carbon dioxide further (1). Markers credit volcanic origin, oceans dissolving carbon dioxide, photosynthesis adding oxygen and carbon being locked away. A common error is to omit photosynthesis.
Related dot points
- The greenhouse effect and human causes of climate change, the products of combustion, and the pollutants from burning fuels and their effects.
A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on climate and air quality, covering the greenhouse effect and human-caused climate change, the products of complete and incomplete combustion, and the pollutants from burning fuels.
- The treatment of water to make it safe to drink, the testing of water purity, and solubility including the idea of saturated solutions.
A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on water, covering the stages of water treatment, testing water purity, and solubility including dissolving, saturated solutions and how temperature affects solubility.
- Thermal decomposition of limestone, the reactions of the limestone cycle, and the uses of limestone, quicklime, slaked lime and cement.
A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on limestone, covering thermal decomposition, the reactions of the limestone cycle, and the uses of limestone, quicklime, slaked lime and cement.
- The carbon cycle and the nitrogen cycle, the role of decomposers and named bacteria, and how human activity affects these cycles and biodiversity.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on nutrient cycles, covering the carbon cycle, the nitrogen cycle and the bacteria involved, the role of decomposers, and how human activity affects the cycles and biodiversity.