The air contains greenhouse gases such as CO2, which are now known to be responsible for global warming because their concentration has risen continuously for a number of years. In contrast to the atmosphere, the concentration of CO2 in the oceans is sixty times higher.
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.
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 mix-ing 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 re-searchers 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 spec-trometers. 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.
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 anthropogenic warming causes additional critical feedback on the CO2 balance.”