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Prospects for ocean sequestration of carbon dioxide

Prospects for ocean sequestration of carbon dioxide. 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 ).

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Prospects for ocean sequestration of carbon dioxide

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  1. Prospects for ocean sequestration of carbon dioxide

  2. 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.

  3. 90 90.5 Surface waters 1000 50 Land versus ocean • 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.

  4. Where will the CO2 go in the end? • The model run at right shows what happens to atmospheric and deep ocean CO2 concentrations in response to a release to the atmosphere that increases for 150 years and then stops abruptly. • 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.

  5. Options for ocean sequestration 1) CO2 capture followed by deep ocean sequestration. 2) Iron fertilization to enhance marine biological sink.

  6. Capture of CO2. • The most expensive part of capture-sequestration technology. • Options • pre-combustion capture (conversion of carbon-containing fuels to H2 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 CO2 emission avoided and be about 80% efficient. Transport and storage add a further ~$10 per tonne.

  7. Density of liquid CO2 compared to seawater. Liquid CO2 is very compressible. Injected at ocean depths between 2 and 3 km (pressure 20-30MPa) it would be more dense than the surrounding seawater. Seawater CO2 @10°C @2°C @0°C @-2°C

  8. Three options for disposal of CO2 in the deep ocean

  9. Deep disposal -- unknowns and uncertainties • How efficiently will it sequester CO2? • 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.

  10. 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 CO2!

  11. SOIREE; Feb 1999 Location

  12. “HNLC” zones marked by surface phosphate.

  13. Subtropical front Polar front North South 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 CO2 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.

  14. “HNLC” zones marked by surface phosphate.

  15. 1000 900 800 No fertilization Equatorial Pacific 700 Southern Ocean 600 CO2 in the atmosphere (ppm) 500 400 300 1950 2000 2050 2100 2150

  16. Summary • Capture and sequestration: • High capacity • Relatively expensive Iron Fertilization: • Low capacity (realistically only a few percent of global emissions). • Cheap (estimated cost ~ $3 per tonne of CO2) and low-tech.. • The amount sequestered is not easily monitored. 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).

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