Irina Marinov

An in-depth look at how the oceans take up carbon dioxide.

Carbon dioxide is a well known greenhouse gas and a major contributor to global warming. As researchers and policy-makers struggle to find ways to reduce atmospheric concentrations of the gas, some scientists are switching their gaze from the sky to the sea. The world's oceans absorb and store a sizeable amount of atmospheric CO₂, but the exact mechanisms behind the process are not yet fully understood.

On page 964 of this issue, Irina Marinov, now based at the Massachusetts Institute of Technology, and her colleagues at Princeton University in New Jersey provide fresh insight into the oceans' ability to soak up CO₂. Marinov and her team focused on the Southern Ocean, the expanse of water that surrounds Antarctica and absorbs the lion's share of CO₂. Their work involved modelling CO₂ uptake in the ocean — but the model was inspired by the combination of two factors.

A meeting about iron fertilization of the ocean, which Marinov attended in 2003, provided the first spark. This technique involves seeding certain sections of the ocean with iron to encourage the growth of carbon-loving phytoplankton. The organisms use up and break down CO₂ and other nutrients, lowering the concentration of CO₂ in the water and so allowing more atmospheric CO₂ to dissolve in the sea.

The meeting described how researchers had been experimenting with iron fertilization in the Southern Ocean, where lack of iron prevents phytoplankton from growing. The scientists chose where to seed the ocean based on the biology and chemistry of the water. It occurred to Marinov that they might be overlooking one important factor: ocean circulation patterns.

That thought led Marinov to the second key element in her team's model: computer simulations of sections of the Southern Ocean. She was aware of simple mathematical models developed by Robbie Toggweiler and his colleagues at the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, which divided the ocean up according to circulation patterns. Most scientists tend to treat a body of water such as the Southern Ocean as a uniform system, but Toggweiler's work implied that various parts of an ocean behave differently.

So Marinov and her team combined the two ideas to produce a new model of the Southern Ocean. “The mathematical techniques we used were simple, standard even, but we used them in a more creative way,” she says.

In the model, the researchers removed most of the nutrients and associated CO₂ from the surface water to simulate increased photosynthesis by phytoplankton. This allowed them to work out the maximum amount of CO₂ that a specific body of water could take up. They then deployed the mathematical models of ocean circulation to see what would happen.

After several months of running simulations, the group found that nutrient depletion was more efficient at drawing down atmospheric CO₂ in the most southerly regions of the Southern Ocean. In this region, the circulation pattern moves the surface water down into the ocean's depths, where the sequestered carbon might be trapped for a relatively long time.

The study has implications for the design of future iron-fertilization experiments, Marinov says. “Our conclusion is that you need to focus on the Antarctic,” she says.