Carbon, an essential element, occurs in organic substances such as carbohydrates, proteins, fats and nucleic acids such as deoxyribonucleic acid (DNA). Most carbon on Earth was originally released from the interior of the Earth as gaseous carbon dioxide, which now constitutes about 0.03-0.04 per cent by volume of the air. This gas helps insulate the Earth, retaining enough of the Sun’s heat to keep the planet warm and sustain life.
Carbon dioxide is used in photosynthesis by plants to make carbohydrates. This locked carbon is transferred to animals that eat the plants. During respiration, all organisms break down food releasing energy and carbon dioxide, which is released back into the air every time an animal exhales. Some of the carbon dioxide from respiration is used by plants in photosynthesis, while some of the oxygen made in photosynthesis can be used in respiration. For part of the day, the rate of photosynthesis exceeds the rate of respiration leading to an excess of oxygen. During night, photosynthesis stops and respiration continues so that a balance between carbon dioxide and oxygen production is achieved. Many plants in middle latitudes remain dormant through the cold winter, photosynthesis slows, but animal and plant respiration continues – thus increasing atmospheric concentration of carbon dioxide. It decreases again in spring as plant growth resumes.
When organisms die, their bodies are broken down by decomposers, which convert the carbon in their cells into carbon dioxide. Sometimes when the dead matter is buried beneath mud or acidic water, it prevents further decomposition and over millions of years the material gets converted into fossil fuels – coal, gas and oil. Carbon in this form remains locked up in the fuel deposits as hydrocarbons and till recent times did not take part in the carbon cycle. With the invention of modern devices such as the internal-combustion engine that used these fuels, combustion caused the carbon to oxidise (react with oxygen from the atmosphere) and emit carbon dioxide gas. Carbon dioxide released by forest and grass fires is removed again as vegetation recovers.
The Earth’s oceans play an important role in the carbon cycle. Like forests, they are huge carbon ‘sinks’ because their surface waters absorb vast amounts of carbon dioxide from the atmosphere. The carbon is used in photosynthesis by aquatic plants or it may become incorporated into the shells of marine organisms as carbonate compounds. Decomposition and respiration also occur in the ocean, returning carbon dioxide to the atmosphere. Algae are as crucial to these processes as are green plants on land.
Carbon is a natural ingredient of the air we breathe. It enters the air principally as carbon dioxide (CO2) through the carbon cycle. Each year, volcanic eruptions release about 110,000 tons of carbon into the atmosphere. This is the only source of ‘new’ carbon dioxide and it is not enough to unbalance the carbon cycle. In addition, a small amount enters the air as carbon monoxide (CO) from the action of certain bacteria on carbon and as methane (CH4). Carbon monoxide is quickly oxidised to carbon dioxide. Methane is also oxidised, but more slowly than carbon dioxide and water. At any time, the atmosphere contains about 803 billion tons of carbon, almost all of it as carbon dioxide. Much of the extra carbon released today dissolves into the ocean or is absorbed by green plants, but in some parts of the world forests are being destroyed to provide farmland and by commercial logging. As huge tracts are cleared, yet more carbon dioxide is released by burning or by decaying trees.
Methane is another source of atmospheric carbon. It is a natural constituent of the air, present in very small amounts. At present the methane concentration is about 1.8 parts per billion (p.p.b.) of air, equal to about 0.000002 per cent. It is also one of the gases that we release into the air. Rice paddies are the principal source, followed by cattle and sheep. In both cases the methane is produced by bacteria in the mud of rice paddies and in the digestive system of farm animals. Leaking gas pipes, landfill sites, and burning of plant wastes also contribute. Both carbon dioxide and methane are so-called ‘greenhouse gases’. Greenhouse gases trap solar energy in the atmosphere, warming the planet. Without them, the Earth would be 30°C colder.
If the amount of greenhouse gases in the atmosphere increases, scientists believe the radiation they absorb will warm the air, producing a general rise in average temperatures – global warming. Average temperatures have risen over the last century and it is possible that the increasing concentration of greenhouse gases has contributed to this warming. Rising levels of carbon dioxide may affect the world’s climate by causing weather patterns to change. Climate changes have occurred many times in Earth’s history, and sometimes very rapidly, but plants may have trouble adjusting to the present rate of change. It has been estimated that plants need to move 90 kilometres towards the poles for every 1°C rise in temperature. Trees expand into new areas at a rate of 4 to 200 kilometres per century, so it could take them more than a century to respond to a warming of 4°C – even assuming there were no barriers to prevent them colonising new land. Middle latitudes might experience longer growing seasons and higher crop productivity, but if the rate of evaporation were to rise faster than precipitation, the ground could become drier, with more droughts. Some deserts would shrink, however, including the Sahara. Major crop growing areas would shift to cooler areas, where the soil might not be suitable for the new crops. Extreme weather would become more common and widespread; storms would be much more powerful.
Inputs from Ecology and Environment by Sally Morgan and Mike Allaby.