Understanding the marine ecosystem that drives biogeochemical general circulation

The annual amount of carbon dioxide (CO2) generated by anthropogenic activities is said to be approximately 7.3 gigatons*1. Of this amount, the ocean's uptake is approximately 2 gigatons. The Carbon Cycle Research Group of the Global Warming Research Program has developed a general circulation*2 model that represents the marine ecosystem dynamics, and is engaged in precise estimation of the amount of CO2 uptaken by the oceans. Dr. Yamanaka describes the importance of clarifying the marine ecosystems and carbon cycle process*3, and presents the group's future plans.

Dr. Yasuhiro Yamanaka Group Leader
Carbon Cycle Research Group, Global Warming Research Program

Prediction research using a new model that representsthe marine ecosystem more precisely

 
Our research group studies changes to the marine ecosystems such as plankton and to CO2 circulation due to global warming, and how the CO2 absorption in the oceans shifts accordingly. Until now, we used a biogeochemical general circulation model in which the activity of plankton is simplified. Currently, we have been developing a new biogeochemical general circulation model that can represent the marine ecosystem and sinking particles more precisely by classifying the various groups of plankton. One factor is an increase in the "coccolithophorids" plankton that favors warm ocean areas with small amounts of nutrients. This increase is due to warming on the ocean surface water that leads to an increase in CO2 concentration*4 and thus likely reduces the amount of CO2 uptaken in the ocean.

 
Participation of the ocean carbon cycle model in our international research project

 
According to the IPCC Third Assessment Report published in 2001, the amount of CO2 uptaken in the ocean was "2 gigatons ± 0.6" after applying the values estimated by ten research groups from countries around the world. Our group was among the ten, and one of our members was among the few contributing authors from Japan in the field of atmosphere and ocean science for the Third Assessment Report.

The Groups from different countries studying the carbon cycle formed a partnership to establish an international research project called OCMIP*5. We are involved in the task force that will determine the model and calculation method and expression of the analytical results to be used within the organization.

In the IPCC Fourth Assessment Report, a predicted uptake of anthropogenic CO2 will be presented using the aforementioned new biogeochemical general circulation model that represents the marine ecosystem and sinking particles more precisely.

 
Accurately estimating the CO2 cyclesby setting a more realistic model
 
One of the new research subjects we are engaged in is an experiment focused on the "copepods" zooplankton, which is born in the ocean deep layer at a depth of 400 m-1,500 m and rises to the ocean surface as it grows. This plankton matures by eating phytoplankton near the ocean surface during spring and summer, and it returns to the ocean deep layer to breed in the autumn. Through numerical calculation using the ecosystem model, we found that carbon transport associated with the seasonal migration of copepod is equal to an approximate 10% of marine snow. We will study changes brought about by global warming on marine resources such as fish.

  • Image of the carbon cycle in the ocean

  • Differences in the partial pressure of CO2 between atmosphere and ocean surface
    (Unit: ppm)
The positive value near the equator indicates that the CO2 concentration in the ocean is high and that CO2 is released into the atmosphere. The negative value at the high latitudes indicates that CO2 is uptaken by the ocean.

*1 1 gigaton=1 billion tons
*2 A part of the organic matter produced by phytoplankton. Some parts become an organic matter that exists in a dissolved form in the ocean. The other part settles into the deep layer of the ocean in particle form and eventually decomposes into inorganic matter, which is used again by phytoplankton. The accumulation of these many processes is called general circulation.
*3 Explanation-Why is it important to clarify the marine ecosystem and carbon cycle process to predict global change?
It is believed that oceans contain approximately 50 times more CO2 than that in the atmosphere. Marine ecosystems, including the phytoplankton, significantly contribute to this fact. Phytoplankton takes in nutrients from the ocean (nitrate, phosphate, silicate, etc.), performs photosynthesis, and produces organic matter. In the photosynthesis process, CO2 in the ocean is uptaken by phytoplankton. Thus CO2 concentrations in the ocean are reduced, and atmospheric CO2 is more readily dissolved in the ocean. Phytoplankton that becomes fecal pellets after being consumed by zooplankton falls to the ocean deep layer; it is called marine snow. In this process, CO2 is also carried to the ocean deep water, creating a difference in CO2 concentrations between the ocean surface and deep layers. In the absence of plankton activity, the CO2 concentration would be uniform throughout the ocean, and atmospheric CO2 would be calculated at twice the current amount.
*4 Cells of the coccolithophorids are covered by distinctive disc-shaped plates made of calcium carbonate. As coccolithophorids create calcium carbonate, they also produce CO2, rising the partial pressure of CO2 in the sea water.
*5 Ocean Carbon-Cycle Model Intercomparison Project
Frontier Newsletter/No.23
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