Isotopes are chemically identical forms of the same element but with different atomic weights. Three isotopes of carbon are important in studies of the carbon cycle: the most abundant isotope 12C which makes up 98.9% of ordinary carbon, 13C present at about 1.1% and the radioactive isotope 14C which is present only in very small quantities. About 10 kg of 14C is produced in the atmosphere each year by the action of particle radiation from the Sun; half of this will decay into nitrogen over a period of 5730 years (the 'half-life' of 14C).
When carbon in carbon dioxide is taken up by plants and other living things, less 13C is taken up in proportion than 12C. Fossil fuel such as coal and oil was originally living matter so also contains less 13C (by about 18 parts per 1000) than the carbon dioxide in ordinary air in the atmosphere today. Adding carbon to the atmosphere from burning forests, decaying vegetation or fossil fuel will therefore tend to reduce the proportion of 13C.
Because fossil fuel has been stored in the Earth for much longer than 5730 years (the half-life of 14C), it contains no 14C at all. Therefore, carbon from fossil fuel added to the atmosphere reduces the proportion of 14C the atmosphere contains.
By studying the ratio of the different isotopes of carbon in the atmosphere, in the oceans, in gas trapped in ice cores and in tree rings, it is possible to find out where the additional carbon dioxide in the atmosphere has come from and also what amount has been transferred to the ocean. For instance, it has been possible to estimate for different times how much carbon dioxide has entered the atmosphere from the burning or decay of forests and other vegetation and how much from fossil fuels.
Similar isotopic measurements on the carbon in atmospheric methane provide information about how much methane from fossil fuel sources has entered the atmosphere at different times.
Further information regarding the broad partitioning of added atmospheric carbon dioxide between the atmosphere, the oceans and the land biota as presented in Table 3.1 comes from comparing the trends in atmospheric carbon dioxide concentration with the trends in very accurate measurements of the atmospheric oxygen/nitrogen ratio (Figures 3.3 and 3.4). This possibility arises because the relation between the exchanges of carbon dioxide and oxygen with the atmosphere over land is different from that over the ocean. On land, living organisms through photosynthesis take in carbon dioxide from the atmosphere and build up carbohydrates, returning the oxygen to the atmosphere. In the process of respiration they also take in oxygen from the atmosphere and convert it to carbon dioxide. In the ocean, by contrast, carbon dioxide taken from the atmosphere is dissolved, both the carbon and the oxygen in the molecules being removed. How such measurements can be interpreted for the period 1990-4 is shown in Figure 3.4. These data are consistent with budget for the 1990s shown in Table 3.1.
Figure 3.4 Partitioning of fossil fuel carbon dioxide uptake using oxygen measurements. Shown is the relationship between changes in carbon dioxide and oxygen concentrations. Observations are shown by solid circles. The arrow labelled 'fossil fuel burning' denotes the effect of the combustion of fossil fuels based on the O2 : CO2 stoichiometric relation of the different fuel types. Uptake by land and ocean is constrained by the stoichiometric ratio associated with these processes, defining the slopes of the respective arrows.
Fossil fuel burning
Fossil fuel burning
360 365 370 375
CO2 concentration (ppm)
360 365 370 375
CO2 concentration (ppm)
The global land-atmosphere flux in Table 3.1 represents the balance of a net flux due to land-use changes which has generally been positive or a source of carbon to the atmosphere and a residual component that is, by inference, a negative flux or carbon sink. The estimates of land-use changes (Table 3.1) are dominated by deforestation in tropical regions although some uptake of carbon has occurred through forest regrowth in temperate regions of the northern hemisphere and other changes in land management. The main processes that contribute to the residual carbon sink are believed to be the carbon dioxide 'fertilisation' effect (increased carbon dioxide in the atmosphere leads to increased growth in some plants - see box in Chapter 7 on page 199), the effects of increased use of nitrogen fertilisers and of some changes in climate. The magnitudes of these contributions (Table 3.1) are difficult to estimate directly and are subject to much more uncertainty than their total, which can be inferred from the requirement to balance the overall carbon cycle budget.
A clue to the uptake of carbon by the land biosphere is provided from observations of the atmospheric concentration of carbon dioxide which, each year, show a regular cycle; the seasonal variation, for instance, at the observatory site at Mauna Loa in Hawaii approaches about 10 ppm (Figure 3.3). Carbon dioxide is removed from the atmosphere during the growing season and is returned as the vegetation dies away in the winter. In the northern hemisphere therefore a minimum in the annual cycle of carbon dioxide occurs in the northern summer. Since there is a larger amount of the terrestrial biosphere in the northern hemisphere the annual cycle there has a much greater amplitude than in the southern hemisphere. Estimates from carbon cycle models of the uptake by the land biosphere are constrained by these observations of the seasonal cycle and the difference between the hemispheres.5
The carbon dioxide fertilisation effect is an example of a biological feedback process. It is a negative feedback because, as carbon dioxide increases, it tends to increase the take-up of carbon dioxide by plants and therefore reduce the amount in the atmosphere, decreasing the rate of global warming. Positive feedback processes, which would tend to accelerate the rate of global warming, also exist; in fact there are more potentially positive processes than negative ones (see box on page 48-9). Although scientific knowledge cannot yet put precise figures on them, there are strong indications that some of the positive feedbacks could be large, especially if carbon dioxide were to continue to increase, with its associated global warming, through the twenty-first century into the twenty-second. These feedbacks are often called climate/carbon-cycle feedbacks as they all result from changes in the climate affecting the performance of the carbon cycle. In later chapters, it will be seen that these feedbacks can assert large influence on future concentrations of carbon dioxide and hence on future climate.
Carbon dioxide provides the largest single contribution to anthropogenic radiative forcing. Its radiative forcing from pre-industrial times to the present is shown in Figure 3.11. A useful formula for the radiative forcing R from atmospheric carbon dioxide when its atmospheric concentration is C ppm is: R = 5.3 ln(C/C0) where C0 is its pre-industrial concentration of 280 ppm.
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