Carbon is a vital element, the source of life itself, constantly moving between living things, the atmosphere, the oceans and rocks. We have already disturbed this natural cycle; we will have to find ways to redress the balance

Figure 1: A variety of processes, driven by nature and human needs, move carbon around the world between the atmosphere, plants, animals, the oceans and rock

CARBON makes up less than 1 per cent of our planet, but it is the key element for life on Earth. Plants, animals, microorganisms, our food and our bodies are all based on compounds of this versatile element - and carbon compounds in the atmosphere made the planet warm enough for life to evolve. Today, carbon-based fossil fuels provide three-quarters of our energy needs, but this may change in future. Scientists are concerned that our dependence on fossil fuels is distorting the natural movement of carbon through the world's ecosystems - known as the carbon cycle. The "common currency" of the carbon cycle is the gas carbon dioxide. Although it makes up only 0.03 per cent of the atmosphere, this gas is the source of most carbon for living things as well as the product of burning fuel or decomposing organic matter and its concentration in the atmosphere has increased rapidly due to recent human activity.

The idea of a global carbon cycle was first proposed by the German chemist Justus von Liebig in 1840. During the 19th century, scientists such as the great French mathematician Jean-Baptiste Fourier and the British physicist John Tyndall contributed greatly to our knowledge of how carbon dioxide keeps the Earth warm. The Swedish chemist Svante Arrhenius predicted today's concern about the greenhouse effect as long ago as 1896.

The carbon cycle can be divided into three parts, each consisting of carbon pools, that store carbon for different residence times. The biological part of the cycle has the shortest storage time. Plants, the atmosphere, and the surface layers of the oceans each contain approximately equal amounts of carbon - between about 500 and 700 billion tonnes. Animals, ourselves included, hold far less - between 1 and 2 billion tonnes. As organisms grow, die and decompose, over days, years or even hundreds of years, carbon moves between these pools. Pools are known as sinks or sources, depending on whether they are net "uptakers" or "providers" of carbon. But in spite of these large and fairly rapid movements of carbon, on a global scale the system is in dynamic equilibrium, which is to say that the pools remain about the same size.

Figure 2: Where the carbon is: The arrows show how much of it moves between these pools in a year. The biggest pool, the rocks, holds 75 million billion tonnes

The geochemical part of the carbon cycle stores carbon for far longer. There are two main pools: the deep ocean water, with an estimated 36 000 billion tonnes, and rocks, especially limestone, which hold an estimated 90 million billion tonnes. Carbon moves to other parts of the cycle very gradually when deep ocean currents surface again after hundreds or thousands of years. Volcanic eruptions, weathering of rocks and burning fossil fuels such as oil, coal and gas all release carbon to the biological cycle, usually by way of the atmosphere, after many millions of years underground.

Intermediate between biological and geochemical carbon in terms of its "cycling time" is the carbon in soil. Worldwide, this amounts to about 1600 billion tonnes, between two and three times as much as is held in plants. Soil carbon is fairly stable and does not normally interchange very quickly with the other pools.

Plants are the major force behind the global carbon cycle. Through photosynthesis, they convert carbon dioxide into their stems, trunks, leaves and roots. By fixing carbon in this way, the plants themselves grow, and carbon then enters the food chains as animals eat the plants. About twice as much carbon dioxide is taken up by photosynthesis on land as in the oceans and rivers. Land plants contain 250 times as much carbon as aquatic plants, most of it in the form of trees.

Both plants and blue-green algae (cyano-bacteria) are able to make their own organic food by photosynthesis, converting solar energy into chemical energy in carbohydrate molecules. Organisms that can make their own food are known as autotrophs; most use photosynthesis, but a few live on sulphur or nitrogen compounds. All other organisms rely on finding their food ready-made, as sugars or other carbon compounds: these organisms are termed heterotrophs. However they feed, once plants and animals die, the carbon they have stored is normally released back into the atmosphere by decomposition, by another group of heterotrophs.

Carbon turnover

Fluxes and flows

CARBON is constantly moving from one pool to another. On land, carbon dioxide from the atmosphere is held by plants and animals. In the oceans, only a small proportion of the carbon is held in living organisms. Most of it is in the debris left behind by these creatures, as well as some dissolved carbon dioxide, bicarbonate, in the main. Plankton (both plant and animal), coral and other creatures grow shells and skeletons of the mineral calcite, calcium carbonate. They convert the carbon dioxide dissolved in the oceans into carbonate (CO₃^2-). When they die some of their shells and skeletons accumulate on the ocean floor, eventually becoming chalk or limestone. This removes carbon from the seas steadily over millions of years, until the processes of mountain formation and erosion expose the rocks to wind and weather on land.

