Some tens of thousands of years after the burning of fossil fuels has ceased, ~8% of fos sil fuel CO2 emissions (assumed to be 4167 Pg C here) will remain in the atmosphere. The atmospheric CO2 concentration is 435 ppm, compared with 376 ppm in 2003 (Keeling and Whorf, 2005) and a pre-Industrial value of 278 ppm (Enting et al., 1994). This would probably give half as much climate change as has already occurred to date. Is this the 'end of the road', or does the geologic carbon sink have any further cards to play?
Estimates of the evolution of the amount of carbon in the ocean and atmosphere through Earth history have both reservoirs generally paralleling each other over very long periods of time (>1 million years); i.e. CO2 and DIC tend to increase and decrease together (Fig. 6.1). In contrast, our model has so far predicted that when atmospheric CO2 declines, the ocean inventory increases (Fig. 6.8b and c); an antiphased relationship. It would not be unreasonable to conclude from this that we are missing (at least) one important mechanism. We now come to the final geologic (carbonate) carbon sink and one of the most fundamental regulatory mechanisms of the Earth system - the weathering of silicate rocks.
The reaction involved in the weathering of calcium silicate minerals (particularly the feldspar family, which are the most abundant group of minerals in continental rocks) can be written as:
This differs from the weathering of carbonate rocks (in contrast to the weathering reaction listed in Section 6.2.2) in one fundamental regard; it takes two moles of CO2 to weather each mole of CaAl2Si2O8 and release a single mole of calcium ions (plus 2 of bicarbonate ions). The calcium ion is subsequently removed from solution in the same precipitation reaction as before, meaning that only one mole of CO2 is released back to the ocean (and atmosphere). The weathering of silicate rocks is thus a net sink for atmospheric CO2 (Berner, 1992) (Fig. 6.7c) - i.e. one mole of CO2 is being sequestered for each mole of calcium silicate mineral weathered. In the long term, the rate of silicate weathering should balance the rate of volcanic release of CO2 to the atmosphere (Berner and Caldeira, 1997). If this mechanism is then already busy removing volcanic CO2 emissions, how can it help in removing the final fraction of anthropogenic CO2 from the atmosphere?
The rate at which the weathering reaction proceeds depends on a variety of variables. The ones that interest us here are ambient temperature and CO2 concentration (which is enhanced in soils through the metabolic activity of plants, animals and microbes) (Berner, 1990, 1992). Now we can see how the ultimate fate of fossil fuel CO2 and the final 'geologic' (carbonate) carbon sink arises - a faster rate of weathering of silicate rocks under a fossil fuel-elevated CO2 atmosphere (and a warmer, wetter climate), which acts to remove the excess carbon from the atmosphere and sequesters it in marine carbonates. In fact, silicate weathering could remove not only the remaining ~8% fraction of fossil fuel CO2 left in the atmosphere, but also the fossil fuel CO2 stored (as bicarbonate ions - see Fig. 6.8c). No trace of our meddling with the environment would remain, except for a slightly more weathered continental surface than before and a fresh thick layer of carbonates covering the ocean floor. Unfortunately, the planetary cleaners will not finish their work any time soon - the timescale for this process is counted in hundreds of thousands of years (Berner and Caldeira, 1997).
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