3.1 Introduction

The concentration of CO₂ in the atmosphere has risen from close to 280 parts per million (ppm) in 1800, at first slowly and then progressively faster to a value of 367 ppm in 1999, echoing the increasing pace of global agricultural and industrial development. This is known from numerous, well-replicated measurements of the composition of air bubbles trapped in Antarctic ice. Atmospheric CO₂ concentrations have been measured directly with high precision since 1957; these measurements agree with ice-core measurements, and show a continuation of the increasing trend up to the present.

Several additional lines of evidence confirm that the recent and continuing increase of atmospheric CO₂ content is caused by anthropogenic CO₂ emissions – most importantly fossil fuel burning. First, atmospheric O₂ is declining at a rate comparable with fossil fuel emissions of CO₂ (combustion consumes O₂). Second, the characteristic isotopic signatures of fossil fuel (its lack of ¹⁴C, and depleted content of ¹³C) leave their mark in the atmosphere. Third, the increase in observed CO₂ concentration has been faster in the northern hemisphere, where most fossil fuel burning occurs.

Atmospheric CO₂ is, however, increasing only at about half the rate of fossil fuel emissions; the rest of the CO₂ emitted either dissolves in sea water and mixes into the deep ocean, or is taken up by terrestrial ecosystems. Uptake by terrestrial ecosystems is due to an excess of primary production (photosynthesis) over respiration and other oxidative processes (decomposition or combustion of organic material). Terrestrial systems are also an anthropogenic source of CO₂ when land-use changes (particularly deforestation) lead to loss of carbon from plants and soils. Nonetheless, the global balance in terrestrial systems is currently a net uptake of CO₂.

The part of fossil fuel CO₂ that is taken up by the ocean and the part that is taken up by the land can be calculated from the changes in atmospheric CO₂ and O₂ content because terrestrial processes of CO₂ exchange involve exchange of oxygen whereas dissolution in the ocean does not. Global carbon budgets based on CO₂ and O₂ measurements for the 1980s and 1990s are shown in Table 3.1. The human influence on the fluxes of carbon among the three “reservoirs” (atmosphere, ocean, and terrestrial biosphere) represent a small but significant perturbation of a huge global cycle (Figure 3.1).

Figure 3.1: The global carbon cycle: storages (PgC) and fluxes (PgC/yr) estimated for the 1980s. (a) Main components of the natural cycle. The thick arrows denote the most important fluxes from the point of view of the contemporary CO₂ balance of the atmosphere: gross primary production and respiration by the land biosphere, and physical air-sea exchange. These fluxes are approximately balanced each year, but imbalances can affect atmospheric CO₂ concentration significantly over years to centuries. The thin arrows denote additional natural fluxes (dashed lines for fluxes of carbon as CaCO3), which are important on longer time-scales. The flux of 0.4 PgC/yr from atmospheric CO₂ via plants to inert soil carbon is approximately balanced on a time-scale of several millenia by export of dissolved organic carbon (DOC) in rivers (Schlesinger, 1990). A further 0.4 PgC/yr flux of dissolved inorganic carbon (DIC) is derived from the weathering of CaCO3, which takes up CO₂ from the atmosphere in a 1:1 ratio. These fluxes of DOC and DIC together comprise the river transport of 0.8 PgC/yr. In the ocean, the DOC from rivers is respired and released to the atmosphere, while CaCO3 production by marine organisms results in half of the DIC from rivers being returned to the atmosphere and half being buried in deep-sea sediments - which are the precursor of carbonate rocks. Also shown are processes with even longer time-scales: burial of organic matter as fossil organic carbon (including fossil fuels), and outgassing of CO₂ through tectonic processes (vulcanism). Emissions due to vulcanism are estimated as 0.02 to 0.05 PgC/yr (Williams et al., 1992; Bickle, 1994). (b) The human perturbation (data from Table 3.1). Fossil fuel burning and land-use change are the main anthropogenic processes that release CO₂ to the atmosphere. Only a part of this CO₂ stays in the atmosphere; the rest is taken up by the land (plants and soil) or by the ocean. These uptake components represent imbalances in the large natural two-way fluxes between atmosphere and ocean and between atmosphere and land. (c) Carbon cycling in the ocean. CO₂ that dissolves in the ocean is found in three main forms (CO₂, CO32-, HCO3-, the sum of which is DIC). DIC is transported in the ocean by physical and biological processes. Gross primary production (GPP) is the total amount of organic carbon produced by photosynthesis (estimate from Bender et al., 1994); net primary production (NPP) is what is what remains after autotrophic respiration, i.e., respiration by photosynthetic organisms (estimate from Falkowski et al., 1998). Sinking of DOC and particulate organic matter (POC) of biological origin results in a downward flux known as export production (estimate from Schlitzer, 2000). This organic matter is tranported and respired by non-photosynthetic organisms (heterotrophic respiration) and ultimately upwelled and returned to the atmosphere. Only a tiny fraction is buried in deep-sea sediments. Export of CaCO3 to the deep ocean is a smaller flux than total export production (0.4 PgC/yr) but about half of this carbon is buried as CaCO3 in sediments; the other half is dissolved at depth, and joins the pool of DIC (Milliman, 1993). Also shown are approximate fluxes for the shorter-term burial of organic carbon and CaCO3 in coastal sediments and the re-dissolution of a part of the buried CaCO3 from these sediments. (d) Carbon cycling on land. By contrast with the ocean, most carbon cycling through the land takes place locally within ecosystems. About half of GPP is respired by plants. The remainer (NPP) is approximately balanced by heterotrophic respiration with a smaller component of direct oxidation in fires (combustion). Through senescence of plant tissues, most of NPP joins the detritus pool; some detritus decomposes (i.e., is respired and returned to the atmosphere as CO₂) quickly while some is converted to modified soil carbon, which decomposes more slowly. The small fraction of modified soil carbon that is further converted to compounds resistant to decomposition, and the small amount of black carbon produced in fires, constitute the “inert” carbon pool. It is likely that biological processes also consume much of the “inert” carbon as well but little is currently known about these processes. Estimates for soil carbon amounts are from Batjes (1996) and partitioning from Schimel et al. (1994) and Falloon et al. (1998). The estimate for the combustion flux is from Scholes and Andreae (2000). ‘t’ denotes the turnover time for different components of soil organic matter.

