C23: Earth's Atmosphere
Early atmosphere, how it changed and the greenhouse effect
The Early Atmosphere
The Earth's early atmosphere was formed by intense volcanic activity. It contained large amounts of carbon dioxide and water vapour, with nitrogen and small amounts of ammonia and methane. There was little or no oxygen.
- Volcanoes released gases including CO₂, water vapour, nitrogen, ammonia and methane.
- This is similar to the atmospheres of Venus and Mars today (mostly CO₂).
- There was no oxygen in the early atmosphere — it is wrong to think oxygen was always present.
Worked Example 1: Describing the early atmosphere
Question: Describe the composition of the Earth's early atmosphere and explain how it was formed.
Answer: The early atmosphere was formed by volcanic activity. It contained large amounts of carbon dioxide and water vapour, along with nitrogen and small amounts of ammonia and methane. There was little or no oxygen. The gases came from volcanoes which released them during intense volcanic activity as the Earth cooled.
How the Atmosphere Changed
The atmosphere changed over billions of years through several key processes: condensation of water vapour, dissolving of CO₂ in oceans, and photosynthesis by plants and algae.
1. Water vapour condensed to form oceans
As the Earth cooled, water vapour in the atmosphere condensed to form liquid water, creating the oceans.
2. Carbon dioxide was removed
- CO₂ dissolved in the oceans.
- Dissolved CO₂ formed carbonate sediments on the sea floor (e.g. calcium carbonate from shells and skeletons of marine organisms).
- This locked up CO₂ in rocks, reducing the amount in the atmosphere.
3. Photosynthesis produced oxygen
Algae and plants evolved and carried out photosynthesis. This removed CO₂ from the atmosphere and produced oxygen. Over time, oxygen built up in the atmosphere.
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
carbon dioxide + water → glucose + oxygen
carbon dioxide + water → glucose + oxygen
4. The ozone layer formed
As oxygen levels increased, some oxygen formed ozone (O₃) in the upper atmosphere. The ozone layer shields the Earth's surface from harmful UV radiation, allowing more complex organisms to evolve and survive on land.
Worked Example 2: Explaining atmospheric change
Question: Explain how the amount of oxygen in the atmosphere increased over time.
Answer: Oxygen increased because algae and plants evolved and carried out photosynthesis. Photosynthesis takes in CO₂ and releases oxygen. Over millions of years, as more plants and algae grew, the oxygen produced built up in the atmosphere. The formation of the ozone layer then protected organisms from UV radiation, allowing life to thrive and continue producing oxygen.
Worked Example 3: Explaining the decrease in CO₂
Question: Describe two processes that reduced the amount of CO₂ in the atmosphere.
Answer: (1) CO₂ dissolved in the oceans and formed carbonate sediments on the sea floor, which locked CO₂ away in rocks. (2) Photosynthesis by algae and plants removed CO₂ from the atmosphere and used it to make glucose and other organic compounds.
Current Composition of the Atmosphere
The current atmosphere is approximately: nitrogen ~78%, oxygen ~21%, argon ~0.96%, CO₂ ~0.04%, plus trace amounts of other gases and water vapour.
| Gas | Approximate % | Origin / Role |
|---|---|---|
| Nitrogen (N₂) | ~78% | Present from volcanic activity; largely inert |
| Oxygen (O₂) | ~21% | Produced by photosynthesis |
| Argon (Ar) | ~0.96% | Noble gas, from radioactive decay |
| Carbon dioxide (CO₂) | ~0.04% | Residual from early atmosphere; increased by human activity |
Worked Example 4: Comparing early and current atmospheres
Question: Compare the composition of the early atmosphere with the current atmosphere.
Answer: The early atmosphere had lots of CO₂ and water vapour but little or no oxygen. The current atmosphere has mostly nitrogen (~78%) and oxygen (~21%), with only ~0.04% CO₂ and very little water vapour. CO₂ decreased dramatically due to dissolving in oceans and photosynthesis. Oxygen increased due to photosynthesis by plants and algae.
The Greenhouse Effect
The greenhouse effect is a natural process that keeps the Earth warm enough to support life. Greenhouse gases (CO₂, methane and water vapour) absorb and re-radiate infrared radiation from the Earth's surface, trapping heat in the atmosphere.
- Short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface.
- The Earth radiates longer-wavelength infrared radiation.
- Greenhouse gases absorb some of this infrared radiation and re-radiate it back towards the surface, warming the Earth.
- Without the greenhouse effect, the Earth would be too cold for life.
Practice Questions
F H
Q1. Describe the composition of the Earth's early atmosphere and explain how it was formed.
Show Answer
The early atmosphere was formed by volcanic activity. It contained large amounts of carbon dioxide and water vapour, nitrogen, and small amounts of ammonia and methane. There was little or no oxygen.
F H
Q2. Explain how the amount of oxygen in the atmosphere increased from the early atmosphere to today.
Show Answer
Oxygen increased because algae and plants evolved and carried out photosynthesis, which produces oxygen as a by-product. Over millions of years, the oxygen built up in the atmosphere. The formation of the ozone layer from oxygen then protected organisms from UV radiation, allowing more complex life to evolve on land and continue producing oxygen through photosynthesis.
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Q3. Explain two ways in which carbon dioxide was removed from the early atmosphere.
Show Answer
(1) CO₂ dissolved in the oceans and formed carbonate compounds, which were deposited as carbonate sediments (e.g. from shells and skeletons of marine organisms). (2) Photosynthesis by plants and algae absorbed CO₂ from the atmosphere and used it to make glucose and other organic molecules.
F H
Q4. State the approximate percentages of nitrogen, oxygen, argon and CO₂ in the current atmosphere.
