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.

Atm osphere 750

LandB iota550

Soils andDetritus1500

rivers0.7 W arm surfa ce ocean

850 B iota 2

In term ediate and deepwaters 38000

231

22

0.2

1820

0.2

50.7

100.7 50 5070 71.5

Steady-state pre-industrial carbon cycle

Biota 1

1

Cold surfaceocean 150

19

20

109

19.2

50.2

90.590

50

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 CO2 concentrations in response to a release to the atmosphere that increases for 150 years and then stops abruptly.

Where will the CO2 go in the end?

1) CO2 capture followed by deep ocean sequestration.

Options for ocean sequestration

2) Iron fertilization to enhance marine biological sink.

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.

Seawater

@2°C@0°C

@-2°C

CO2

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.

@10°C

Three options for disposal of CO2 in the deep ocean

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.

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!

SOIREE; Feb 1999Location

2

1

0 .5

0 .25

P O 4 uM :

“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 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.

Atm osphere

NA DWAAIW

AABW

M ain therm ocline

Polar frontSubtropicalfront

North South

2

1

0 .5

0 .25

P O 4 uM :

“HNLC” zones marked by surface phosphate.

300

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1950 2000 2050 2100 2150

No fertilization

Equatorial Pacific

Southern Ocean

CO

2 in t

he a

tmosp

here

(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 CO2) 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 CO2. 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.

Atm osphere

NA DWAAIW

AABW

M ain therm ocline

Polar frontSubtropicalfront

North South