The Global Carbon Budget

Over the past 200 years humans have introduced ~ 400 petagrams of carbon (PgC) to the atmosphere through deforestation and the burning of fossil fuels. Part of this carbon was absorbed by the oceans and terrestrial biosphere. Table 2.1, section 1, shows the global budget recently compiled by the IPCC Third Assessment Report (Prentice et al. 2001). The global carbon budget quantifies the relative importance of these two reservoirs today, and the budget uncertainty reflects our understanding of the exchanges between these reservoirs. The IPCC assessment partitions the uptake into net terrestrial and oceanic components based primarily on observations of the concurrent global trends of atmospheric CO2 and oxygen. Since the compilation of the IPCC report, new evidence from model studies and oceanographic observations shows that this budget should be slightly revised to account for a previously ignored oceanic oxygen flux. This flux is the result of enhanced oceanic mixing, as inferred from observed changes in oceanic heat content (Le Quéré et al. 2003). The revised values are presented in section 2 of Table 2.1.

Section 3 of Table 2.1 shows the breakdown of the net land-atmosphere flux through the 1990s into emissions from changes in land use and a residual terrestrial sink, based on the updated land use change emissions of Houghton (2003). This breakdown has recently been challenged based on new estimates of land use change determined from remote sensing data (Table 2.1, sections 4 and 5). These new estimates lie at the lower end of the range of estimates based on data reported by individual countries (Houghton 2003). If correct, they imply a residual terrestrial sink in the 1990s that is about 40 percent smaller than previous estimates.

This budget, of course, does not attempt to represent the richness of the global carbon cycle. The land-atmosphere-ocean system is connected by a multitude of exchange fluxes. The dynamical behavior of this system is determined by the relative sizes of the different reservoirs and fluxes, together with the biogeochemical processes and human driving forces controlling these exchanges. Colorplate 1 shows the globally aggregated layout of the carbon cycle, together with the pools and exchange fluxes that are relevant on timescales of up to a few millennia. Panel a in Colorplate 1 presents a basic picture of the global carbon cycle, including the preindustrial (thin) and anthropogenic (bold) ocean-atmosphere and land-atmosphere exchange fluxes. The anthropogenic fluxes are average values for the 1980s and 1990s. Panel a also shows components of the long-term

Table 2.1. The global carbon budget (PgC y-1)

1980s

1990s

1. Prentice et al. 2001

Atmospheric increase

+3.3 ± 0.1

+3.2 ± 0.1

Emissions (fossil fuel, cement)

+5.4 ± 0.3

+6.3 ± 0.4

Ocean-atmosphere flux

-1.9 ± 0.6

-1.7 ± 0.5

Net land-atmosphere flux

-0.2 ± 0.7

-1.4 ± 0.7

Land use change

+ 1.7 (+0.6 to +2.5)

-

Residual terrestrial sink

-1.9 (-3.8 to +0.3)

-

2. Le Quere et al. 2003

Ocean corrected

-1.8 ± 0.8

-1.9 ± 0.7

Net land-atmosphere flux

-0.3 ± 0.9

-1.2 ± 0.8

3. Houghton 2003

Land use change

+2.0 (+0.9 to +2.8)

+2.2 (+1.4 to +3.0)

Residual terrestrial sink

-2.3 (-4.0 to -0.3)

-3.4 (-5.0 to -1.8)

4. DeFries et al. 2002

Land use change

+0.6 (+0.3 to +0.8)

+0.9 (+0.5 to +1.4)

Residual terrestrial sink

-0.9 (-3.0 to 0)

-2.1 (-3.4 to -0.9)

5. Achard et al. 2002

Land use change

+ 1.0 ± 0.2

Residual terrestrial sink

-2.2 (-3.2 to -1.2)

Note: Positive values represent atmospheric increase (or ocean/land sources); negative numbers represent atmospheric decrease (sinks). Residual terrestrial sink determined by difference (net land/atmosphere flux minus land use change).

Note: Positive values represent atmospheric increase (or ocean/land sources); negative numbers represent atmospheric decrease (sinks). Residual terrestrial sink determined by difference (net land/atmosphere flux minus land use change).

geological cycle and the composite estimates of CO2 emissions from geological reservoirs (i.e., fossil fuels and the production of lime for cement). Panels b and c provide more detailed pictures of the ocean and terrestrial fluxes, respectively. Individual component pools and fluxes, including key climatic and anthropogenic drivers, are discussed later in this chapter and in subsequent chapters.

Although Colorplate 1 focuses primarily on fluxes directly related to CO2, a number of non-CO2 trace gases also play significant roles in the global carbon cycle (e.g., CO, CH4, non-methane hydrocarbons) and/or climate forcing (e.g., CH4, N2O, chloro-fluorocarbons). The global cycles of these trace gases, which share many of the processes driving the CO2 cycle, are briefly discussed later in this chapter.

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