Other rocks also lock away carbon, notably those that produce fossil fuels. This carbon too originates in living things - plants provide the carbon for coal, whereas oil and gas come mainly from marine animals.

The flow of carbon is not just into the geochemical part of the cycle. Weathering, the slow breakdown of rocks by organic acids in soil, results in a release of carbon dioxide to the atmosphere from carbonates. On a longer timescale, carbon comes into the atmosphere from rocks, for example when volcanoes erupt.

Over the past 150 years, since the Industrial Revolution in the West, humans have added carbon to the atmosphere from two main sources. First, by burning fossil fuels, and, secondly, by changes in land use, especially the destruction of forests. The rate of carbon dioxide emission from both these sources has increased dramatically since 1945 - a man-made perturbation on a colossal scale. The growth in the demand for energy in both developed and developing countries has had the most effect.

Carbon dioxide levels in the atmosphere in the past 30,340 and 150 000 years

We know how much carbon is released from the burning of fossil fuel because the coal, oil and gas industries keep records: almost 6 billion tonnes of carbon goes up in smoke each year, and the total is increasing. Because oceans and plants take in carbon naturally, the increase in carbon in the atmosphere is in fact only about half the value predicted from these figures. But we need to understand much more about the role of the world's many ecosystems in the global carbon cycle, as well as the effects of human disturbance. Scientists remain uncertain about the turnover of carbon worldwide, especially the contribution from changes to forests, grasslands and agriculture. When farmers convert forest to cropland, the carbon in the vegetation is oxidised through burning and decomposition, adding carbon dioxide to the atmosphere. Carbon in organic matter in soil may also be oxidised and lost to the atmosphere as cultivation continues, especially if the soil erodes at the same time. The effect is reversed when trees and forest grow again on abandoned land, but no one is sure how much regeneration is going on.

Burning vegetation plays an important part in the global carbon cycle, although fire often merely speeds up the release of carbon that would otherwise be produced later by decomposition. Many people in developing countries use wood or agricultural waste as fuel and both forests and grasslands are burned for farming. Vast areas of grasslands are cleared and burned annually, especially in the tropics, giving a large seasonal flow of carbon to the atmosphere. Some scientists claim that it is as large as the amount of carbon released every year by felling and burning tropical forests. But unlike forests, grasslands can recover quickly provided they are carefully managed.

If we carry on burning fossil fuel and destroying vegetation on the Earth's surface at current rates, the carbon dioxide in the atmosphere could reach three times today's level by the year 2100. Even if politicians agree to act, the concentration of carbon dioxide may still double. It will take major changes in the use of energy worldwide and in land management to keep the carbon in the atmosphere at a steady and acceptable level. Every kilowatt-hour of electricity used in Britain today releases one kilogram of carbon dioxide, and each litre of petrol and diesel burned by private and public transport adds another 2.5 kilograms. And every hectare of tropical forest burned gives between 350 and 700 tonnes of carbon dioxide.

Carbon in the atmosphere

The greenhouse effect

ENVIRONMENTAL scientists have been measuring the amount of carbon dioxide in the atmosphere since 1958 at the Mauna Loa observatory in Hawaii. They chose this site because it is remote from the major sources of carbon dioxide - people and industry - so that the records present a fairly "true" picture of global changes in atmospheric carbon dioxide. More recently, Mauna Loa has joined a network of monitoring stations which record changes in carbon dioxide along the entire length of the Pacific Ocean, from Alaska to the South Pole. Superimposed on the gradual increase in carbon dioxide is a seasonal "heartbeat"; during each year photosynthesis and respiration shuttle carbon between the atmosphere and vegetation in the land masses, which are predominately in the Northern Hemisphere.

Scientists have gained information about the world's carbon dioxide from further back in time from the bubbles of air trapped deep in the ice of Antarctica. This archaeological air, hundreds of thousands of years old, is an invaluable guide to the Earth's atmospheric and climatic history. By measuring the gases in these tiny bubbles, scientists can calculate how our atmospheric carbon dioxide has varied, in and out of Ice Ages, since before the dawn of human civilisation.

Human activities are "short-circuiting" the carbon cycle in two main ways. By using fossil fuels, we are speeding up the return to the atmosphere of carbon that would otherwise stay locked up in rocks for millions of years. And by destroying forests we are releasing carbon dioxide into the atmosphere much faster than would happen naturally by decomposition. Scientists and politicians are now concerned about the influence of this extra carbon dioxide on the Earth's climate through the greenhouse effect (see Inside Science Number 13).

