5.4 Ocean Biogeochemical Changes
The observed increase in atmospheric carbon dioxide (CO2; see Chapter 2) and the observed changes in the physical properties of the ocean reported in this chapter can affect marine biogeochemical cycles (here mainly carbon, oxygen, and nutrients). The increase in atmospheric CO2 causes additional CO2 to dissolve in the ocean. Changes in temperature and salinity affect the solubility and chemical equilibration of gases. Changes in circulation affect the supply of carbon and nutrients from below, the ventilation of oxygen-depleted waters and the downward penetration of anthropogenic carbon. The combined physical and biogeochemical changes also affect biological activity, with further consequences for the biogeochemical cycles.
The increase in surface ocean CO2 has consequences for the chemical equilibrium of the ocean. As CO2 increases, surface waters become more acidic and the concentration of carbonate ions decreases. This change in chemical equilibrium causes a reduction of the capacity of the ocean to take up additional CO2. However, the response of marine organisms to ocean acidification is poorly known and could cause further changes in the marine carbon cycle with consequences that are difficult to estimate (see Section 7.3.4 and Chapter 4 of the Working Group II contribution to the IPCC Fourth Assessment Report).
Dissolved oxygen (O2) in the ocean is affected by the same physical processes that affect CO2, but in contrast to CO2, O2 is not affected by changes in its atmospheric concentration (which are only of the order of 10–4 of its mean concentration). Changes in oceanic O2 concentration thus provide information on the changes in the physical or biological processes that occur within the ocean, such as ventilation (here used to describe the rate of renewal of thermocline waters), mode water formation, upwelling or biological export and respiration. Furthermore, changes in the oceanic O2 content are needed to estimate the CO2 budget from atmospheric O2/molecular nitrogen (N2) ratio measurements. However, the method currently estimates the change in air-sea fluxes of O2 indirectly based on heat flux changes (see Section 7.3.2).
This section reports observed changes in biogeochemical cycles and assesses their consistency with observed changes in physical properties. Changes in oceanic nitrous oxide (N2O) and methane (CH4) have not been assessed because of the lack of large-scale observations. Observations of the mean fluxes of N2O and CH4 (including CH4 hydrates) are discussed in Chapter 7.