This chapter summarises current knowledge of the global carbon cycle, with special reference to the fate of fossil fuel CO₂ and the factors that influence the uptake or release of CO₂ by the oceans and land. These factors include atmospheric CO₂ concentration itself, the naturally variable climate, likely climate changes caused by increasing CO₂ and other greenhouse gases, changes in ocean circulation and biology, fertilising effects of atmospheric CO₂ and nitrogen deposition, and direct human actions such as land conversion (from native vegetation to agriculture and vice versa), fire suppression and land management for carbon storage as provided for by the Kyoto Protocol (IPCC, 2000a). Any changes in the function of either the terrestrial biosphere or the ocean - whether intended or not - could potentially have significant effects, manifested within years to decades, on the fraction of fossil fuel CO₂ that stays in the atmosphere. This perspective has driven a great deal of research during the years since the IPCC WGI Second Assessment report (IPCC, 1996) (hereafter SAR) (Schimel et al., 1996; Melillo et al., 1996; Denman et al., 1996). Some major areas where advances have been made since the SAR are as follows:

Observational research (atmospheric, marine and terrestrial) aimed at a better quantification of carbon fluxes on local, regional and global scales. For example, improved precision and repeatability in atmospheric CO₂ and stable isotope measurements; the development of highly precise methods to measure changes in atmospheric O₂ concentrations; local terrestrial CO₂ flux measurements from towers, which are now being performed continuously in many terrestrial ecosystems; satellite observations of global land cover and change; and enhanced monitoring of geographical, seasonal and interannual variations of biogeochemical parameters in the sea, including measurements of the partial pressure of CO₂ (pCO₂) in surface waters.
Experimental manipulations, for example: laboratory and greenhouse experiments with raised and lowered CO₂ concentrations; field experiments on ecosystems using free-air carbon dioxide enrichment (FACE) and open-top chamber studies of raised CO₂ effects, studies of soil warming and nutrient enrichment effects; and in situ fertilisation experiments on marine ecosystems and associated pCO₂ measurements.
Theory and modelling, especially applications of atmospheric transport models to link atmospheric observations to surface fluxes (inverse modelling); the development of process-based models of terrestrial and marine carbon cycling and programmes to compare and test these models against observations; and the use of such models to project climate feedbacks on the uptake of CO₂ by the oceans and land.

As a result of this research, there is now a more firmly based knowledge of several central features of the carbon cycle. For example:

Time series of atmospheric CO₂, O₂ and ¹³CO₂ measurements have made it possible to observationally constrain the partitioning of CO₂ between terrestrial and oceanic uptake and to confirm earlier budgets, which were partly based on model results.
In situ experiments have explored the nature and extent of CO₂ responses in a variety of terrestrial ecosystems (including forests), and have confirmed the existence of iron limitations on marine productivity.
Process-based models of terrestrial and marine biogeochemical processes have been used to represent a complex array of feedbacks in the carbon cycle, allowing the net effects of these processes to be estimated for the recent past and for future scenarios.

Table 3.1: Global CO₂ budgets (in PgC/yr) based on intra-decadal trends in atmospheric CO₂ and O₂. Positive values are fluxes to the atmosphere; negative values represent uptake from the atmosphere. The fossil fuel emissions term for the 1980s (Marland et al., 2000) has been slightly revised downward since the SAR. Error bars denote uncertainty (± 1s), not interannual variability, which is substantially greater.

	1980s	1990s

Atmosphere increase	3.3 ± 0.1	3.2 ± 0.1
Emissons (fossil fuel, cement)	5.4 ± 0.3	6.3 ± 0.4
Ocean-atmosphere flux	-1.9 ± 0.6	-1.7 ± 0.5
Land atmsphere fluux*	-0.2±0.7	-.1.4±0.7
*partitioned as follows
Land use change	1.7 (0.6 to 2.5)	NA
Residual terrestrial sink	-1.9 (-3.8 to 0.3)	NA

* The land-atmosphere flux represents the balance of a positive term due to land-use change and a residual terrestrial sink. The two terms cannot be separated on the basis of current atmospheric measurements. Using independent analyses to estimate the land-use change component for the 1980s based on Houghton (1999), Houghton and Hackler (1999), Houghton et al. (2000), and the CCMLP (McGuire et al., 2001) the residual terrestrial sink can be inferred for the 1980s. Comparable global data on land-use changes through the 1990s are not yet available.

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