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Chemistry · GCSE · The School
Today’s air, and where it came from
The atmosphere is about 78% nitrogen, 21% oxygen and 0.04% carbon dioxide. It did not start that way. The early atmosphere was mostly carbon dioxide from volcanoes, with almost no oxygen. Photosynthesis by early organisms put oxygen in over hundreds of millions of years and took carbon dioxide out, locking much of it into limestone and fossil fuels.
The greenhouse effect, mechanically
Short-wavelength radiation from the Sun passes through the atmosphere and warms the surface. The surface re-emits it as longer-wavelength infrared, and greenhouse gases — carbon dioxide, methane, water vapour — absorb that infrared and re-radiate it in all directions, including back down. The energy comes in easily and leaves less easily, so the surface sits warmer.
It is a natural effect being amplified
Without any greenhouse effect the Earth would average around −18°C and be uninhabitable. The concern is not that the effect exists but that burning fossil fuels has raised carbon dioxide sharply, strengthening it. That distinction matters: the argument is about a change in degree, and saying so is more accurate than treating greenhouse gases as simply bad.
Pollutants are a separate problem
Carbon monoxide is toxic and colourless. Sulfur dioxide and nitrogen oxides cause acid rain and breathing problems. Particulates cause global dimming and lung damage. None of these are greenhouse gases, and carbon dioxide is not toxic at atmospheric levels. Two different problems from the same chimney, and mixing them up produces confident wrong answers in exams.
Carbon footprint, and the honest difficulty
A footprint is the total greenhouse gas over the whole life of a product or service, which is why it is hard to measure — it includes extraction, manufacture, transport, use and disposal. Answers should name a real reduction and its drawback: renewable energy needs storage, carbon capture costs energy, and behaviour change is slow.
Evidence, and why early atmosphere is uncertain
There were no instruments, so the early atmosphere is reconstructed from rock and ice evidence rather than observed. That is why the theory is stated with hedges and why alternatives exist. Saying what the evidence IS — trapped gases in ice cores, oxidised iron in ancient rock — is worth more than repeating the conclusion.