The oceans are a huge reservoir of dissolved carbon, containing about 50 times more CO2 than the atmosphere. Greenhouse gas emissions and climate change is concerned largely with the amount of CO2 in the atmosphere, but the overall levels that may be expected in the future will be strongly influenced by the ocean’s uptake potential. Unravelling the complex interplay between physical, biological and chemical process in the sea, how this couples to the atmosphere and climate, and possible feedbacks is pivotal in providing robust and credible prognoses for climate in an uncertain future.
For nearly 30 years, it has been recognized that high latitude seas play an important role in the uptake and sequestration of CO2 into the oceans depths. Several inter-related factors cause this. Firstly the solubility of gasses (CO2 included) in high latitude seas is roughly twice that for tropical seas. High latitude seas – particularly in the North Atlantic – are also areas of deep water formation. That is, where surface waters become dense enough to sink to depths greater than 1000 m. Indeed, waters formed in the North Atlantic flood all of the world’s oceans basins. The second important element involves biological processes. Phytoplankton, in the presence of light and sufficient nutrients, take up CO2 and convert it to organic matter – most of which goes to fuel the marine food web leading to fish, whales, seabirds and other top consumers. However, a significant fraction of this production rains out of the surface ocean in the form of dead particulate material as so-called marine snow. The process by which CO2 is removed from the surface ocean and transported to depth as particulate organic matter is known as the biological pump.
To put some figures on this, the net primary production of the oceans is about 50 GTC / year (about the same as on land), of which about 10 GTC / year sinks out of the surface ocean. Compare this to the 8 GTC / year being released into the atmosphere by human activities. Of the 10 GTC / year that sink out of the surface ocean, only about 2 to 3 GTC reach the deep ocean where it can remain for 100’s of years. This difference between what leaves the surface and what becomes sequestered at depth is one of the biggest unknowns in the global carbon cycle.