The global budget of natural and anthropogenic carbon dioxide

We now synthesize much of the knowledge outlined in previous sections on the global budget of CO2. Firstly, the relative sizes of the natural reservoirs are considered and then the natural flows between them, followed by how anthropogenic CO2 partitions between the boxes. Finally, likely future levels of atmospheric CO2 are discussed in terms of possible scenarios of fossil fuel consumption.

Reservoir sizes

A simplified version of the carbon cycle is given in Fig. 7.9. By far the largest reservoir is in marine sediments and sedimentary materials on land (20000000 GtC), mainly in the form of CaCO3. However, most of this material is not in contact with the atmosphere and cycles through the solid Earth on geological timescales (see Section 4.1). It therefore plays only a minor role in the short-term cycle of carbon considered here. The next largest reservoir is seawater (about 39 000 GtC), where the carbon is mainly in the dissolved form as HCO- and CO2-. However, the deeper parts of the oceans, which contain most of the carbon (38 100 GtC), do not interact with the atmosphere at all rapidly, as discussed in

Fig. 7.7 Latitudinal change in CO2 emissions from 1980 to 1989 as seen in 5° latitude bands. After Andres et al. (2000). With kind permission of Cambridge University Press.
Year

Fig. 7.8 Fossil-fuel emissions and the rate of increase of CO2 concentrations in the atmosphere. Vertical arrows define El Niño events (see text for discussion). Data from IPCC (2001). With permission of the Intergovernmental Panel on Climate Change.

Atmosphere

749 (354 ppm CO2)

Fossil fuels and shales

12000

(7500 recoverable)

2000 Land biosphere

1020 Oceans 38100

Key:

Reservoirs Natural fluxes Anthropogenic fluxes

Sediments 20000000

Fig. 7.9 A simplified version of the global carbon cycle for the 1980s. The numbers in boxes indicate the reservoir size in GtC. Arrows represent fluxes and the associated numbers indicate the magnitude of the flux in GtC yr-1. After IPCC (2001). With permission of the Intergovernmental Panel on Climate Change.

Section 7.2.2. The reservoir of carbon in fossil fuels and mudrocks is also substantial and a major portion of the latter is thought to be recoverable and thus available for burning. The smallest reservoirs are the land biosphere (2000 GtC) and the atmosphere (749 GtC, equivalent to an atmospheric concentration of about 354 ppm). It is the small size of the latter which makes it sensitive to even small percentage changes in the other larger reservoirs, where these changes result in emissions to the atmosphere, as, for example, in the burning of fossil fuels.

Natural fluxes

It is often assumed that natural flows between the major reservoirs are balanced two-way fluxes when averaged over the whole year and the total surface of the reservoir. For example, the land biosphere and the oceans exchange approximately 120 and 90GtCyr-1 respectively in both directions with the atmosphere. There is, however, some uncertainty about this assumption over periods of years and it is surely wrong on longer timescales. Evidence for short-term imbalance comes from careful inspection of the atmospheric record (as shown in Fig. 7.8). At the end of the record (in the early 1990s) the rate of increase of atmospheric CO2 is significantly smaller than for previous years. The explanation for this decrease in the rate of change is very unlikely to be alterations in anthropogenic inputs to the atmosphere, since there is no evidence that fossil fuel burning or land clearance has appreciably altered compared with earlier years. The cause of this decrease in the rate of change appears to be small alterations in the natural fluxes between the atmosphere and land surfaces and the oceans. The large two-way fluxes between these latter reservoirs and the atmosphere mean that only a small imbalance between the up and down fluxes is enough to lead to an observable change in the atmospheric CO2 concentration. The reasons for such imbalances are largely unknown at present, although possible relationships with, for example, El Niño events (as indicated in Fig. 7.8) are the subject of considerable research effort. El Niño events occur every few years in the tropical Pacific ocean when abnormally warm water near the surface in the eastern part leads to alterations in water circulation and heat exchange with the atmosphere. These cause disruption to fisheries along the coast of Ecuador and Peru, and the altered atmospheric circulation leads to changes in climate throughout the Pacific region and in many other parts of the world.

On the timescale of thousands of years it is clear that changes in the land and ocean reservoirs have led to imbalances in their CO2 fluxes with the atmosphere. The best evidence for this comes from ice cores and the record of atmospheric composition preserved in them. Figure 7.10 shows how atmospheric CO2 concentrations and earth-surface temperatures have changed over the last 420000 years, as recorded in the Vostok ice core from Antarctica. There have clearly been dramatic changes in atmospheric CO2 levels over this period and the most likely explanation for these shifts is that they arise from (temporary) imbalances between the inter-reservoir fluxes.

