The composition of the atmosphere has changed over billions of years. It is changing again as a result of burning fuels.
The simplified model used at GCSE is that for the last 200 million years the atmosphere has been roughly 80% nitrogen and 20% oxygen, with small amounts of carbon dioxide, water vapour and noble gases.
The exact figures usually quoted are 78% nitrogen and 21% oxygen, with argon making up most of the remaining 1%.
Evidence for the first billion years is limited, so theories have changed and remain uncertain.
One model, used at GCSE, is that intense volcanic activity released gases, producing an early atmosphere containing mainly carbon dioxide with little or no oxygen, rather like Mars and Venus today. Volcanoes also produced nitrogen, water vapour, and small amounts of methane and ammonia. As the Earth cooled the water vapour condensed to form the oceans.
Carbon dioxide dissolved in the newly formed oceans, and carbonates were precipitated as sediments that became rock.
Later, sea creatures used the dissolved carbonates to make shells and skeletons, which became limestone. Carbon was also locked away as the remains of plants and animals were buried and formed fossil fuels. And photosynthesis removed carbon dioxide continuously.
About 2.7 billion years ago photosynthetic microorganisms, cyanobacteria in particular, evolved and began photosynthesising, taking in carbon dioxide and releasing oxygen: 6CO2 + 6H2O → C6H12O6 + 6O2.
Those microorganisms drove the early rise in oxygen over the following hundreds of millions of years. Plants evolved very much later and added to it, and eventually oxygen reached a level that could support animal life.
Carbon dioxide, methane and water vapour are greenhouse gases. They let most of the short-wavelength radiation arriving from the Sun through, but absorb some of the longer-wavelength infrared radiated from the Earth’s surface and re-emit it in all directions, including back towards the surface.
The effect is what keeps the Earth warm enough for life. The concern is not the greenhouse effect itself but the increase in it.
Carbon dioxide is increased by burning fossil fuels and by deforestation, which both removes the trees that would otherwise absorb it and releases the carbon stored in the vegetation and soil.
Methane is increased by cattle farming and by rice paddy fields, and released from landfill and from thawing permafrost.
There is broad scientific consensus that human activity is causing the global temperature to rise. Peer review, and evidence from many independent lines, is what gives that conclusion its weight.
The consequences include rising sea levels from melting ice and thermal expansion, changes in the frequency and severity of some extreme weather events, changes to where crops can be grown, and changes to the distribution of wild species.
Media reports are often simplified or biased, sometimes because the science is complex and sometimes because the information is partial.
A carbon footprint is the total amount of greenhouse gases emitted over the full life cycle of a product, service or event. It is usually expressed as an equivalent mass of carbon dioxide (CO2e).
It can be reduced by using less energy and less meat, by using renewable energy, by capturing and storing carbon, and by carbon taxes or caps. The difficulties are cost, the absence of international agreement, the lack of alternative technologies in some sectors, and the need for individuals to change their habits.
Burning fuels releases more than carbon dioxide and water. Fuel impurities, incomplete combustion and the high temperatures inside an engine each produce their own pollutants:
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