Variability in feedback timescales

Modelling studies have suggested that the ocean will ultimately absorb up to 70-85% of the CO2 released by human activity (Le Quere and Metzl, 2004). Including a carbon system feedback where carbonate sediments in the ocean are dissolved by the lowered pH of the waters suggests that the ocean may be the ultimate storage place for as much as 90% of the anthropogenic CO2 (Archer et al., 1997). The dissolution of carbonate sediments reverses Eq. 3.3 and increases the carbon storage capacity of the ocean. However, because of the slow mixing time to get the anthropogenic CO2 into the deep ocean, this capacity may not be realized for hundreds or thousands of years. When considering the role of carbon system and carbon-climate feedbacks, it is important to understand the timescale of these processes. With typical lifetimes ranging from weeks to months, biological processes have the potential to respond very quickly to carbon system or climate changes. Large-scale circulation changes are likely to be relevant on annual to decadal timescales, and sediment dissolution processes are presumed to be relevant on centennial to millennial timescales.

As long as atmospheric CO2 continues to rise, the ocean will continue to take up CO2. Long-term feedbacks like dissolution of carbonate sediments may enhance the oceanic uptake but most indications are that the shorter-term feedbacks may reduce the rate of CO2 uptake on the decadal to centennial timescales. Although there are considerable differences in the relative magnitudes of each of the individual feedback processes described in Table 3.5, all the models showed a net decrease in the overall uptake of CO2 by the ocean over time (Greenblatt and Sarmiento, 2004). In the OCMIP-2 models, which utilized a constant biological activity and circulation, the estimated oceanic uptake of anthropogenic CO2 by 2100 ranged from 4 to 8 Pg C/year, depending on the CO2 emission scenario used in the model (Watson and Orr, 2003). This estimate has a factor of 2-4 times higher than the current value of 2 ± 0.5 Pg C/year but still lags behind the projected rate of CO2 emissions. This means that a larger fraction of the CO2 emissions will be retained by the atmosphere in the future, thus enhancing the overall climate change impact.

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