Prospects for ocean sequestration of carbon dioxide Andrew Watson School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK

Carbon cycle in the 21st Century. We are currently releasing about 7.5 Gt carbon per year from fossil fuels (6 GtC yr -1 ) and deforestation (1.5 GtC yr -1 ) Projected temperature increases are between 1.5 and 5.8°C by the end of this century. The first priority is to reduce our emissions. Reductions of 50% overall are needed to avoid the prospect of extreme climate change. The developed countries must be prepared to cut by much higher amounts -- maybe 90%. Given the difficulties involved in making such deep cuts by energy efficiencies alone, sequestration strategies need to be investigated and implemented if feasible.

Surface waters 1000 The land biota contain about 550Gt C carbon. We might conceivably enhance this by 20%. This represents only about 20 years of carbon emissions. Sequestration in the land biota is not therefore a long-term solution to global warming. The oceans contain ~40,000 Gt C and have a much greater capacity than the land biosphere. Land versus ocean

After a few hundred years, almost all the the CO2 released into the atmosphere ends in the deep sea, where it causes a relatively small increase in the total carbon content. The model run at right shows what happens to atmospheric and deep ocean CO 2 concentrations in response to a release to the atmosphere that increases for 150 years and then stops abruptly. Where will the CO 2 go in the end?

1) CO 2 capture followed by deep ocean sequestration. Options for ocean sequestration 2) Iron fertilization to enhance marine biological sink.

Capture of CO 2. The most expensive part of capture-sequestration technology. Options pre-combustion capture (conversion of carbon- containing fuels to H 2 by catalysed reaction with water for instance). Post-combustion capture -- removal from flue gas. Post-combustion removal at large power stations would cost between $30-40 per tonne of CO 2 emission avoided and be about 80% efficient. Transport and storage add a further ~$10 per tonne.

CO 2 Density of liquid CO 2 compared to seawater. Liquid CO 2 is very compressible. Injected at ocean depths between 2 and 3 km (pressure 20-30MPa) it would be more dense than the surrounding

Three options for disposal of CO 2 in the deep ocean

Deep disposal -- unknowns and uncertainties How efficiently will it sequester CO 2 ? Depends on location and depth of release -- for well-chosen sites, sequestration for > several hundred years. Deeper is better. Pacific probably better than Atlantic. What will be the effect on ocean biology? Deep lakes would kill nearby benthic fauna Mid-water releases would disperse more rapidly, but reduce pH near to outlets, impacting meso- pelagic fauna.

Iron fertilization Experiments during the 1990s have shown that in the equatorial Pacific and Southern Ocean, substantial plankton blooms can be stimulated by addition of nanomolar concentrations of iron to surface water. Plankton have a very low requirement for iron with C:Fe ratios ~ 250,000. Potentially therefore, 1 mole of iron will remove 250,000 moles CO 2 !

SOIREE; Feb 1999 Location

HNLC zones marked by surface phosphate.

Where is a good place to fertilise? All the HNLC zones appear to be iron limited. However, only in the Southern Ocean is it likely that CO 2 will be permanently removed from the atmosphere by iron fertilization. This is because the other zones have light water that is trapped at the surface for decades. During this time it will receive sufficient iron from atmospheric dust to be fertilised naturally. Polar front Subtropical front NorthSouth

HNLC zones marked by surface phosphate.

No fertilization Equatorial Pacific Southern Ocean CO2 in the atmosphere (ppm)

Summary Capture and sequestration: High capacity Relatively expensive Iron Fertilization: Both methods have as yet unknown impacts on the ocean environment. There is strong, emotive resistance from environmentalists to the use of the oceans as a dumping ground (e.g. Brent Spar). Low capacity (realistically only a few percent of global emissions). Cheap (estimated cost ~ $3 per tonne of CO 2 ) and low-tech.. The amount sequestered is not easily monitored.

Northern Hemisphere Data from instruments (red) or from tree rings, corals and historical records (blue)

Drawdown of atmospheric CO2 via iron-enhanced marine production The Southern Ocean would be much the most effective as a sink for CO 2. Other regions, the very slow transport is into the Southern Ocean means that the water upwelling there is trapped at the surface and eventually naturally fertilised by iron in dust. Polar front Subtropical front NorthSouth