Ocean Storage

What is Ocean Storage of CO2? Over the last two centuries, human activities such as fossil fuel emissions, biomass burning, and land-use changes have profoundly impacted the global carbon cycle. Present atmospheric CO2 levels are now higher than this planet has experienced for at least the last 20 million years. One method being considered to reduce atmospheric concentrations of CO2 is to remove it from the atmosphere and to store it in natural reservoirs out of contact with the atmosphere, such as deep geological formations or the deep ocean. This is often referred to as ocean carbon sequestration, or ocean storage of carbon.

Why the Oceans? The oceans have an enormous natural capacity to absorb and store carbon. The ocean contains an estimated 40,000 GtC (GtC = a billion tons of carbon). To put this in perspective, the atmosphere contains approximately 750 GtC and the land contains about 2200 GtC. This means that if we were to take all the atmospheric CO2 and put it in the deep ocean, the concentration of CO2 in the deep ocean would change by less than 2%.

What are the concerns? There are many scientific, legal, political, economic, and ethical issues that must be thoroughly investigated to determine the best balance between minimizing the most serious effects of human-induced climate change and protecting the natural environment.

At present, mitigation options such as ocean carbon sequestration are considered to be too costly. It is estimated that deep-ocean storage of CO2 would cost between $100-300 per ton of carbon, similar to the price of geological sequestration. To be economically competitive with current energy prices, the price would have to decline to approximately $10 per ton. The legality of injecting CO2 into the ocean is unclear. Some international conventions encourage investigating the possibility of storing carbon in reservoirs such as deep aquifers or the ocean, while others label CO2 an industrial waste and thus ban dumping it in the ocean. The acceptability of ocean storage of CO2 revolves around society's perception of the risks and benefits of such an activity. Several recent attempts to conduct small-scale research experiments in the ocean have been abandoned because of strong public objection, suggesting that public opinion may be the most important consideration in this debate.

Accelerating the natural uptake of CO2 by marine phytoplankton may lead to large changes in the ocean ecosystem. Marine phytoplankton assimilate carbon from seawater as they grow. When these organisms die, they sink to the deep ocean carrying the carbon with them. This is referred to as the "biological pump" for carbon. In many regions of the ocean, phytoplankton growth is limited by lack of an essential micro-nutrient, iron. Four small-scale experiments over the last 10 years have shown that introducing iron to these regions can stimulate phytoplankton growth to 20-30 times the natural rate. A number of private companies are attempting to use this technology to increase the rate at which carbon is transferred from the surface to deep ocean and to sell the resulting decrease in atmospheric carbon as credits in the developing carbon markets. Scientists argue, however, that this method is inefficient at sequestering atmospheric carbon, because very little of the carbon assimilated in the organism actually makes it to the deep ocean. Instead, as the organism sinks through the water column, it decays and releases the carbon back into the water at shallow and intermediate depths, where it can readily be mixed back up into contact with the atmosphere, thus short-circuiting the biological pump to the deep sea. It is estimated that fertilizing the entire Southern Ocean with iron for 100 years would only reduce atmospheric CO2 levels by about 20-30%. Fertilization would also lead to significant ecological perturbations. When organisms die and decompose, oxygen is consumed. Creating an unnatural abundance of decomposing organisms would lead to low oxygen levels that could be devastating to marine life.

Scientific research on direct injection of CO2 into the deep ocean is still extremely limited, and scientists do not fully understand the chemical behaviour of CO2 in the deep ocean or the efficiency of this technique for isolating CO2 from the atmosphere over several centuries. Models of ocean circulation indicate that CO2 injected at a depth of 3000 meters would remain out of contact with the atmosphere for about 200 years. As the depth of injection decreases, so does the storage efficiency. Between 800 and 3000 meters, a stream of liquid CO2 is less dense than the seawater around it and it tends to rise to the surface, slowly mixing and dissolving into the surrounding water. Below about 3000 meters, the liquid CO2 reacts with the seawater to form a clathrate, a solid, ice-like substance that is denser than the surrounding water. These chemical reactions at depth could yield a number of benefits for reducing potential environmental and biological impacts, yet there is still much that is not known. Much has been written and theorized about how CO2 would behave at depth, the consequences of this for chemical interactions with the water and sediments, and the possible effects on marine organisms. But the few ocean experiments that have been conducted yielded many unexpected results and scientists argue that the only way to understand these new and complex interactions is through continued, small-scale experiments.