Much of the concern stems from uncertainty about the complex interaction between living things and the environment. Horticulturists have long known that they can "force" tomatoes to grow faster in glasshouses by increasing the levels of carbon dioxide there. Might not extra carbon dioxide in the atmosphere work like a fertiliser for many plants, increasing their rate of growth worldwide? If so, this might have a balancing effect on emissions of carbon dioxide by storing some of the extra carbon as wood. But this fertilisation can only happen when plants have good supplies of water and nutrients as well, and predictions suggest that in a greenhouse world rainfall will be lower in many regions important for food production, such as the grain belt of the US.

As yet, our understanding of many ecosystems is simply not good enough to say for certain whether plants will benefit from extra carbon dioxide in the atmosphere. Overall, the balance of opinion among scientists is that the greenhouse effect and global warming will be bad for vegetation worldwide (see Inside Science Number 21), either by making large areas of land arid, or through changes in climate too rapid for plants to adapt to. Greenhouse warming could even disturb the large and fairly stable pools of carbon in soil organic matter. If parts of the Earth's surface become warmer, the bacteria that convert dead organic matter in the soil into carbon dioxide could become much more active.

Whiffs of methane

Cows and coal mines

CARBON DIOXIDE is not the only gas in the global carbon cycle. Methane is also important, but until recently scientists knew much less about it than carbon dioxide. Although there is very little methane in the atmosphere in terms of the carbon it contains, it is increasing faster than carbon dioxide, and each molecule of methane has the same greenhouse effect as 11 molecules of carbon dioxide. Methane comes from agriculture, produced especially in rice paddies, and by cattle. It also leaks from pipelines, coal mines and rubbish tips. Bacteria in the soil are another source, although some of them can also break down methane. Controlling emissions of methane and the other greenhouse gases may also form a part of our response to the greenhouse effect, but the most immediate problem concerns the level of carbon dioxide in the atmosphere.

Carbon budgeting

Preparing for the future

THE Intergovernmental Panel on Climate Change has already reported on the likely consequences of the greenhouse effect and must now agree on proposals for international policy measures. The United Nations Conference on Environment and Development, held in 1992 in Rio de Janeiro, Brazil, prepared the way for international agreement on controlling the carbon cycle with a Climate Convention which by 1994 had been signed by more than 50 countries. Each country has agreed to limit its emissions of carbon dioxide by the year 2000.

Some politicians and economists propose internationally agreed taxes on carbon dioxide emissions as a way of controlling the greenhouse effect. The carbon polluters must be made to pay for their misdeeds. If carbon taxes are introduced, it will be essential to know whether, on balance, countries or regions are producing or absorbing carbon dioxide. National data on the use of fossil fuel should be combined with information on land management and the state of natural ecosystems, to estimate "carbon budgets".

But these efforts may well be hampered by the shortage of good data, particularly in developing countries in the tropics, where the information on carbon flows is very imprecise. Scientists have only crude estimates, if any, of the amount of carbon in the wood of each country's forests; they know even less about the carbon flows in the extensive areas of tropical grasslands. New techniques such as remote sensing of the Earth's vegetation using satellites, together with computer modelling of the carbon flows in natural ecosystems, are essential for serious future negotiations over national and international responses to global warming. Methods suggested include using less fossil fuel, and being more efficient with that we do use, stopping or slowing the destruction of forests and encouraging reforestation and revegetation in general.

In the short term, the simplest action that we could take is to use our energy more efficiently, so that we use less fossil fuel to produce the same amount of energy services, such as electrical power, heating and transport. Less straightforward, but important, is that we should stop or limit the destruction of forests and other types of vegetation, and encourage the re- planting and rehabilitation of degraded forests and grasslands.

To take up all the carbon from fossil fuel burning worldwide we would need to plant trees over an area of at least half the size of Australia. Although this sounds enormous, it is about equal to the area of degraded or abandoned land worldwide. And there are many other good reasons to plant trees, for example to protect against floods and erosion, and to provide fuel, so such measures should be seen as a low-risk insurance policy.

A futuristic way to tackle the global carbon problem in the future could be to establish ponds or reactors known as photo-bioreactors next to power stations. Here single-celled algae would grow and take in carbon dioxide directly from the exhaust gases. But to stop them decomposing and releasing the carbon dioxide, the algae would need to be stored permanently or converted into stable useful products such as plastics. They could even be reused as fuel.

Other technological solutions focus on converting carbon dioxide in the atmosphere into other forms. Energy from the Sun could be harnessed to convert carbon dioxide directly into useful substances such as petrochemicals. Throughout the world the fossil fuel industries are investigating the possibilities of storing compressed carbon dioxide in disused oil and gas wells.

