The chemistry of the atmosphere
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.
The natural effect is needed: the Royal Society says that without it Earth would be around −18°C and we would be unlikely to survive on it. The worry is a change in degree, so how much has carbon dioxide risen? It is measured in parts per million, and 1% is 10,000 parts in a million. Climate.gov gives 422.8 parts per million as the global average for 2024. That is 422.8 ÷ 10,000 = 0.04228%, which rounds to the 0.04% used earlier, and we use 420. Bubbles of air sealed in ice cores, with other evidence, show 280 or less before the Industrial Revolution began in the mid-1700s, so we use 280. The rise is 420 − 280 = 140 parts per million. A percentage rise is the rise divided by the starting value, times 100, so 140 ÷ 280 × 100 = 50%. Carbon dioxide is about 50% higher than before the Industrial Revolution, as Climate.gov also says. Climate.gov says the rise is mostly from burning fossil fuels. Carbon dioxide absorbs infrared, so more of it strengthens the natural effect.
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.