Perhaps the most important concern is the effect that ocean storage of carbon might have on deep marine organisms. As the concentration of CO2 increases, the pH of the water decreases and the water becomes more acidic. Deep-sea organisms generally have slow metabolisms and would be incapable of adapting to rapid changes in environmental conditions. There is little research on the effects of decreased pH on marine organisms, especially in the deep-sea. Ocean models suggest that the natural uptake of atmospheric CO2 by the oceans will decrease pH levels by 0.4 - 1 pH units over the next 500 years. Since the industrial revolution, the pH of ocean water has already decreased by about 0.1 pH units. Some scientists suggest that the most severe impacts of deep-sea injection of CO2 may be avoided, either by engineering the injection system so that the CO2 stream entering the water is not so concentrated, or through injecting the CO2 sufficiently deep that it forms an ice-like clathrate that dissolves very slowly.

What if we do nothing? Because of the natural uptake of CO2 by the surface ocean, about 85% of the CO2 released from fossil fuel burning will eventually end up in the ocean. Some scientists argue that it is not a question of whether we want to put anthropogenic CO2 in the oceans - it's already happening. The large majority of marine life resides in the upper ocean and could be strongly affected by not only climate-induced changes such as increased temperatures and modified circulation, but also by increasing acidity. Organisms may be able to adapt to this slow invasion of CO2, although the ultimate effects on the ecosystem composition and food-web are unknown.

Summary: No one wants to pollute the oceans and endanger marine life. But in the process of polluting the atmosphere, we have already set in motion the large-scale penetration of CO2 into the surface oceans. The natural interactions and exchanges between the ocean and atmosphere are not fully understood. We do know, however, that we are powerless to stop or slow them. We are faced not with the choice to pollute or not to pollute, but rather with deciding which scenario of pollution will cause the least damage. Is it better to let the CO2 continue to penetrate into the surface ocean where the majority of marine life lives ? Will organisms and the food-web be able to adapt to this slow change in the environment ? What will the ultimate effect of those changes be on the ecosystem, on climate, on fisheries, on human health ? Would it be better to try to protect the surface ocean by artificially placing the CO2 in the deep-ocean ? What affect would this have on marine organisms there ? How effective would this storage be ? Would the CO2 eventually reach the surface ocean anyway ? Would removing CO2 from the atmosphere encourage society to continue polluting the atmosphere, or would it allow us to alleviate some of the worst climate-change effects while transitioning to cleaner energy sources ? Science is currently unable to adequately address these questions. Because of the global impact of this issue, it cannot be addressed by any one nation or by special-interest sectors such as the energy industry or environmental groups. It must be addressed at an international and intergovernmental level to provide sound, un-biased answers that society needs in order to make the appropriate choices.

Further Reading: Peer-reviewed scientific literature (see also Bibliography section)

Alendal, G., and H. Drange, Two-phase, near-field modelling of purposefully released CO2 in the ocean. Journal of Geophysical Research, 106, 1085-1096, 2000.

Boyd, P.W., et al., A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization, Nature, 407, 695-702, 2000.

Brewer, P.G., F.M. Orr, G. Friederich, K.A. Kvenvolden, et al., Gas hydrate formation in the deep sea: in situ experiments with controlled release of methane, natural gas, and carbon dioxide. Energy & Fuels, v12, 183-188, 1998.

Brewer, P.G., G. Friederich, E.T. Peltzer, and F.M. Orr, Jr., Direct experiments on the ocean disposal of fossil fuel CO2. Science, 284, 943-945, 1999.

Caldeira, K., and G.H. Rau, Accelerating carbonate dissolution to sequester carbon dioxide in the ocean: Geochemical implications, Geophysical Research Letters, vol. 27, 225-228, 2000.

