The carbon cycle and climate change

Stores, transfers, and a cycle in approximate balance

Before industrialisation, the global carbon cycle was in approximate long-term dynamic balance, although stores and atmospheric carbon dioxide varied.

Carbon is the element every organic molecule is built on, and there is a fixed amount of it. It is not made or destroyed by anything biology does; it is moved between places where it is held. Those places are called stores, or reservoirs, and the processes that move carbon between them are transfers, or fluxes. Keep the two terms distinct: a store is a place where carbon is held, and a transfer is a process that moves it.

The stores are the atmosphere, where carbon is carbon dioxide gas; living organisms, where it is carbohydrate, lipid and protein; dead organic matter in soil; the oceans, where it is dissolved carbon dioxide, hydrogencarbonate ions and the tissues of marine organisms; sedimentary rock, chiefly limestone, which is much the largest store of all; and fossil fuels. The transfers are the processes you already know, viewed from the outside.

Photosynthesis takes carbon dioxide out of the air and fixes it into organic molecules, which is the only sizeable route from the atmosphere into living things. Feeding moves that carbon along a food chain, which is a transfer between organisms rather than into or out of the air. Respiration returns carbon dioxide to the air from every living thing, producers and consumers alike. Decomposition is respiration too, carried out by bacteria and fungi on dead organisms and on faeces, and it is the route by which carbon locked into a body gets back into circulation instead of accumulating. The sea both dissolves carbon dioxide from the air and releases it back, and the two happen at rates that depend on temperature: cold water holds more dissolved gas than warm water.

Two of the transfers are slow enough to be worth separating out. Marine organisms build shells and skeletons of calcium carbonate, and when they die those settle, compact and become limestone, which takes carbon out of circulation for many millions of years; weathering of that rock and volcanic activity slowly return it. And where dead organic matter fell into conditions with little or no oxygen, decomposition was slowed and altered rather than stopped, so organic matter accumulated faster than it was broken down and was buried, compressed and heated into fossil fuels. Coal formed mostly from the trees of swamp forests some three hundred million years ago; oil and natural gas formed mostly from marine plankton. Every one of them is carbon that photosynthesis removed from the air and that was buried before decomposition returned it.

Carbon store

A place where carbon is held, such as the atmosphere, living organisms, soil, the oceans, sedimentary rock or fossil fuels.

Carbon transfer

A process that moves carbon from one store to another, such as photosynthesis, respiration, decomposition or combustion.

Fossil fuel

Coal, oil or natural gas, formed over millions of years from the remains of organisms that were buried before decomposers could break them down.

The net transfer added by human activity

The natural transfers above continue. In addition, human activity has greatly accelerated the transfer of geological carbon to the atmosphere. Combustion of fossil fuels returns to the atmosphere, in a couple of centuries, carbon that took hundreds of millions of years to be buried, and the rate at which it is being returned is about ten thousand million tonnes of carbon a year. Deforestation adds to it twice over: burning or rotting the felled trees releases the carbon they held, and the forest that would have gone on fixing carbon is no longer there to do it. Cement manufacture, which drives carbon dioxide out of limestone, adds a further few per cent.

The relative sizes matter. Photosynthesis and respiration move of the order of a hundred and twenty gigatonnes of carbon a year in each direction, and the human addition is of the order of eleven, so the addition is under a tenth of the natural gross flux. Those are recent published estimates and should be quoted with a year and a source, since both are revised. The natural fluxes approximately cancel over the long term; the human addition does not, so the difference accumulates year on year. Land plants and the oceans absorb roughly half of the addition, so about half remains in the atmosphere each year and the concentration rises.

Population size is one of several contributing factors. The human population was about one thousand million in 1800, two thousand million by 1930, and passed eight thousand million in 2022, and energy use per person rose over the same period as well. Population growth has slowed, since the global growth rate peaked in the 1960s and family sizes have fallen almost everywhere, but the population is still rising and the total demand for energy, food and land rises with it. Population is not the whole account, though. Environmental pressure depends on population, consumption per person, technology and land-use choices. All four bear on the pressures treated in this unit and in the biodiversity lesson: land cleared for farming, forests felled for timber, fisheries taken past what they can replace, and now the composition of the atmosphere.

The figures below are approximate annual averages for the 2010s. Published global carbon-budget estimates are revised every year, so quote the year and the source with any flux you use.

What a changing atmosphere does to living things

Carbon dioxide is a greenhouse gas: it is transparent to the short-wavelength radiation arriving from the Sun and absorbs some of the long-wavelength radiation the warmed Earth emits, so raising its concentration raises the mean surface temperature. Methane, released by cattle, rice paddies, landfill and thawing permafrost, does the same job far more strongly per molecule while being much less abundant. The ecological question is not whether the temperature changes but what a change does to organisms whose tolerances evolved under the old one.

Distribution. Every species has a range of conditions it can survive in, so a change in climate moves the places where those conditions are found. In practice ranges have been moving polewards and uphill. British butterflies and dragonflies have extended their northern limits by tens of kilometres over a few decades, tree lines on mountains have crept upwards, and fish stocks in the North Sea have shifted north. Species already at the top of a mountain or the edge of a continent have nowhere to move to, which is why arctic and alpine species are the ones most often listed as at risk.

Timing. Many life cycles are triggered by temperature, and warming brings events forward. Trees come into leaf earlier, insects emerge earlier and birds lay earlier. The problem is that not everything shifts by the same amount, because different species take their cue from different things: a bird that times laying by day length, which has not changed, can find its chicks hatching after the peak of the caterpillars they were meant to eat. That mismatch is one of the clearest measured effects on abundance.

The oceans. About a quarter of the carbon dioxide released dissolves in seawater, where it forms carbonic acid and lowers the pH, a change usually called ocean acidification. It makes building calcium carbonate shells and skeletons harder, which affects corals, molluscs and much of the plankton at the base of marine food chains. Warming does more immediate damage to reefs: above a threshold temperature corals expel the photosynthetic protoctists living in their tissues and bleach, and repeated bleaching kills them. A reef is a habitat for a very large number of other species, so losing it is a loss of species diversity as well as of habitat diversity.

Biodiversity overall. The three effects work together. Species with narrow tolerances, small ranges or poor powers of dispersal lose out; generalists with wide tolerances spread; and communities that took a long time to assemble come apart at different rates. The result is not simply fewer species everywhere but a redistribution, with local diversity rising in some places and falling in others while the number of species on the planet falls. Habitat fragmentation increases the effect, since a species whose range must shift may be unable to cross a city or a motorway, which is one argument for wildlife corridors.