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Analogous simulations have been performed with several ocean carbon models
(Figure 3.10c,d). To compute the impact of increasing
CO2 alone (no climate change), OCMIP models were forced to follow
the atmospheric CO2 concentration derived from the IS92a scenario
as in the DGVM experiment (Figure 3.10a,b) (Orr
and Dutay, 1999). All models agreed in projecting that the annual ocean-atmosphere
flux of CO2 continues to become larger, reaching -6.7 to -4.5 PgC/yr
by 2100 (Figure 3.10c). Since surface conditions
(temperature, wind speed, alkalinity) were prescribed, the range in model estimates
stems only from different representations of physical transport processes.
Several atmosphere-ocean models were used to project the effect of climate change
(Maier-Reimer et al., 1996; Sarmiento et al., 1998; Matear and Hirst, 1999;
Joos et al., 1999b; Bopp et al., 2001). These models include most processes
previously discussed, including all processes associated with carbonate chemistry
and gas exchange, physical and biological transport of CO2, and changes
in temperature, salinity, wind speed, and ice cover. They account for simple
changes in biological productivity, but not for changes in external nutrient
supply, species composition, pH, or Redfield ratios, all of which could be involved
in more complex biological feedbacks. Coupled models estimate the impact of
climate change as a departure, reported in per cent, from a “control”
experiment modelling the effect of increasing atmospheric CO2 alone.
In climate change simulations, warming of surface waters and increased stratification of the upper ocean produced an overall positive feedback that reduced the accumulated ocean uptake of CO2 by 6 to 25% between 1990 and the middle of the 21st century, as compared with the CO2-only case. In the first part of the simulation, the climate-mediated feedback is mainly due to the temperature effect on CO2 solubility (Sarmiento and Le Quéré, 1996; Matear and Hirst, 1999). Towards the mid-century, the impact of circulation changes becomes significant in most models, with the net effect of further reducing ocean CO2 uptake. To investigate the effect of climate change on the IS92a scenario, the average of the OCMIP CO2-only projections (mean of results in Figure 3.10c) was used as a baseline and the reduction in atmosphere-ocean CO2 flux caused by climate change (in per cent since the beginning of the simulation) was applied to this curve (Figure 3.10d). The range in model results (Figure 3.10d) must be attributed to uncertainties related to climate change feedback, and not to uncertainties in the modelling of physical transport as shown in Figure 3.10c.
The range of model estimates of the climate change impact is dependent on the choice of scenario for atmospheric CO2 and on assumptions concerning marine biology (Joos et al., 1999b). At high CO2 concentrations, marine biology can have a greater impact on atmospheric CO2 than at low concentrations because the buffering capacity of the ocean is reduced (see Box 3.3) (Sarmiento and Le Quéré, 1996). Although the impact of changes in marine biology is highly uncertain and many key processes discussed in Section 3.2.3.1 are not included in current models, sensitivity studies can provide approximate upper and lower bounds for the potential impact of marine biology on future ocean CO2 uptake. A sensitivity study of two extreme scenarios for nutrient supply to marine biology gave a range of 8 to 25% for the reduction of CO2 uptake by mid-century (Sarmiento et al., 1998). This range is comparable to other uncertainties, including those stemming from physical transport (Figure 3.10c).
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