The composition of the atmosphere, how it evolved over time, the greenhouse gases and the greenhouse effect, and the link between human activity, climate change and the carbon footprint.

A six-mark Level of Response question. Reward describing the changes: the early atmosphere was produced mainly by volcanic activity and was largely carbon dioxide, with little or no oxygen and small amounts of other gases (such as water vapour, nitrogen, methane and ammonia). As the Earth cooled, the water vapour condensed to form the oceans. Carbon dioxide decreased in two main ways: a lot of it dissolved into the newly formed oceans, where it formed carbonate precipitates and sediments (later forming carbonate rocks), and it was used up by photosynthesis when plants and algae evolved. Oxygen increased because algae (and later plants) carried out photosynthesis, which produces oxygen as a product, so over hundreds of millions of years the oxygen level rose to about 20 percent. Markers reward the early carbon-dioxide-rich volcanic atmosphere, water vapour condensing to form oceans, carbon dioxide dissolving in the oceans and being locked into carbonates, photosynthesis removing carbon dioxide, and photosynthesis releasing oxygen. A common error is to reverse the oxygen and carbon dioxide changes.

A C6.2 structured question. Reward the greenhouse mechanism: the Sun's short-wave (visible) radiation passes through the atmosphere and warms the Earth's surface; the warm surface re-emits energy as long-wave (infrared) radiation; greenhouse gases such as carbon dioxide, methane and water vapour absorb this outgoing long-wave radiation and re-emit some of it back towards the Earth, which keeps the Earth warmer than it would otherwise be (the greenhouse effect). Then explain the human contribution: burning fossil fuels (such as coal, oil and gas) for energy and transport releases carbon dioxide, and deforestation removes trees that would otherwise take in carbon dioxide by photosynthesis, so the level of carbon dioxide in the atmosphere rises. Markers credit the absorption of long-wave (infrared) radiation by greenhouse gases keeping the Earth warm, and the human causes (combustion of fossil fuels and deforestation) increasing carbon dioxide. A common error is to say greenhouse gases absorb the incoming radiation from the Sun rather than the outgoing radiation from the Earth.