Carbon Uptake, Transport and Storage by Oceans and the Consequences of Change

This chapter explores the greatest biospheric reservoir of carbon on planet Earth – the oceans. When in balance, there is a large flux of CO2 between the oceans and the atmosphere of almost 90 Gt C yr1 due to a combination of primary production and particle sinking (the biological pump) and ocean circulation and mixing (the solubility pump). Climate change will tend to suppress ocean-carbon uptake through reductions in CO2 solubility, suppression of vertical mixing by thermal stratification and decreases in surface salinity. It is envisaged that climate-driven changes in any of these physical mechanisms will have a subsequent impact on the phytoplankton and their ability to draw carbon from the atmosphere and into the ocean. This will increase the fraction of anthropogenic CO2 emissions that remain in the atmosphere this century and produce a positive feedback to climate change.

Increased burning of fossil fuel, cement manufacturing and land-use change since the industrial revolution has increased atmospheric CO2 and caused an imbalance in the exchange of CO2 between atmosphere and ocean, resulting in more ocean uptake. Oceans have taken up around 25% of the anthropogenic CO2 produced in the last 200 years and through this have buffered climate change. However, this has already lead to a profound change in ocean carbonate chemistry (a 30% increase in hydrogen ions), coined ‘‘ocean acidification’’, and this change will increase in magnitude in the future as anthropogenic CO2 emissions increase and more CO2 dissolves in the surface of oceans. The atmospheric CO2 increase alone will lead to continued uptake by the ocean, although the efficiency of this uptake will decrease as the carbonate buffering mechanism in the ocean weakens. Research so far indicates that these changes to ocean pH, and bicarbonate and carbonate ion saturation, will have a profound impact on ocean biology, both in pelagic (free-floating) and benthic (seafloor) realms.

Ocean productivity is far from uniform and may cause impacts when vast numbers of phytoplankton cells are concentrated in high-biomass, sometimes harmful or toxic, algal blooms. The most significant harm caused by highbiomass blooms is oxygen depletion, usually caused when dead phytoplankton cells sink down the water column and are decomposed by bacteria, using oxygen to do so. The degree of depletion is determined by the quantity of organic matter accumulated, the stability of the water column, and the bathymetry (depth of the water column), the first two being sensitive to climate change