Carbon is the chemical element that allows life to exist, the core component of molecules in our cells and bodies.
It eventually returns to the environment when we die, however, and the element must be recycled – via the carbon cycle – so it can be used again and again by living things.
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What’s special about carbon?
The element forms strong chemical bonds with others and produces many important molecules. One atom combines with two oxygen atoms to make carbon dioxide (CO2), for example, while long chains of carbon with added hydrogen atoms (hydrocarbons) or water (carbohydrates) form molecules whose bonds contain the energy – as fuel or food – that powers everything from petrol cars to the biochemical reactions in living cells.
Where does carbon come from?
Carbon ultimately originates from massive, dying stars. These burning balls of plasma forge elements that are scattered across the universe when one explodes in a spectacular supernova. The resulting interstellar dust – containing carbon and other elements – clumps together at the birth of star systems (including our solar system) and give rise to planets and life. We are made of stardust.
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Where is carbon stored?
Carbon is stored in reservoirs around the globe, including the atmosphere and oceans, lithosphere (Earth’s rocky, outer layer) and biosphere (the total collection of living things). A reservoir becomes a carbon ‘source’ if it releases material – such as volcanoes or humans emitting CO2 into the air – or ‘sink’ if it absorbs more than it produces. Carbon sinks can include the sea, soil and forests created by plants and other things that use light and CO2 to make food via photosynthesis.
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How does the carbon cycle work?
The carbon cycle is the continuous movement of carbon between reservoirs. For example, volcanic activity moves carbon from the lithosphere to the atmosphere; photosynthetic organisms shift carbon from the atmosphere to the biosphere; and burning food (by natural respiration) or fossil fuels (by artificial combustion) releases CO2 into the atmosphere, and some of that gas dissolves in oceans.
These movements are driven by processes that work on two different timescales. Fast processes often occur over years or decades because they involve biological activity, such as photosynthesis. Slow processes typically take place over hundreds of millions of years as they require geological activity: rocks are dissolved by rainwater (made slightly acidic by CO2) through weathering, for instance, which causes carbon to flow via rivers to the sea as minerals such as calcium carbonate. Some marine organisms have shells or skeletons made of calcium carbonate, too, which sink to form sediments that turn into rock. The formation of sedimentary rock, whether from organisms or weathering, returns carbon to the lithosphere.
How are fossil fuels formed?
Organic matter such as plant material – containing energy-rich carbon molecules – can be fossilised by being buried in layers of sediments under conditions of high heat, pressure and lack of oxygen. Without decay, that carbon isn’t broken down, which transforms the material into solid coal, liquid oil or natural gas.
Why can’t the carbon cycle prevent climate change?
Burning fossil fuels releases gases such as CO2 that create a blanket that traps heat from sunlight in the atmosphere, causing the greenhouse effect that leads to global warming. But given that carbon continually flows between reservoirs, can’t the cycle compensate for excess emissions? Well, it can… eventually.
Atmospheric CO2 concentrations were around 280 parts per million (ppm) before the Industrial Revolution (circa 1760-1840) and have now reached over 420ppm, and after 2030 it’s likely that surface temperatures will have risen by 1.5°C. This warming reflects the fact carbon is released from sources faster than any sink or slow process can counter. So while the carbon cycle may well stabilise the climate in the far future, Earth would go through a period that’s too hot for humans before it returns to being habitable for other living things.
Main image: Mangrove forests are carbon ‘sinks’, absorbing CO2 / Getty
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