Show Answer
Nitrogen ~78%, oxygen ~21%, argon ~0.96%, CO₂ ~0.04%.
F H
Q5. Describe the greenhouse effect and name three greenhouse gases.
Show Answer
The greenhouse effect is a natural process that warms the Earth. Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth's surface. The Earth emits longer-wavelength infrared radiation. Greenhouse gases absorb this infrared radiation and re-radiate it in all directions, including back towards the surface, keeping the Earth warm. Three greenhouse gases are carbon dioxide, methane and water vapour.
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Q6. Explain the role of the ozone layer in the evolution of life on Earth.
Show Answer
As oxygen levels increased in the atmosphere, some oxygen formed ozone (O₃) in the upper atmosphere. The ozone layer absorbs harmful UV radiation from the Sun. Before the ozone layer formed, UV radiation would have damaged organisms, preventing complex life from surviving on land. Once the ozone layer was in place, it protected the Earth's surface, allowing organisms to live on land and evolve into more complex life forms.
Exam tip: When explaining how the atmosphere changed, structure your answer chronologically: (1) water vapour condensed → oceans, (2) CO₂ dissolved → carbonate sediments, (3) photosynthesis → oxygen increased, (4) ozone formed → protected life. Always emphasise that the early atmosphere had no oxygen. For the greenhouse effect, make sure you describe the process rather than just naming it.
🔢 Maths Skills
Mathematical Skills
You need to interpret data on atmospheric gas percentages, calculate changes in gas composition over time, and use pie charts to represent atmospheric composition.
Maths Example
The early atmosphere was approximately 95% CO₂. Today it is 0.04% CO₂. By what factor has the CO₂ percentage decreased?
Factor = 95 ÷ 0.04 = 2375. The CO₂ percentage has decreased by a factor of about 2375.
Maths Example 2
If the atmosphere has 78% nitrogen and 21% oxygen, what percentage is other gases?
Other gases = 100% − 78% − 21% = 1%
⚠️ Common Misconceptions
Watch Out!
1. Wrong: The early atmosphere contained oxygen Correct: The early atmosphere had LITTLE OR NO OXYEN — oxygen only appeared after photosynthetic organisms evolved
2. Wrong: CO₂ was removed from the atmosphere only by photosynthesis Correct: CO₂ was removed by BOTH dissolving in oceans (forming carbonate sediments) AND photosynthesis — dissolving was actually the main process initially
3. Wrong: The greenhouse effect is caused by human activity Correct: The greenhouse effect is NATURAL and essential for life — human activities are ENHANCING it, causing additional warming
4. Wrong: Ozone (O₃) and oxygen (O₂) are the same thing Correct: Ozone (O₃) is a different molecule from oxygen (O₂) — ozone is formed from oxygen in the upper atmosphere and blocks UV radiation
✍️ 6-Mark Question
Extended Answer
6 marks: Describe how the Earth's atmosphere has changed from its early composition to its current composition. Explain the processes that caused these changes.
The Earth's early atmosphere was formed by volcanic activity and contained large amounts of carbon dioxide and water vapour, with nitrogen and small amounts of ammonia and methane. There was little or no oxygen. The atmosphere changed through several key processes. First, as the Earth cooled, water vapour in the atmosphere condensed to form the oceans. Second, carbon dioxide dissolved in the oceans and reacted to form carbonate sediments on the sea floor, which locked up large amounts of CO₂ in rocks. Marine organisms also used CO₂ to form shells and skeletons made of calcium carbonate. Third, algae and plants evolved and carried out photosynthesis, which removed CO₂ from the atmosphere and produced oxygen as a by-product. Over millions of years, the oxygen built up in the atmosphere. Finally, as oxygen levels increased, some oxygen formed ozone (O₃) in the upper atmosphere, creating the ozone layer which blocked harmful UV radiation and allowed more complex life to evolve on land. The result is today's atmosphere of approximately 78% nitrogen, 21% oxygen, 0.96% argon and 0.04% CO₂.
Mark scheme: 1 mark for early atmosphere composition (CO₂, water vapour, no oxygen); 1 mark for water vapour condensing to form oceans; 1 mark for CO₂ dissolving in oceans / forming carbonate sediments; 1 mark for photosynthesis removing CO₂ and producing oxygen; 1 mark for ozone layer formation protecting life; 1 mark for current atmosphere composition
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
Ice cores from Antarctica contain trapped air bubbles that show atmospheric CO₂ levels over the past 800,000 years:
- Before 1850: CO₂ varied between 180 ppm and 280 ppm (natural cycles)
- 1850 (Industrial Revolution): CO₂ = 280 ppm
- 2024: CO₂ = 420 ppm
(a) Describe the trend shown by the data.
(b) A scientist claims that "the current increase in CO₂ is just part of a natural cycle." Evaluate this claim using the data.
(c) Explain how the data links to concerns about global warming.
Answers: (a) Before 1850, CO₂ naturally varied between 180 and 280 ppm in cycles. After 1850, CO₂ rose rapidly and continuously to 420 ppm — far above any level in the past 800,000 years. (b) The claim is not supported by the data. While CO₂ did vary naturally before 1850, it never exceeded 280 ppm in 800,000 years. The current level of 420 ppm is 50% higher than the natural maximum, and the rate of increase is unprecedented. The timing coincides with the Industrial Revolution and increased fossil fuel burning, strongly suggesting human activity is the cause, not a natural cycle. (c) CO₂ is a greenhouse gas that absorbs infrared radiation and re-radiates it back to Earth, warming the atmosphere. The data shows CO₂ has risen to levels never seen in 800,000 years, which means more heat is being trapped. This enhanced greenhouse effect is causing global temperatures to rise faster than natural cycles would predict, leading to climate change, sea level rise and habitat loss.
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