We contribute to this cycle every time we breathe. Using the oxygen we take in from the atmosphere, carbon from our food is burnt and turned into carbon dioxide that we then exhale; in this way we are provided with the energy we need to maintain our life. Animals contribute to atmospheric carbon dioxide in the same way; so do fires, rotting wood and decomposition of organic material in the soil and elsewhere. To offset these processes of respiration whereby carbon is turned into carbon dioxide, there are processes involving photosynthesis in plants and trees which work the opposite way; in the presence of light, they take in carbon dioxide, use the carbon for growth and return the oxygen back to the atmosphere. Both respiration and photosynthesis also occur in the ocean.
Figure 3.1 is a simple diagram of the way carbon cycles between the various reservoirs – the atmosphere, the oceans (including the ocean biota), the soil and the land biota (biota is a word that covers all living things – plants, trees, animals and so on – on land and in the ocean, which make up a whole known as the biosphere). The diagram shows that the movements of carbon (in the form of carbon dioxide) into and out of the atmosphere are quite large; about one-fifth of the total amount in the atmosphere is cycled in and out each year, part with the land biota and part through physical and chemical processes across the ocean surface. The land and ocean reservoirs are much larger than the amount in the atmosphere; small changes in these larger reservoirs could therefore have a large effect on the atmospheric concentration; the release of just two per cent of the carbon stored in the oceans would double the amount of atmospheric carbon dioxide.
It is important to realise that on the timescales with which we are concerned anthropogenic carbon emitted into the atmosphere as carbon dioxide is not destroyed but redistributed among the various carbon reservoirs. Carbon dioxide is therefore different from other greenhouse gases that are destroyed by chemical action in the atmosphere. The carbon reservoirs exchange carbon between themselves on a wide range timescales determined by their respective turnover times – which range from less than a year to decades (for exchange with the top layers of the ocean and the land biosphere) to millennia (for exchange with the deep ocean or long-lived soil pools). These timescales are generally much longer than the average time a particular carbon dioxide molecule spends in the atmosphere, which is only about four years. The large range of turnover times means that the time taken for a perturbation in the atmospheric carbon dioxide concentration to relax back to an equilibrium cannot be described by a single time constant. Although a lifetime of about a hundred years is often quoted for atmospheric carbon dioxide so as to provide some guide, use of a single lifetime can be very misleading.
Before human activities became a significant disturbance, and over periods short compared with geological timescales, the exchanges between the reservoirs were remarkably constant. For several thousand years before the beginning of industrialisation around 1750, a steady balance was maintained, such that the mixing ratio (or mole fraction, for definition see Glossary) of carbon dioxide in the atmosphere as measured from ice cores (see Chapter 4) kept within about ten parts per million of a mean value of about 280 parts per million (ppm).
The Industrial Revolution disturbed this balance and since its beginning in about 1700 approximately 600 thousand million tonnes (or gigatonnes, Gt) of carbon have been emitted into the atmosphere from fossil fuel burning. This has resulted in a concentration of carbon dioxide in the atmosphere that has increased by about thirty per cent, from 280 ppm around 1700 to a value of over 370 ppm at the present day (Figure 3.2(b)). Accurate measurements, which have been made since 1959 from an observatory near the summit of Mauna Loa in Hawaii, show that carbon dioxide is currently increasing on average each year by about 1.5 ppm, although there are large variations from year to year (Figure 3.2(c)). This increase spread through the atmosphere adds about 3.3 Gt to the atmospheric carbon reservoir each year.
Carbonade Flamande or Carbonnade
