The oceans exchange large amounts of carbon dioxide (CO₂) with the atmosphere.

• The net ocean-atmosphere exchange of CO₂ is currently estimated as an ocean uptake of 2.2 ± 0.5 gigatonnes of carbon per year.
• The oceans remove about a quarter of atmospheric CO₂ emissions from human activities.
• The partial pressure of CO₂ (pCO₂) in the surface ocean is expected to increase to double its pre-industrial value by the middle of this century, driven by increasing concentrations of atmospheric CO₂.

The role of the ocean in removing carbon dioxide from the atmosphere and the physical, chemical and biological processes responsible for this, are vulnerable to the effects of climate change.

• Rising sea surface temperatures will decrease the solubility of CO₂ in sea water. Consequently, the oceans will draw down CO₂ from the atmosphere less efficiently.
• Increases in the sea water CO₂ levels will themselves change the chemical buffering capacity of sea water, reducing its capacity for CO₂ uptake.
• The likely consequence of these two effects is that the fraction of CO₂ emissions taken up by the oceans will decrease in future, accelerating the rate of atmospheric CO₂ increase.
• The impacts of changes in other processes that influence the air-sea exchange of CO₂, such as stratification, upwelling, ocean circulation and primary production, are less well known.
• Recent studies suggest that the efficiency of CO₂ uptake by the oceans may be decreasing in some oceanic regions (e.g. Southern Ocean), but not in others.

Continental shelf seas play a key role in the global carbon cycle, linking the terrestrial, oceanic and atmospheric carbon pools.

• Evidence from measurements and modelling suggests that the North West European Shelf acts primarily as a sink for atmospheric CO₂.

Monitoring changes in surface-ocean CO₂ and the exchange of CO₂  between ocean & atmosphere can provide an early warning of changes in the ability of the ocean to absorb CO₂, and consequent feedbacks to the global climate system.

• The UK research community runs pCO₂ monitoring systems on five national research vessels and two Voluntary Observing Ships. Funding is unsure beyond 2010/11.

What is already happening: Low

Given rising atmospheric CO₂ and increasing global surface ocean temperatures confidence in the ocean surface CO₂ response is of low to medium:

• solubility response to temperature is well known (high confidence)
• carbonate chemistry response to a shift in pCO₂ is well known (high confidence)
• primary production responses are not well understood (low confidence)
• the influence of other processes (e.g. mixing, upwelling, stratification, circulation) are not well understood (low confidence)

Estimates of current global CO₂ uptake by the oceans are based on both direct and indirect measurements (medium confidence)
Rates of change in surface ocean pCO₂ and oceanic CO₂ uptake are documented for a few regions for relatively short time intervals, but are not well known globally and for longer periods (low confidence).

What could happen: Low

Assessment of future changes in CO₂ uptake is based on modelling studies. Most of these studies take little or no account of feedback processes. Confidence in these assessments is low.

The top priority knowledge gaps that need to be addressed in the short term to provide better advice to be given to policy makers are:

1. Relative importance of, and understanding of interactions between, processes which cause change in the oceanic uptake of atmospheric CO₂.
2. Mechanisms and magnitude of associated feedback processes on the climate system
3. Long-term monitoring of oceanic CO₂ uptake and ocean acidification by measurements of oceanic CO₂ parameters and of the atmospheric CO₂ and oxygen contents.

Can we value the role of the oceans CO₂ storage capacity?

Only a preliminary valuation of the oceans' storage capacity for CO₂ has been undertaken. The 'goods and services' approach being used is common to socio-economic analysis of the environment. Ocean CO₂ uptake is considered as part of the service 'gas and climate regulation'. Its economic value is estimated using marginal damage costs avoided, based on current carbon market values. The approach is popular in the environmental economics literature which gives it a high confidence but arguments regarding the discount rate to use, reduce confidence, resulting in a medium-to-high confidence for the method. The application of these methods to carbon cycling in the oceans is still in its infancy, so current estimates should be treated cautiously.

An assessment by Beaumont et al. (2008) of the 'goods and services' provided by marine biodiversity in UK waters gave a figure for 'gas and climate regulation' of between about £0.5 billion and £9 billion per annum. However, this is considered an underestimate because primary production by marine phytoplankton was the only process considered and confidence in the cost estimate should be considered low. Furthermore, the current role of the biological carbon pump in shelf seas for cycling anthropogenic CO₂ is not determined, so this estimate only relates to natural cycling of CO₂.

Hardman-Mountford, N., Litt, E., Mangi, S., Dye, S., Schuster, U., Bakker, D., and Watson, A. (2009) Ocean uptake of carbon dioxide (CO2), MCCIP Briefing Notes, 9pp. www.mccip.org.uk