The Anthropogenic Ocean Carbon Sink Alan Cohn March
The Anthropogenic Ocean Carbon Sink Alan Cohn March 29, 2006 http: //science. hq. nasa. gov/oceans/images/carbon_pyramid. jpg
Due to oceanic and land sinks, less than 1/2 of CO 2 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/Carbon_cycle-cute_diagram. jpeg
O 2/N 2 technique used in 2001 IPCC report has come under scrutiny due to inconsistencies in observed oceanic oxygen concentrations with CO 2 inventory. • More recent efforts have based CO 2 inventories on “age” of water, carbonate chemistry
Mc. Neil et al. (2003) used CFC concentrations to estimate age of water masses • Problems with this method. • CFCs around for limited time dating prior to ~30 yrs impossible • Don’t account for Revelle factor
Revelle factor is defined as • Describes how partial pressure of CO 2 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
Sabine et al. (2004) used direct measurements of DIC to assess changes in anthropogenic ocean CO 2 • Used carbon tracer ∆C* method to separate anthropogenic component http: //www. sciencemag. org/cgi/content/full/sci; 305/5682/367
North Atlantic had highest vertically integrated CO 2 concentrations with 23% of global oceanic CO 2 • Mostly due to the rapid sinking of cold water http: //www. sciencemag. org/cgi/content/full/sci; 305/5682/367
Southern Ocean south of 50°S only contains 9% of global inventory • Southern Hemisphere oceans, however, contain about 60% of total oceanic CO 2 inventory, mostly due to immense area http: //www. sciencemag. org/cgi/content/full/sci; 305/5682/367
Highest concentrations of anthropogenic CO 2 are found near surface, since CO 2 enters ocean by airsea gas exchange • Variations in surface CO 2 concentration related to how long water has been exposed to atmosphere and Revelle factor
Capacity for ocean waters to take up anthropogenic CO 2 is inversely related to the Revelle factor • Highest anthropogenic CO 2 concentrations found in subtropical Atlantic due to low Revelle factor • North Pacific has high Revelle factor lower anthropogenic CO 2 concentrations
Revelle factor limits uptake so that ocean CO 2 inventory is significantly lower than what it would be if one was to neglect its influence
CO 2 concentrations at depth determined by how rapidly near-surface anthropogenic CO 2 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 CO 2 concentration when water was last in contact with surface also important
High anthropogenic CO 2 concentrations off Antarctica due to: • High winds • Low initial anthropogenic CO 2 content of water • Rapid sinking Deepwater has low concentrations because: • High Revelle factor • Limited contact with surface • Dilution with older waters
Sabine et al. suggest that land has been net source while ocean is only true net sink • They estimate atmospheric CO 2 would be about 55 ppmv higher today if it weren’t for oceanic uptake • What if oceanic uptake slows down? http: //www. windows. ucar. edu/earth_science/images/ocean_currents 1. jpg
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
Negative feedbacks are mostly chemical • Warming More stratification Transport into the interior slows down • Mc. Neil et al. suggest that this will not have much effect on oceanic uptake • Chemical: Greater PCO 2 of surface ocean Decrease in carbonate ion concentration Increase in Revelle factor decreased ability to absorb CO 2
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 CO 2 flux • Scenario A 1 B: Balanced energy sources – Fossil Fuel emissions increase until 2050, then decrease • Scenario A 2: Business-as-usual – Emissions increase exponentially
In balanced energy sources model, mixing of CO 2 in deep ocean maintains slower surface CO 2 increase Oceanic sink steadily increases Atmospheric CO 2 concentrations in 2100 of 661 ppmv Temperature increase of 1. 21 K Oceanic CO 2 fraction in 2100, compared to land & airborne CO 2, is 24%
In business-as-usual scenario, carbon sequestration in land ocean can’t keep up with emissions Capacity of sinks decreases as CO 2 increases. Atmospheric CO 2 concentrations in 2100 of 792 ppmv Temperature increase of 1. 42 K Oceanic CO 2 fraction in 2100 is 21%
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 (g. C/m 2) http: //www. pnas. org/cgi/content/full/102/32/11201
Because they did not account for the Revelle factor, they may have overestimated oceanic uptake and underestimated atmospheric CO 2 concentrations! More model studies are need that account for ocean acidification Caldeira & Wickett, Nature, 2003
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 CO 2 in the oceans, Global Biogeochemical Cycles, 10, 809 -837. Houghton et al. (2001) Climate Change 2001: Synthesis Report, Cambridge Univ. Press, Cambridge, U. K. Mc. Neil et al. (2003) Anthropogenic CO 2 Uptake by the Ocean Based on the Global Chlorofluorocarbon Data Set, Science, 299, 235 -239. Sabine et al. (2004) The Oceanic Sink for Anthropogenic CO 2, Science, 305, 367 -371.
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