Climate Change 2001:
Working Group I: The Scientific Basis
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3.6.2.2 Uptake and release of anthropogenic CO2 by the land

The most comprehensive model-based estimates of the terrestrial components of the anthropogenic CO2 budget are those that have been produced by the CCMLP. McGuire et al. (2001) used two TBMs and two DGVMs driven by changes in atmospheric CO2, then changes in CO2 with historical changes in climate (from observations), and finally changes in CO2 and climate with land-use change from Ramankutty and Foley (2000) (Figure 3.8; Table 3.4). In these simulations, CO2 fertilisation accounted for a land-atmosphere flux of -0.9 to -3.1 PgC/yr, land-use change a positive flux of 0.6 to 1.0 PgC/yr, and climate variability a small additional effect of uncertain sign, -0.2 to 0.9 PgC/yr during the 1980s. The total land-atmosphere flux simulated for the 1980s amounted to -0.3 to -1.5 PgC/yr, which is consistent with or slightly more negative than the observationally-based estimate of -0.2 ± 0.7 PgC/yr (Table 3.1). Net uptake by all models reported in McGuire et al. (2001) is shown to be occurring mainly in tropical, temperate and boreal forests - consistent with forest inventory data (Section 3.5.4) - while some regions (notably semi-arid tropical and sub-tropical regions) show net carbon loss. The model estimates of the CO2 source due to land-use change are substantially smaller than the estimate of Houghton (1999) (Section 3.4.2). This divergence primarily reflects disagreements between the Houghton (1999) and Ramankutty and Foley (2000) data sets as to the timing of tropical deforestation in different regions (see Section 3.4.2).

Figure 3.8: Modelled fluxes of anthropogenic CO2 over the past century. (a) Ocean model results from OCMIP (Orr and Dutay, 1999; Orr et al., 2000); (b), (c) terrestrial model results from CCMLP (McGuire et al., 2001). Positive numbers denote fluxes to the atmosphere; negative numbers denote uptake from the atmosphere. The ocean model results appear smooth because they contain no interannual variability, being forced only by historical changes in atmospheric CO2. The results are truncated at 1990 because subsequent years were simulated using a CO2 concentration scenario rather than actual measurements, leading to a likely overestimate of uptake for the 1990s. The terrestrial model results include effects of historical CO2 concentrations, climate variations, and land-use changes based on Ramankutty and Foley (2000). The results were smoothed using a 10-year running mean to remove short-term variability. For comparison, grey boxes denote observational estimates of CO2 uptake by the ocean in panel (a) and by the land in panel (b) (from Table 3.1). Land-use change flux estimates from Houghton et al. (1999) are shown by the black line in panel (c). The grey boxes in panel (c) indicate the range of decadal average values for the land-use change flux accepted by the SRLULUCF (Bolin et al., 2000) for the 1980s and for 1990 to 1995.

There is no general agreement on how to model the linkage between reactive nitrogen deposition and vegetation productivity, and recent model estimates of the additional effect of anthropogenic nitrogen fertilisation on the global carbon cycle vary widely. The anthropogenic nitrogen input itself (Holland et al., 1999), the fate of anthropogenic nitrogen in the ecosystem (Nadelhoffer et al., 1999; Jenkinson et al., 1999), and changes in ecosystem nitrogen fixation (Vitousek and Field, 1999) represent major sources of uncertainty. Estimates of the anthropogenic nitrogen effect range from -0.2 PgC/yr (Nadelhoffer et al., 1999) to -1.1 or -1.4 PgC/yr (Holland et al., 1997). The model with the smallest CO2 fertilisation effect (-0.9 PgC/yr) in the McGuire et al. (2001) study has been shown to respond strongly to anthropogenic nitrogen input, yielding a combined (CO2 and nitrogen) fertilisation effect of -1.5 PgC/yr. A modelling study by Lloyd (1999) suggests that CO2 and nitrogen fertilisation effects may by synergistic. Evaluation of model results on carbon-nitrogen coupling against experimental results is a current research focus.



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