FY 2006

Ocean Storage

Caldeira, K., M. Akai, P. Brewer, B. Chen, P. Haugan, T. Iwama, P. Johnston, H. Kheshgi, Q. Li, T. Ohsumi, H. Pörtner, C. Sabine, Y. Shirayama, and J. Thomson

Chapter 6 in IPCC Special Report on Carbon Dioxide Capture and Storage, Metz, B., O. Davidson, H.C. de Coninck, M. Loos, and L.A. Meyer (eds.), Prepared by Working Group III of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 277–317 (2005)

Executive Summary

Captured CO₂ could be deliberately injected into the ocean at great depth, where most of it would remain isolated from the atmosphere for centuries. CO₂ can be transported via pipeline or ship for release in the ocean or on the sea floor. There have been small-scale field experiments and 25 years of theoretical, laboratory, and modelling studies of intentional ocean storage of CO₂, but ocean storage has not yet been deployed or thoroughly tested.

The increase in atmospheric CO₂ concentrations due to anthropogenic emissions has resulted in the oceans taking up CO₂ at a rate of about 7 GtCO₂ yr⁻¹ (2 GtC yr⁻¹). Over the past 200 years the oceans have taken up 500 GtCO₂ from the atmosphere out of 1300 GtCO₂ total anthropogenic emissions. Anthropogenic CO₂ resides primarily in the upper ocean and has thus far resulted in a decrease of pH of about 0.1 at the ocean surface with virtually no change in pH deep in the oceans. Models predict that the oceans will take up most CO₂ released to the atmosphere over several centuries as CO₂ is dissolved at the ocean surface and mixed with deep ocean waters.

The Earth's oceans cover over 70% of the Earth's surface with an average depth of about 3,800 metres; hence, there is no practical physical limit to the amount of anthropogenic CO₂ that could be placed in the ocean. However, the amount that is stored in the ocean on the millennial time scale depends on oceanic equilibration with the atmosphere. Over millennia, CO₂ injected into the oceans at great depth will approach approximately the same equilibrium as if it were released to the atmosphere. Sustained atmospheric CO₂ concentrations in the range of 350 to 1000 ppmv imply that 2,300 ± 260 to 10,700 ± 1,000 Gt of anthropogenic CO₂ will eventually reside in the ocean.

Analyses of ocean observations and models agree that injected CO₂ will be isolated from the atmosphere for several hundreds of years and that the fraction retained tends to be larger with deeper injection. Additional concepts to prolong CO₂ retention include forming solid CO₂ hydrates and liquid CO₂ lakes on the sea floor, and increasing CO₂ solubility by, for example, dissolving mineral carbonates. Over centuries, ocean mixing results in loss of isolation of injected CO₂ and exchange with the atmosphere. This would be gradual from large regions of the ocean. There are no known mechanisms for sudden or catastrophic release of injected CO₂.

Injection up to a few GtCO₂ would produce a measurable change in ocean chemistry in the region of injection, whereas injection of hundreds of GtCO₂ would eventually produce measurable change over the entire ocean volume.

Experiments show that added CO₂ can harm marine organisms. Effects of elevated CO₂ levels have mostly been studied on time scales up to several months in individual organisms that live near the ocean surface. Observed phenomena include reduced rates of calcification, reproduction, growth, circulatory oxygen supply and mobility as well as increased mortality over time. In some organisms these effects are seen in response to small additions of CO₂. Immediate mortality is expected close to injection points or CO₂ lakes. Chronic effects may set in with small degrees of long-term CO₂ accumulation, such as might result far from an injection site, however, long-term chronic effects have not been studied in deep-sea organisms.

CO₂ effects on marine organisms will have ecosystem consequences; however, no controlled ecosystem experiments have been performed in the deep ocean. Thus, only a preliminary assessment of potential ecosystem effects can be given. It is expected that ecosystem consequences will increase with increasing CO₂ concentration, but no environmental thresholds have been identified. It is also presently unclear, how species and ecosystems would adapt to sustained, elevated CO₂ levels.

Chemical and biological monitoring of an injection project, including observations of the spatial and temporal evolution of the resulting CO₂ plume, would help evaluate the amount of materials released, the retention of CO₂, and some of the potential environmental effects.

For water column and sea floor release, capture and compression/liquefaction are thought to be the dominant cost factors. Transport (i.e., piping, and shipping) costs are expected to be the next largest cost component and scale with proximity to the deep ocean. The costs of monitoring, injection nozzles etc. are expected to be small in comparison.

Dissolving mineral carbonates, if found practical, could cause stored carbon to be retained in the ocean for 10,000 years, minimize changes in ocean pH and CO₂ partial pressure, and may avoid the need for prior separation of CO₂. Large amounts of limestone and materials handling would be required for this approach.

Several different global and regional treaties on the law of the sea and marine environment could be relevant to intentional release of CO₂ into the ocean but the legal status of intentional carbon storage in the ocean has not yet been adjudicated.

It is not known whether the public will accept the deliberate storage of CO₂ in the ocean as part of a climate change mitigation strategy. Deep ocean storage could help reduce the impact of CO₂ emissions on surface ocean biology but at the expense of effects on deep-ocean biology.