Changes to the Ocean Carbon Cycle

Over several thousands of years, around 90% of the anthropogenic CO2 emissions will end up in the ocean [4]. Because of the slow mixing time of the ocean the current oceanic uptake fraction is only about one-third of this value [1], without which atmospheric CO2 would be about 55 ppm higher today than what is currently observed (385 ppm).

The Southern Ocean is estimated to account for around 25% of the anthropogenic CO2 taken up by all the oceans while the North Atlantic is estimated to account for 40% [1]. Unlike the Southern Ocean which has a strong biological pump, the North Atlantic CO2 sink is thought to be mainly due to the physical pump, with the 'biological pump' contributing only around 10% [68]. As the surface ocean CO2 concentrations continue to increase the ocean's ability to absorb more CO2 from the atmosphere will slow down.

Whilst there were indications that this might be occurring in the analysis of 1990s oceanographic cruises by Sabine et al. [1], more recent analysis of CO2 in the NE Atlantic [69] and Southern Ocean [11] show a decrease in CO2 uptake over the last 1 2 decades. Whether this decrease in the efficiency of the ocean sink for anthropogenic CO2 is decadal variation awaits further long time series study. If the ocean CO2 sink is becoming less efficient then more CO2 will remain in the atmosphere exacerbating global warming.

The 'biological pump' removes carbon from surface waters to the deep ocean via the organic or 'soft' tissue pump (which decreases CO2 of surface water, increasing its ability to absorb atmospheric CO2) while the inorganic or 'hard' CaCO3 pump increases CO2 of the surface water and decreases its ability to absorb atmospheric CO2. Decreasing calcification and CaCO3 export rates could therefore play a direct role in ameliorating future global change. However, decreasing primary production and export rates (the soft tissue pump) would have the opposite effect, resulting in less atmospheric CO2 draw down by surface waters. To add to the complexity of these key mechanisms in the carbon cycle, there may be strong association between 'soft' and 'hard' pumps with a 'ballasting' of organic matter by carbonate particles, making the organic matter sink faster than it would on its own. A decrease in CaCO3 production [70] would then lead to a reduction in the efficiency with which organic matter is transported to depth, weakening the biological pump and resulting in higher surface ocean CO2. This would reduce fossil fuel CO2 uptake by the ocean and exacerbate future climate change [71]. Although we have a poor understanding of the importance of these two mechanisms experiments looking at the calcification and primary production of coccolithophores in 27 m3 seawater enclosures (mesocosms) found a shift in the ratio of organic carbon to calcium carbonate production and vertical flux with rising atmospheric CO2 [72].

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