Could Warmer Oceans Make Atmospheric Carbon Dioxide Rise Faster Than Expected?

In the global carbon cycle the sea absorbs a proportion of the atmospheric CO₂ but also releases CO₂ into the atmosphere again. About half of the anthropogenic emission of CO₂ is absorbed naturally by the oceans. Thus it is all the more important to understand how the exchange of CO₂ between the ocean and the atmosphere functions with regard to a world that is warming up. The newly available study shows that the ocean was able to store more CO₂ during the ice age than it can today.

Practically static bodies of water

Together with North American colleagues, an ETH Zurich research team made measurements on sea bed sediments. These sediments originate from mountains lying at a depth of about three kilometres below the water surface of the sub-Arctic Pacific Ocean. At that point the water temperatures are close to freezing and the conditions are very stable, because there is practically no mixing between the deep bodies of water and the surface water. The circulation of the water is measured using the radio-carbon method, which is based on the radioactive decay of the carbon isotope 14C. Measurements showed that the deep Pacific water has not been at the surface for more than 2,000 years.

Tiny single-celled organisms betray the CO₂ level

To find out how the situation has changed compared to the last ice age, the researchers studied mud from the sub-Arctic Pacific Ocean lying approximately one metre below the present sea bed and about 20,000 years old. Tiny single-celled organisms with limestone shells known as foraminifera were selected from this mud under a microscope and afterwards measured with mass spectrometers. These foraminifers locked in the carbon isotope signature of the seawater of their day -- like in a time capsule. The research team has now been able to measure the 14C content precisely. This enabled them to show that the water in the ocean depths exchanged less CO₂ with the atmosphere than at present.

A sobering result

The team also looked for key indicators that provide some information about the chemical composition of the ice age water. They found unusually clear evidence that this water captured more CO₂ from the atmosphere than the water at the present day. The latest research results show that the oceans are generally able to fix more CO₂ when they are cold.

Oceans that warm up as a result of climate change release more CO₂ into the atmosphere. This discovery has far-reaching consequences for the climate. The ocean warming caused by humans contributes to the formation of additional greenhouse gases, mainly CO₂. Consequently the positive feedback with the atmosphere associated with the latter leads to an even greater acceleration in global warming.

Samuel Jaccard, Research Assistant at ETH Zurich and one of the two principal authors of the study thinks that: "With a system as complex as the climate, even if we cannot draw conclusions directly from the natural cold past that are applicable to the warm future modified by humans, our results show that anthropo-genic warming causes additional critical feedback on the CO₂ balance."

Reference: Galbraith, E.D., Jaccard, S.L., Pedersen, T.F., Sigman, D.M., Haug, G.H., Cook, M., Southon, J.R., Francois, R. Carbon dioxide release from the North Pacific abyss during the last deglaciation, Nature, 449, 890-894.