The Anthropogenic Ocean Carbon Sink

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The Anthropogenic Ocean Carbon Sink
Alan Cohn March 29, 2006
http//science.hq.nasa.gov/oceans/images/carbon_py
ramid.jpg
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Due to oceanic and land sinks, less than 1/2 of
CO2 emissions from industrial period remain in
atmosphere.

• How much is due to land sink and how much is due
  to ocean sink?
• How have sinks changed over time and how will
  they change in future?

http//upload.wikimedia.org/wikipedia/en/5/55/Carb
on_cycle-cute_diagram.jpeg
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O2/N2 technique used in 2001 IPCC report has come
under scrutiny due to inconsistencies in observed
oceanic oxygen concentrations with CO2 inventory.

• More recent efforts have based CO2 inventories on
  age of water, carbonate chemistry

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McNeil et al. (2003) used CFC concentrations to
estimate age of water masses

• Problems with this method.
• CFCs around for limited time
• ? dating prior to 30yrs
• impossible
• Dont account for Revelle factor

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Revelle factor is defined as

• Describes how partial pressure of CO2 in seawater
  changes for a given change in DIC

• Proportional to ratio btwn DIC and alkalinity
  (oceanic charge balance).

• Low Revelle factors generally in warm tropical
  and subtropical waters

• High Revelle factors in cold high latitude waters

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Sabine et al. (2004) used direct measurements of
DIC to assess changes in anthropogenic ocean CO2

• Used carbon tracer ?C method to separate
  anthropogenic component

http//www.sciencemag.org/cgi/content/full/sci305
/5682/367
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North Atlantic had highest vertically integrated
CO2 concentrations with 23 of global oceanic CO2

• Mostly due to the rapid sinking of cold water

http//www.sciencemag.org/cgi/content/full/sci305
/5682/367
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Southern Ocean south of 50S only contains 9 of
global inventory

• Southern Hemisphere oceans, however, contain
  about 60 of total oceanic CO2 inventory, mostly
  due to immense area

http//www.sciencemag.org/cgi/content/full/sci305
/5682/367
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Highest concentrations of anthropogenic CO2 are
found near surface, since CO2 enters ocean by
air-sea gas exchange

• Variations in surface CO2 concentration related
  to how long water has been exposed to atmosphere
  and Revelle factor

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Capacity for ocean waters to take up
anthropogenic CO2 is inversely related to the
Revelle factor

• Highest anthropogenic CO2 concentrations found in
  subtropical Atlantic due to low Revelle factor
• North Pacific has high Revelle factor ? lower
  anthropogenic CO2 concentrations

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Revelle factor limits uptake so that ocean CO2
inventory is significantly lower than what it
would be if one was to neglect its influence
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CO2 concentrations at depth determined by how
rapidly near-surface anthropogenic CO2 is
transported into the ocean interior

• Transport occurs along surfaces of constant
  density, or isopycnals
• Deepest penetration at mid-latitude convergence
  zones
• Low vertical penetration in upwelling regions
  like equatorial Pacific

Atmospheric CO2 concentration when water was last
in contact with surface also important
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High anthropogenic CO2 concentrations off
Antarctica due to

• High winds
• Low initial anthropogenic CO2 content of water
• Rapid sinking

Deepwater has low concentrations because

• High Revelle factor
• Limited contact with surface
• Dilution with older waters

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Sabine et al. suggest that land has been net
source while ocean is only true net sink

• They estimate atmospheric CO2 would be about
  55ppmv higher today if it werent for oceanic
  uptake
• What if oceanic uptake slows down?

http//www.windows.ucar.edu/earth_science/images/o
cean_currents1.jpg
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Already, there are signs of slowing

• Uptake fraction has decreased from 28-34 to 26

• Ocean has slow mixing time
• ? may not be able to keep up with emissions

• If given thousands of years, ocean would uptake
  90

• Positive and Negative feedbacks may take effect

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Negative feedbacks are mostly chemical

• Warming ? More stratification ? Transport into
  the interior slows down

• McNeil et al. suggest that this will not have
  much effect on oceanic uptake

• Chemical Greater PCO2 of surface ocean ?
  Decrease in carbonate ion concentration ?
  Increase in Revelle factor ? decreased ability to
  absorb CO2

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Fung et al. modeled future oceanic uptake using 2
different emissions scenarios

• Includes simplified form of solubility carbon
  pump, organic and inorganic carbon pump, and
  air-sea CO2 flux

• Scenario A1B Balanced energy sources
• Fossil Fuel emissions increase until 2050, then
  decrease

• Scenario A2 Business-as-usual
• Emissions increase exponentially

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In balanced energy sources model, mixing of CO2
in deep ocean maintains slower surface CO2
increase
? Oceanic sink steadily increases
? Atmospheric CO2 concentrations in 2100 of 661
ppmv
? Temperature increase of 1.21 K
Oceanic CO2 fraction in 2100, compared to land
airborne CO2, is 24
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In business-as-usual scenario, carbon
sequestration in land and ocean cant keep up
with emissions
? Capacity of sinks decreases as CO2 increases.
? Atmospheric CO2 concentrations in 2100 of 792
ppmv
? Temperature increase of 1.42 K
Oceanic CO2 fraction in 2100 is 21
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Models did not include an increasing Revelle
factor!

• Their results were based on a slowed ocean
  circulation

• Also accounted for increased biological uptake

Difference between coupled and uncoupled
carbon-climate system (gC/m2)
http//www.pnas.org/cgi/content/full/102/32/11201
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Because they did not account for the Revelle
factor, they may have overestimated oceanic
uptake and underestimated atmospheric CO2
concentrations!
More model studies are need that account for
ocean acidification
Caldeira Wickett, Nature, 2003
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References

• Fung et al. (2005) Evolution of carbon sinks in
  a changing climate. PNAS, 102, 11201-11206.
• Gruber, N., Sarmiento, J.L., and T. Stocker
  (1996) An improved method for detecting
  anthropogenic CO2 in the oceans, Global
  Biogeochemical Cycles, 10, 809-837.
• Houghton et al. (2001) Climate Change 2001
  Synthesis Report, Cambridge Univ. Press,
  Cambridge, U.K.
• McNeil et al. (2003) Anthropogenic CO2 Uptake by
  the Ocean Based on the Global Chlorofluorocarbon
  Data Set, Science, 299, 235-239.
• Sabine et al. (2004) The Oceanic Sink for
  Anthropogenic CO2, Science, 305, 367-371.