While examining Fig. 7.10, it is worth noting that the excursion in atmospheric CO2 over this 420000-year period (about 110ppm) is only marginally greater than that achieved by human activities over the last 200 years (90ppm), as shown in Figs 7.1 and 7.2. A second point to note from Fig. 7.10 is the close

Age (years x 103 BP)

Age (years x 103 BP)

Fig. 7.10 CO2 concentration in the atmosphere and estimated temperature changes during the past 420000 years, as determined from the Vostock ice core from Antarctica. BP, before present. Data from Petit et al. (1999).

correlation between the CO2 and the temperature records. This supports the notion of CO2 as an important greenhouse gas (Section 7.2.4), i.e. when CO2 is low the temperature is cool (as in glacial periods) and vice versa. Closer inspection of more detailed ice core records indicates that change in CO2 is apparently not the initiator of the temperature change, which probably arises from alterations in the Earth's orbit and/or changes in the amount of energy coming from the sun. However, orbital or insolation changes cannot account for the magnitude of temperature changes recorded in ice cores, suggesting that CO2 variations act to amplify the orbital and solar perturbations.

Anthropogenic fluxes

The primary human-induced flux to the atmosphere is that from fossil fuel burning, cement production and so forth, and, as shown in Fig. 7.9, for the 1980s its average value was 5.4 ± 0.3 GtC yr-1. Of this input, an amount equivalent to 3.3 ± 0.1 GtCyr-1 remains in the atmosphere and leads to the observed year-by-year rise in CO2 concentrations.

Change in land use arising from human activities leads to an addition to the atmosphere of 1.7 ± 1.1 GtCyr-1. On the other hand, fertilization of terrestrial plant growth, reforestation and regrowth are estimated to take up 1.9 GtCyr-1, giving a small net land sink of 0.2 ± 0.7 GtC annum-1.

In Table 7.1 these various flows of anthropogenic CO2 are given as a budget. Although the budget achieves balance this should not hide the considerable uncertainties over several of the terms, particularly those for exchanges between the land and the atmosphere. For example, the large error associated with the terrestrial biosphere flux term implies that the land could be a small net source, instead of the small net sink shown in Table 7.1, for anthropogenic CO2.

Emissions and atmospheric carbon dioxide levels in the future

In view of the 'greenhouse' properties of CO2 (Section 7.2.4) and the fact that atmospheric concentrations of the gas have risen substantially as a result of human activities, considerable effort is currently being devoted to the task of predicting what CO2 levels will be in the atmosphere over the next century.

In this chapter we have identified the problems that exist in quantitatively accounting for the CO2 which enters the atmosphere from fossil fuel and other

Table 7.1 Atmospheric sources and sinks of anthropogenic CO2 for the 1980s. All units are GtC yr-1. Data from IPCC (2001).

Sources

Sinks

Fossil fuel burning

5.4 ± 0.3

Atmosphere

3.3 ± 0.1

Oceans

1.9 ± 0.6

Land

0.2 ± 0.7

Total

5.4 ± 0.3

Total

5.4 ± 0.6

human activities at the present time. Thus, for any scenario of future anthropogenic CO2 emissions, there is at least as great an uncertainty over what proportion will remain in the atmosphere as exists for current emissions. In all probability the uncertainty is even greater, since climatological and other global changes, whether human-induced or natural, are likely to alter the rates at which the various environmental reservoirs take up and release CO2.

Estimating the amount of CO2 that will be emitted by human activities over the next 100 years is probably even less certain than calculating how it will partition between the air, ocean and land. Although the factors that determine the amounts of anthropogenic emissions can be identified, their quantification can only be guessed at. The size of the human population is a very important factor. We know it is rising and will almost certainly continue to do so (at an unknown rate). Similarly the standard of living of many people from less developed countries is rising and this will lead to greater use of energy in those parts of the world. How this energy is generated will have a profound bearing on how much CO2 is emitted. The future of CO2 emissions is crucial for policy decisions. For example, will it be necessary to curb future fossil fuel combustion and, if so, when and by how much, in order to prevent or at least ameliorate undesirable alteration in global climate?

Despite these difficulties some attempts have been made to predict atmospheric CO2 levels into the next century. The results of one such study are shown in Fig. 7.11. The different curves correspond to different scenarios of population

1000

2 600

2000

2020

2040 2060

Year

2080

2100

Fig. 7.11 Atmospheric CO2 concentrations calculated for various emission scenarios. From IPCC (2001). With permission of the Intergovernmental Panel on Climate Change.

growth, energy use and mode of production. All predict a substantial increase in atmospheric CO2 during the next 100 years, with levels ranging from 500 to more than 900 ppm by 2100. This factor-of-two range does not represent the whole of the uncertainty since other scenarios outside the range used (both higher and lower) are certainly possible. Furthermore, the environmental model used to simulate how much of the emitted CO2 remains in the atmosphere assumes the environmental system will behave as at present for the whole of the next century.

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