Most of these technology-intensive proposals would require much less land than planting trees, but they may not be practical or economic. Some would simply buy time, storing carbon for perhaps 50 years while we search for a lasting solution.

The answer may be growing all around us. Three-quarters of the world's population already rely on burning plant biomass for much of their energy supply. Although this is not always the case, plants can be grown in a sustainable manner, so that productivity is maintained over many years without damaging the soil structure or depleting its nutrients. Under these conditions, plants take up the same amount of carbon from the atmosphere as they release when later burned.

Replacing fossil fuels by biomass energy may be the most cost-effective response to the greenhouse effect. Modern biomass fuels that are in use already include substitutes for liquid fuels such as petrol or diesel, and the wood or agricultural wastes that many small power stations now use. Advanced biomass technologies could eventually take over from coal and oil as a major energy source all over the world, without distorting the natural flows of the carbon cycle.

2: Products and processes of photosynthesis ALL LIFE requires energy for growth and maintenance; carbon compounds form the energy stores and much of the structural material of life. Through photosynthesis, energy-poor compounds such as carbon dioxide and water are converted to energy-rich carbohydrates and oxygen, and thus solar energy is converted into chemical energy. The photosynthetic process is very similar in all green plants and blue- green algae. Some method of using the Sun's energy must have evolved in the primitive life forms on our planet more than 3 billion years ago. Figure 4: Carbon cycle at local level Photosynthesis is a two stage process: 1. H2O ----- light ----> H+ + O2 + ATP + NADPH [reductant]. 2. CO2 + NADPH + ATP ----> (CH2O) [carbohydrate] The first stage needs energy from sunlight, which splits water molecules, leading to a series of reactions that generate the universal energy compound ATP (adenosine triphosphate) and the reductant NADPH, the reduced form of the compound NADP (nicotinamide adenine dinucleotide phosphate). These two compounds are necessary for the second stage. Plants release oxygen as a by- product of the splitting of water. In the first stage, the green pigment chlorophyll absorbs energy from incoming light and passes it along a chain of carrier molecules embedded in a membrane. The first step in this chain is the splitting of water molecules. Electrons continue to move between carrier molecules within the membrane, transferring energy which eventually drives two important reactions. One is essentially a reduction reaction, producing NADPH from NADP by supplying electrons and hydrogen ions. The other reaction, driven by a pH gradient across the membrane, produces ATP. The second stage of photosynthesis needs both NADPH and ATP in order to reduce carbon dioxide to carbohydrates. This stage does not depend on light, but it uses a series of enzymes. These enzymes form a cycle in which they take up carbon dioxide, make carbon compounds for use elsewhere in the plant and supply the materials needed for the cycle to restart. Overall, photosynthesis is precisely controlled by the intensity of light - affecting mainly the first stage - and the demand for carbon compounds in the plant - in the second stage. Although the second stage of photosynthesis does not need light, the activity of some of the enzymes in the cycle is regulated by light. All our fossil fuels - coal, oil and gas - originated through photosynthesis millions of years ago. Although total fossil fuel resources represent a large pool of carbon, the amount is equivalent to only about 60 years of carbon storage through photosynthesis worldwide. Every year, plants take up one-sixth the total carbon in the atmosphere, or 20 times as much carbon as is released by the burning of fossil fuels. But most of this carbon is later returned to the atmosphere through respiration and breakdown of organic matter (plants, animals and soil).

Further reading

For an up-to-date account of the role of the carbon cycle in climate change, and recommendations for global action, the definitive studies are Climate Change: the 1990 IPCC Scientific Assessment and Climate Change 1992: the Supplementary Report (both edited by J.T. Houghton et al. and published by Cambridge University Press). Basic details of carbon flows within ecosystems are covered well in school textbooks such as Advanced Biology by C.J. Clegg and D.G. McKean (John Murray, 1994) and university-level texts such as Ecology by R.E. Ricklefs (Freeman, 1990). R.A. Houghton wrote an accessible review article in 1990 in Ambio, volume 19(4), pp. 204-209, entitled "The future role of tropical forests in affecting the CO₂ concentration of the atmosphere". More advanced reviews are written by E.T. Sundquist (1993) in Science, volume 259, pp. 934-941 "The global carbon dioxide budget", and edited by T.M.L. Wigley, The Carbon Cycle (Cambridge University Press, 1994). Photosynthesis by D.O. Hall and K.K. Rao (4th edition, Cambridge University Press, 1994) is also recommended reading.

Jonathan Scurlock and David Hall