How the Carbon Cycle Connects Air, Land and Ocean

Carbon dioxide in the atmosphere can enter plants, algae and some bacteria through photosynthesis. These organisms use light energy to build carbon-containing sugars. The carbon becomes part of leaves, roots, wood and other biological molecules.

Animals obtain carbon by eating plants or other animals. Inside cells, respiration breaks down fuel molecules and releases energy. Carbon dioxide is produced and returns to the air or water. Plants respire too, so they both absorb and release carbon.

Dead organisms and waste contain carbon that decomposers can use. Bacteria and fungi break the material down, releasing carbon dioxide through respiration. In waterlogged soils or oxygen-poor environments, some decomposition can also produce methane.

Carbon dioxide moves between the atmosphere and the ocean surface. Some dissolves directly in seawater and takes part in chemical reactions that form bicarbonate and carbonate ions. Marine organisms use dissolved carbon to build tissues and, in some cases, shells.

When organisms die, part of their carbon sinks. Much is recycled near the surface, but some becomes buried in sediment. Over very long periods, carbon can become part of sedimentary rock or fossil fuels. Weathering, erosion and the movement of Earth's crust continue shifting it.

Volcanoes return some geological carbon to the atmosphere. Rock formation and tectonic recycling occur far more slowly than photosynthesis and respiration. The carbon cycle therefore contains fast loops measured in days or years and slow loops measured in thousands or millions of years. A carbon atom can therefore remain in one store for a moment or for longer than recorded human history.

Burning coal, oil and natural gas moves carbon from long-term geological stores into the atmosphere quickly. Cutting forests can release stored carbon and reduce the amount removed by future photosynthesis. Cement production also releases carbon dioxide.

Oceans and land ecosystems absorb part of these emissions, but not all of them. The remaining carbon dioxide increases its concentration in the atmosphere and strengthens the greenhouse effect. Ocean absorption also changes seawater chemistry.

Follow the major transfers:

• •Photosynthesis moves carbon from air into organisms.
• •Respiration and decomposition return carbon.
• •Oceans exchange and store dissolved carbon.
• •Burial stores carbon in sediments and fuels.
• •Combustion rapidly returns geological carbon to the air.

Continuous measurements of atmospheric carbon dioxide began at Mauna Loa in Hawaii in the late 1950s. The record rises steadily year after year, but it also shows a yearly wiggle. Concentrations dip during the northern summer, when vast forests and grasslands take up carbon through photosynthesis, then climb again in autumn and winter as respiration and decay dominate. The Northern Hemisphere has much more land and vegetation than the Southern, so its seasons control the pattern you see in the global record.

Scientists can also tell where the added carbon comes from by studying isotopes, versions of carbon atoms with different numbers of neutrons. Fossil fuels formed so long ago that they contain essentially no radioactive carbon-14, and plant-derived carbon is relatively low in carbon-13. As fossil carbon enters the air, the proportions of both isotopes in atmospheric carbon dioxide shift in exactly the way that source predicts. The slow decline in atmospheric oxygen matches combustion too, because burning uses oxygen as it releases carbon.

When carbon dioxide dissolves in seawater, some of it reacts with water to form carbonic acid, a weak acid that releases hydrogen ions. More dissolved carbon dioxide therefore means more hydrogen ions, and a lower pH. Many of those extra hydrogen ions combine with carbonate ions already in the water, turning them into bicarbonate. The result is a double change: the water becomes slightly more acidic, and the supply of carbonate ions falls. The ocean is still alkaline overall, so the process is called acidification rather than the ocean becoming an acid.

Measurements show that average surface ocean pH has fallen by roughly 0.1 since before the industrial era. That sounds tiny, but pH is a logarithmic scale, so it corresponds to an increase of about a quarter or more in hydrogen ion concentration. The falling carbonate supply matters for organisms such as corals, oysters and tiny swimming snails called pteropods, which build shells and skeletons from calcium carbonate. They can still do so, but it takes more energy, and in very corrosive water existing shells can begin to dissolve.

Over very long timescales, the carbon cycle helps keep Earth's climate within limits suitable for liquid water. The key is the chemical weathering of silicate rocks such as granite and basalt. Rainwater made slightly acidic by carbon dioxide slowly breaks these minerals down, and the dissolved products, including calcium and bicarbonate ions, are carried by rivers to the sea. There, organisms and chemical reactions lock the carbon into calcium carbonate, which settles on the seabed and can eventually become limestone. Each step takes carbon dioxide out of the atmosphere and moves it into rock.

The process responds to temperature. In a warmer, wetter climate, weathering speeds up and draws down more carbon dioxide, which cools the planet. In a cooler climate it slows, letting carbon dioxide from volcanoes build up again. This negative feedback is thought to be one reason Earth has avoided permanent freezing or runaway heating over billions of years. It is extremely slow, however, working over hundreds of thousands of years, so it cannot remove fossil fuel carbon on a timescale that matters to people. Some researchers are testing whether spreading crushed rock on farmland could speed it up.

Carbon dioxide is heavier than the carbon it contains. A carbon atom has a relative mass of 12 and each oxygen atom adds 16, so a molecule of carbon dioxide has a mass of 44. Every kilogram of carbon burned therefore becomes about 3.7 kilograms of carbon dioxide, since 44 divided by 12 is roughly 3.67. A litre of petrol weighs about three quarters of a kilogram and is mostly carbon by mass, so burning it releases roughly 2.3 kilograms of carbon dioxide. The extra mass is oxygen taken from the air as the fuel burns.

That conversion matters whenever you read about the carbon cycle, so check which unit is being used. Scientists who track the whole cycle often count tonnes of carbon, because carbon moves between forms such as sugar, carbonate and gas. Energy and climate reports usually count tonnes of carbon dioxide instead. The same quantity looks 3.67 times larger in the second unit. Recent yearly emissions from fossil fuels are close to 10 billion tonnes of carbon, which is the same thing as roughly 37 billion tonnes of carbon dioxide. Mixing up the two units is an easy way to misread a report by a factor of nearly four.

The carbon cycle links atmosphere, life, ocean and rock through transfers that operate at very different speeds. Natural processes continually exchange carbon, while human activity is moving long-stored carbon into the air unusually quickly. Keep the stores separate from the transfers, and the cycle becomes a readable map rather than an intimidating knot of arrows.