Ocean carbon sink
About 10 Pg C is released annually into the atmosphere by anthropogenic emissions of carbon dioxide (2017-2019). Part of the released CO2 is transferred to the ocean by physical processes. Another part is incorporated in biomass through photosynthesis in the terrestrial and marine environment. However, the carbon stored on land and in the ocean that cannot return as CO2 to the atmosphere over multi-decadal periods is only a small part of the gross primary production. Estimates of the sequestered carbon are: about 3 Pg C/y on land and about 2.4 Pg C/y in the sea .
Global carbon stocks
The major global pools of potentially available C (carbon) include the atmosphere, oceans, fossil fuels, and - collectively - vegetation, soils, and detritus. The oceans are the largest C pool, encompassing an estimated 38 000 petagrams of C (1 petagram C =1 Pg C = 1 megatonne C = 1015 g C) . The geological C pool, composed primarily of fossil fuels, is the next largest pool, estimated at 2000-4000 Pg C. Vegetation (mostly terrestrial, above and below ground) and detritus hold around 2000 Pg C, followed by the atmosphere, which holds about 800 Pg C. The ocean carbon pool consists mainly of dissolved CO2 , bicarbonate (HCO3-), carbonate (CO32-) and carbonic acid (H2CO3).
Ocean CO2 uptake processes
The oceans’ role as a sink for CO2 is driven by two processes: the solubility pump and the biological pump. The solubility pump refers to CO2 transfer through the ocean-atmosphere interface and subsequent mixing in the upper ocean layer and transport to deeper ocean layers by large-scale ocean circulation currents. The name 'solubility pump' points to the strong dependency on the CO2 solubility in seawater and the thermal stratification of the ocean. The biological C pump refers to the uptake of CO2 by marine plankton from the surface waters through photosynthesis. Particulate material of biotic origin (POC, e.g. dead plankton cells, faecal pellets, and PIC, mainly calcium carbonate) is transferred from the ocean surface to deeper ocean layers through several processes: sinking by gravity, advection by downwelling currents and diurnal vertical migration of grazing organisms. Cold, deep waters are generally rich in dissolved inorganic C because of the increased solubility of CO2. The carbon captured in the deep ocean has a turnover time of hundreds to thousands of years. Outgassing to the atmosphere only occurs when deep water wells up to warmer equatorial regions, where the solubility of CO2 is reduced. Just a small non-mineralized fraction of the biomass produced (on the order of 0.2 Pg C/y) reaches the ocean floor and is buried in the sediment, partly as detritus and partly as calcium carbonate.
Ocean carbon sink estimate
The average net carbon uptake from the atmosphere to the global ocean in the period 1994-2007 was estimated to be ca. 2.5 Pg C/y , which equalled nearly 30% of the global carbon emissions during this period. This estimate was based on data-products from observations of surface ocean pCO2 (partial pressure of CO2) and compared to simulation results from global ocean biogeochemical models. There is growing evidence and consistency among methods with regard to the patterns of the multi-year variability of the ocean carbon sink, with a global stagnation in the 1990s and an extra-tropical strengthening of the sink in the 2000s. Explanations for this multi-year variability range from the ocean’s response to changes in atmospheric circulation (especially the variations in the upper ocean overturning) to external forcing through surface cooling associated with volcanic eruptions and variations in atmospheric CO2 growth rate. Fossil fuel CO2 emissions have increased to 10 Pg C /yr in 2019 and the atmospheric CO2 concentration has reached an unprecedented level of 415 parts per million in 2022. However, the fraction of emitted CO2 remaining in the atmosphere has been fairly stable at about 45% on average since 1958. The ocean has sequestered about 25% of cumulative CO2 emissions in the period 2010-2019. The land has sequestered 30% of cumulative emissions over the same period, but has also released a substantial amount (order 50%) of CO2 by land-use change emissions.
Methods for increasing the ocean carbon sink
Several methods have been proposed to increase artificially the ocean carbon sink. We shortly mention here three proposals.
- Large scale cultivation of seaweed in nutrient-rich coastal upwelling zones. The biomass produced is then sunk into the deep ocean where the carbon remains trapped for hundreds to thousands of years. However, scaling up seaweed aquaculture can have negative consequences, including disruption of the natural ecosystem and the diversion of nutrients from wild food webs. See also Seaweed (macro-algae) ecosystem services.
- Ocean fertilization. Soluble iron salts or ferrous dust are added to surface waters where it is currently lacking, particularly in mid-ocean gyres and the Southern Ocean. This artificially increases primary production and biomass in these ocean regions. However, when applied at the required scale, the efficiency of this method is questionable and side effects on remote ecosystems are possible.
- Ocean alkalinization. The concentration of carbonate or hydroxide ions in surface water is artificially raised to shift the associated chemical equilibria in seawater, thus increasing oceanic uptake of atmospheric CO2 and reducing ocean acidification. The feasibility and effectiveness of adding alkalinity at the required scale are questionable and the effects are highly uncertain.
Climate change impact on the carbon sink
Enhanced temperature stratification due to global warming will reduce the mixing of nutrients into the euphotic surface layer of the ocean. High temperatures and low nutrient concentrations give a competitive advantage to small phytoplankton groups that are more labile and less likely to sink than larger plankton species. The carbon sequestration by the biological pump is therefore expected to decrease.
- Blue carbon revenues of nature-based coastal protection
- Ecosystem services
- Governance policies for a blue bio-economy
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