The Missing Carbon Problem (Part 3)

Oceanic sinks and feedbacks to climate change are as variable and hard to measure as their terrestrial counter parts. The oceans are in equilibrium with the partial pressure of CO₂ in seawater balanced by its concentration in the atmosphere. Carbon dioxide is transferred into the atmosphere when its molar concentration in seawater exceeds the pressure from its bulk concentration in the atmosphere. If the inverse is true, we observe a sink of CO₂ in our oceans.

Negative Feedbacks to Climate Change

The primary production of carbon dioxide in shallow oceans near continental margins hosts 30% of the ocean's total primary production. This amount is far larger than the percentage of surface area out of the total ocean that the continental shelf accounts for at roughly 7%. So without a doubt, as ocean water rises, we will see a negative feedback to atmospheric CO₂ concentrations.

Why There is Variance

The variability comes from fluvial inputs, enhanced zones of primary production, vertical mixing of water masses, saturation during night hours, and seasonal variances.

Jeopardizing Any Plausible Sequestration

Several variances seem to downplay any plausible sink. First, during the winter, primary production is negligible and CO₂ accumulates near the surface. Second, during the spring when river discharges are high, fresh water or fluvial input from rivers and estuaries increases CO₂ concentration and jeopardizes the possible sink. Third, when vertical mixing or upwelling occurs, there is over-saturation of carbon dioxide in the surface ocean. These dynamics are highly variable and just as the above cases saturate the surface ocean, there is a counterpart that depletes the CO₂ concentration in the surface ocean.

Understand that Oceans Act Like Sponges

The oceans seem to hold a dynamic equilibrium with the world's carbon cycle. There is also a high degree of variance of CO₂ concentrations in our oceans. An exact figure of the amount of carbon that is sequestered by biological growth alone is poorly understood. Several theoretical approaches to correct for the model's imbalance suggest that vertical mixing of carbonate-rich waters sequesters an alarming amount of CO₂. It is easy to suggest that the missing carbon is fluxing into our oceans where its future long-term residence time is found in the formation of carbonate sediment beds.

What May Come?

Is this the only answer? Well, of course not. But just how quickly and how much carbon can we push into the oceans? Will there be adverse effects like rising pH and disturbances to the food chain? Will there be a dreaded shift to the thermohaline circulation? What kind of climatic impacts could such an event have in today's environment?

References:

1. Chester, Roy. Marine Geochemistry Second Edition. Malden, MA: Blackwell Science Ltd. 2000, 2003

2. Richard Houghton, Senior Scientist, Carbon Research. Understanding the Global Carbon Cycle. Woods Hole Research Center, http://www.whrc.org/carbon/

3. Hesshaimer, Vago, Heimann, Martin, Levin, Ingeborg. Radiocarbon evidence for a smaller oceanic carbon dioxide sink than previously believed. Nature (London). 370 (6486), p. 201-203, 1994.

4. Frankignoulle, Michel , Borges, Alberto V. European Continental shelf as a significant sink for atmospheric carbon dioxide. Global Biochemical Cycles. Pages 569-576, September 1, 2001

5. Houghton, R. A. , Davidson, E. A. , Woodwell, G.M. Missing sinks, feedbacks, and understanding the role of terrestrial ecosystems in the global carbon balance, Global Biochemical Cycles. Vol. 12, No. 1, Pages 25-34, March 1998

6. Tsunogia, Shizuo , Ono, Tsuneo , Watanabe, Shuichi. Increase in total carbonate in the western North Pacific water and a hypothesis on the missing sink of anthropogenic carbon, Journal of Oceanography, Vol. 49, Pages 305-315, 1993

7. Burdige, David J. , Alperin, Marc J., Homstead, Juliana, Martens, Christopher S., The role of benthic fluxes of dissolved organic carbon in oceanic and sedimentary carbon cycling. Geophysical Research Letters, Vol.19, Pages 1851-1854, 1992