Caldeira, K., M.E. Wickett, and P.B. Duffy. Depth, radiocarbon and the effectiveness of direct CO2 injection as an ocean carbon sequestration strategy. Geophysical Research Letters, Geophysical Research Letters, 10.1029/2001GL014234, 2002.

Chisholm, S.W., P.G. Falkowski, and J.J. Cullen, Dis-crediting ocean fertilization. Science, 294, 309-310, 2001.

Coale, K.H. et al., A massive phytoplankton bloom induced by an ecosystem scale iron fertilization experiment in the equatorial Pacific Ocean, Nature, 495-501, 1996.

Drange, H., G. Alendal, and O.M. Johannessen, The Norweigan Sea as a possible site for ocean release of fossil fuel CO2, Geophysical Research Letters, in press, 2002.

Haugan, P. M. and H. Drange, Sequestration oof CO2 in the deep ocean by shallow injection. Nature 357, 318-320, 1992.

Herzog, H. et al., Carbon Sequestration via Direct Injection, in the Encyclopedia of Ocean Sciences, J.H. Steele et al. (eds.), vol. 1, pp. 408-414, Academic Press, London, 2001.

Herzog, H., K. Caldeira and E. Adams, Carbon Sequestration via Direct Injection. In J H Steele, S A Thorpe and K K Turekian (eds) Encyclopedia of Ocean Sciences Vol. 1, pp 408 - 414. London, UK: Academic Press, 2001.

Herzog, H.J., Caldeira, K., and Reilly, J., An issue of permanence: Assessing the effectiveness of temporary carbon storage. Climatic Change, in press (2003).

Hoffert, M.I., K. Caldeira, G. Benford, D.R. Criswell, C. Green, H. Herzog, J.W. Katzenberger, H.S. Kheshgi, K.S. Lackner, J.S. Lewis, W. Manheimer, J.C. Mankins, G. Marland, M.E. Mauel, L.J. Perkins, M.E. Schlesinger, T. Volk, and T.M.L. Wigley, Advanced technology paths to global climate stability: Energy for a greenhouse planet. Science 295, 981-987, 2002.

Marchetti, C., On geo-engineering and the CO2 problem. Climate Change 1, 59-68, 1977.

Martin, J. H., K. H. Coale, K. S. Johnson, S. E. Fitzwater, et al., Testing the Iron Hypothesis in Ecosystems of the Equatorial Pacific Ocean. Nature, 371, 123-129,1994.

Martin, J.H. and S.E. Fitzwater, Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic, Nature, 331, 341-343, 1988.

Martin, J.H., R.M. Gordon and S.E. Fitzwater, The case for iron. Limnology and Oceanography, 36, 1793-1802, 1991.

Rau, G.H., and Caldeira, K. Enhanced carbonate dissolution: A means of sequestering waste CO2 as ocean bicarbonate. Energy Conversion and Management 40, 1803-1813, 1999.

Rau, G.H., and K. Caldeira, Minimizing effects of CO2 storage in oceans (letter). Science 276, 275-276, 2002.

Sarmiento, J.L., and J.C. Orr, Three-dimensional simulations of the impact of Southern Ocean nutrient depletion on atmospheric CO2 and ocean chemistry, Limnology and Oceanography, 36, 1928-1950, 1991.

Wickett, M.E., K. Caldeira and P.B. Duffy, High-resolution simulations of oceanic direct-injection of anthropogenic CO2 and CFC uptake. Journal of Geophysical Research (Oceans), in press.

Further Reading: General interest articles, magazines, news papers, speeches

Brewer, P.G., Contemplating Action: Storing Carbon Dioxide in the Ocean. Roger Revelle Commemorative Lecture, National Academy of Sciences, 1999.

Chang, K., A new strategy to help capture greenhouse gas. In the New York Times Science, July 17, 2001.

Sarmiento, J. and N. Gruber, Sinks for Anthropogenic Carbon. Physics Today, 2002.

Sciama, Y., Stocker le carbone dans les abysses. La Recherche, 355, 93-94, 2002.

Carbon sequestration: Fired up with ideas, The Economist, July 6, 2002.