Summary and Perspectives

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We have looked at the long-term fate of CO2 released to the atmosphere and used an Earth system model to illustrate (and quantify) the role of some of the major geological carbon sinks. After the ocean has absorbed ~66% of the total release, reaction with sea-floor carbonates results in the sequestration of another 11% on a timescale of a few thousand years. Then, reaction with CaCO3 on land consumes a further 15%, but at a slightly slower pace (tens of thousands of years). Finally, the fate of the remaining 8% in the atmosphere (as well as the 92% now in the ocean as bicarbonate) is removal through silicate rock weathering and burial as marine carbonates.

The situation is not quite this straightforward. Rising surface temperatures and changes in ocean circulation could reduce the effectiveness of the initial ocean invasion sink by 10-20%. Additional and as yet poorly quantified complications arise due to a probable reduction in biogenic calcification in the surface ocean that could lead to an acceleration of ocean invasion and the sea-floor CaCO3 neutralization of CO2.

We have also considered different mechanisms of organic carbon deposition and burial in accumulating sediments. There is much greater uncertainty here in what the response will be to future global change. A greater rate of clay deposition and of continental margin sedimentary burial seems possible, and would favour a stronger carbon sink. Acting against this will be a reduction in open-ocean productivity caused by increased ocean stratification. However, regardless of the net impact of these factors, it seems unlikely that the organic carbon sink will be as important in the removal of fossil fuel CO2 from the atmosphere and ocean as changes in carbonate deposition. In contrast, the potential role for peatlands (and incipient coal formation) in the future may be greater.

A clue as to how the geologic carbon sinks might react in the future can be found in the distant past - there is good evidence for a 'catastrophic' release of carbon associated with an event called the 'Paleocene/ Eocene thermal maximum' (PETM), some 55.5 million years ago (Dickens et al., 1995). Associated with this was a CO2 (and maybe CH4)-driven greenhouse warming of the Earth's surface (Bains et al., 1999; Tripati and Elderfield, 2005). Of particular interest to us is that sediment cores document how carbonate accumulation in the deep sea at first declined, and then later recovered in response to this event (Zachos et al., 2005). It has also been suggested that the observed timescale for this recovery (~60,000 years) was accelerated by increased productivity in the ocean (Bains et al., 2000). This would imply that the action of organic carbon sinks in the marine environment might be more important in the future than we speculated earlier (Section 6.3). Paleo-analogues for future global change such as the PETM have a critical role in helping to elucidate the role that the geologic carbon sinks might play in the future.

In the geological carbon sinks, particularly involving silicate rock weathering, the Earth possesses powerful feedback mechanisms that are able to regulate the surface environmental conditions of the planet.

There is therefore no doubt about the long-term survival of the biosphere, despite the currently accelerating rate of greenhouse gas emissions. Indeed, by transforming fossil fuel CO2 into carbonates that are buried on the ocean floor, the geological carbon sink will eventually clean up our mess and return atmospheric composition to the pre-Industrial state. Can we rely on these 'geologic' sinks to stabilize atmospheric CO2 and climate without any societal intervention, leaving us free to continue to burn fossil fuels in a 'business as usual' fashion? Unfortunately, 'Mother Earth' is not a quick healer and the geologic carbon sinks will be of little help in damping the maximum future 'greenhouse' warming of the planet. This warming, along with other global environmental impacts will occur much quicker than the geological sinks can cope with.

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