The Atmosphere: Evolution and Greenhouse Gases
The theory of how the Earth's atmosphere evolved from its violent volcanic origins about 4.6 billion years ago to today's composition, and the mechanism of the greenhouse effect that keeps the planet at a habitable temperature, are covered in detail. It focuses on the sequence of chemical and biological processes that removed carbon dioxide and released oxygen, and on exactly how greenhouse gases interact with infrared radiation.
Method
- Set out the early atmosphere in order: formed mainly by intense volcanic activity, it consisted largely of carbon dioxide, with significant water vapour, nitrogen, and small amounts of methane and ammonia, and almost no oxygen.
- Explain the formation of the oceans: as the Earth's surface cooled below 100 degrees Celsius, the water vapour released by volcanoes condensed, forming the oceans, which then acted as a major store for atmospheric carbon dioxide by dissolving it.
- Explain the biological removal of carbon dioxide in sequence: early algae, and later plants, evolved the ability to photosynthesise, using carbon dioxide and water and light energy to produce glucose and oxygen, steadily drawing down atmospheric carbon dioxide over roughly two billion years.
- Explain how carbon became locked away permanently: dead plant and marine material formed sedimentary carbonate rocks and, over long periods under heat and pressure, fossil fuels, both of which store carbon that would otherwise be in the atmosphere as carbon dioxide.
- Explain the greenhouse effect mechanism precisely: short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth's surface; the surface then re-radiates this energy as longer-wavelength infrared radiation; greenhouse gas molecules absorb some of this infrared radiation and re-emit it in random directions, so a portion is redirected back towards the surface rather than escaping to space.
- Distinguish the natural greenhouse effect, which has kept Earth warm enough to support life for billions of years, from the enhanced (human-caused) greenhouse effect, in which extra emissions of greenhouse gases increase the concentration of these gases and so increase the amount of infrared radiation trapped.
Worked example
Explain, using the idea of infrared radiation, why increasing the concentration of carbon dioxide in the atmosphere is expected to increase the average global temperature.
- State what happens to solar radiation reaching Earth: shorter-wavelength radiation from the Sun passes through the atmosphere largely unabsorbed and warms the Earth's surface.
- State what the warmed surface does: it re-radiates energy as longer-wavelength infrared radiation, moving back out towards space.
- State what carbon dioxide molecules do to this infrared radiation: they absorb some of it, gaining energy, and then re-emit that energy in all directions.
- Explain the consequence of more carbon dioxide molecules being present: a greater proportion of the outgoing infrared radiation is absorbed and re-emitted back towards the surface rather than escaping into space.
- Final answer: with more carbon dioxide in the atmosphere, more outgoing infrared radiation is trapped and redirected back to the surface, so more energy is retained by the Earth system, raising the average global temperature.
Practice questions
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Q1Name the two gases produced by photosynthesis and the substance it consumes alongside carbon dioxide.Show answer
Answer: Photosynthesis consumes carbon dioxide and water, and produces glucose and oxygen (oxygen is the gas released to the atmosphere).
Q2Explain, in one sentence, why the oceans were able to form only after the early Earth had cooled.Show answer
Answer: Water vapour released by volcanic activity could only condense into liquid oceans once surface temperatures had fallen below the boiling point of water.
Q3State two long-term stores of carbon that removed carbon dioxide from the early atmosphere.Show answer
Answer: Sedimentary carbonate rocks and fossil fuels (formed from dead plant and marine material over long periods).
Q4Describe, in terms of wavelength, the difference between the radiation the Earth receives from the Sun and the radiation it re-emits from its surface.Show answer
Answer: The Sun emits mostly shorter-wavelength radiation, which passes through the atmosphere and warms the surface; the warmed surface re-emits longer-wavelength infrared radiation.
Q5Explain why the natural greenhouse effect is necessary for life on Earth, rather than being a purely harmful process.Show answer
Answer: Without any greenhouse gases in the atmosphere, far more infrared radiation would escape directly to space, and the Earth's average surface temperature would be much too low to support liquid water and life; the natural greenhouse effect keeps the planet at a habitable temperature.
Q6Explain, in terms of gas molecules, why methane and water vapour, as well as carbon dioxide, are described as greenhouse gases.Show answer
Answer: All three types of molecule are able to absorb infrared radiation re-radiated from the Earth's surface and re-emit it in all directions, including back towards the surface, so all three contribute to trapping energy within the atmosphere.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Explain how the level of oxygen in the Earth's atmosphere is thought to have increased from almost zero to about 20%, and why this took place over a very long timescale rather than quickly.
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A student claims that greenhouse gases stop heat from the Sun reaching the Earth's surface, and that this is why they are called greenhouse gases. Explain why this claim is scientifically incorrect, describing the correct mechanism of the greenhouse effect.
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Free printable worksheet
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This topic is chapter 28 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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