2.1 The first carbon trail, the carbon cycle

Carbon dioxide, CO₂, is produced and released into the atmosphere when we burn fuels, such as wood harvested from a forest, or coal and other fossil fuels extracted from the ground. Note that the fossil fuels, i.e. coal, oil and natural gas, are derived from fossilised organic matter (once living organisms).

CO₂ is also produced by almost all living things, both animals and plants, at all times of the day and night, in the process called respiration. In our own bodies we tend to equate respiration with the physical process of breathing, the movement of air in and out of our lungs. Within the lungs, oxygen from the air is taken in, and some carbon dioxide is released from the body when we breathe out. When the oxygen reaches the cells of our bodies (and also the cells of other animals and plants), chemical reactions break down some of the complex carbon compounds such as carbohydrates that are stored there. This releases energy for our muscles to use. Oxygen is used in the process, and water and carbon dioxide are produced.

What happens to the carbon dioxide? Plants and certain types of bacteria remove carbon dioxide from the oceans and atmosphere in a process called photosynthesis. This is the process whereby green plants, and a few other organisms, trap energy from sunlight and use carbon dioxide and water to make carbohydrates and oxygen. The carbohydrates are used to store energy for use in the growth and maintenance of the plant, and the oxygen is released to the atmosphere where it can be used again by plants and animals for respiration. The two processes of respiration and photosynthesis underpin the cycling of carbon, on which most life on Earth depends.

Figure 4 shows the fundamentals of the carbon cycle, but in this diagrammatic form it reveals little about the component parts of the cycle and the links between them.

Figure 5 gives a little more detail. Animals are represented by a giraffe, and plants are shown by a tree and some grass. The basic carbon cycle in Figure 4 can be traced in Figure 5 by arrows showing the movement of carbon dioxide. The respiration arrows from the tree and the giraffe to the air indicate production of CO₂, and the photosynthesis arrow shows CO₂ absorbed from the air by the tree. Both plants and animals contain carbon in their tissues in the form of carbohydrates and other carbon-based compounds known collectively as organic compounds. In Figure 5, the arrow from the tree to the giraffe makes the point that animals eat plants and there is a transfer of carbon in the process.

So far, only the biological processes above the ground have been considered. Figure 5 also includes the vital parts of the cycle below the ground surface. While alive, both plants and animals produce waste products, for example in the form of leaf litter from trees and excreta from animals. These wastes and the remains of the dead plants and animals are broken down by micro-organisms living on and in the soil into simpler organic materials. In the process, these micro-organisms also respire and add carbon dioxide to the air. Any remaining organic matter will simply be incorporated into the soil. Figure 6 represents the global carbon cycle and has been broadened to include interactions involving the oceans, aspects of the Earth’s geology, and human activity. The discussion below this figure considers each of these components in turn.

In the oceans, phytoplankton (microscopic floating plants) play the same role as land plants in Figure 5. Carbon dioxide from the air dissolves readily in the surface waters and is used by phytoplankton in photosynthesis. However, as with green plants on the land, phytoplankton respire, so most of the carbon dioxide is returned to the atmosphere again from the surface of the oceans. Arrows show the two-way flows of carbon dioxide between the atmosphere and the oceans. Most of this activity takes place in the top few hundred metres of the oceans, the upper layer through which sunlight can penetrate and that is stirred by waves. Phytoplankton are eaten by zooplankton (microscopic floating animals), and both are eaten in turn by other marine animals. Many aspects of the carbon cycle shown for plants and animals on land are replicated in the ocean, although the details are not shown here. In addition, however, there is a slow trickle of carbon to the bottom of the oceans in the form of the dead remains of plankton and other marine organisms. Some of it remains in the ocean sediments, which eventually form new rock. Thus it is effectively removed from the main carbon cycle and enters what could be called the geological carbon cycle.

In Figure 6, two arrows point to geological mechanisms that remove carbon dioxide from the atmosphere. They indicate the slow processes of rock formation beneath the continents and the oceans, by which organic carbon is incorporated into rock strata, some in the form of coal, oil or gas, or mineral carbon from marine shells that forms rocks such as limestone and chalk (which are composed mainly of calcium carbonate). Other arrows show the return of this buried carbon as carbon dioxide to the atmosphere, through volcanic eruptions and weathering of rocks by erosion.

Two components of the global carbon cycle have now been identified: the biological carbon cycle, involving life on land and in the sea, that continually exchanges carbon with the atmosphere, and a geological component, that incorporates carbon in rocks before it is released to the atmosphere.

Where is most of the carbon on the planet? It lies buried in rock formations as limestone or chalk, or as organic deposits. Only 1 part in 1000 is found in all the other carbon stores, and 90% of that is dissolved in the deep oceans of the world where it does not mix readily with the upper layers. But geological mechanisms function very slowly and it normally takes millions or even hundreds of millions of years for carbon buried as rock to emerge into the air again. Even the carbon in the deep ocean is very slow to mix with the upper ocean, and it may be a thousand years before it reaches the surface again. For this reason, although these stores are massive, the amount of carbon they exchange with the air each year is tiny compared with the non-geological component.

Most of the movement of carbon takes place between four main stores: the atmosphere, living animals and plants, soils, and the upper ocean. Carbon from these four stores is cycled around continually by the natural processes explained above. If the natural processes were left largely undisturbed, the concentration of carbon in each of the stores would remain relatively constant.

However, human activities have been producing large quantities of carbon dioxide. Figure 6 shows two of them: the burning of fossil fuels and the use of fire to clear forests and grasslands.