Much of the carbon dioxide (CO2) was hidden in the ocean, which explains its low atmosphere concentration during the last Ice Age 20,000 years ago, researchers say.

Climate researchers from the Universities of Bern (Switzerland) and Grenoble (France) and the Alfred Wegener Institute for Polar and Marine Research (Germany) found this close connection between CO2 and temperature has existed over the past 800,000 years.

“We have now been able to identify processes in the ocean which are connected to the observed rise in CO2,” said Jochen Schmitt, from Oeschger Centre for Climate Change Research, University of Bern, who led the study, the journal Science reported.

According to Schmitt, during the Ice Age more and more CO2 accumulated in the deep ocean, causing the concentration of atmospheric CO2 to drop, said a university statement.

Only at the end of the Ice Age was this stored CO2 transported back to the sea surface through changing ocean circulation and thus emitted back into the atmosphere, the researchers wrote.

A new method for isotope measurements has now made it possible for the first time “to reliably decode the fingerprint of the CO2 preserved in the ice,” explained Schmitt.

He and his colleague Hubertus Fischer initially developed these new isotope measurement methods for ice cores at the Alfred Wegener Institute and further refined them in many years of research work after moving to Bern.

Using the new method the glaciologists extract the air trapped in the ice core completely and the CO2 contained in the air is thoroughly cleaned.