Scientists have found a release of carbon dioxide stored deep in the ocean helped warm the planet and bring it out of the last ice age.
The findings will help scientists understand how the ocean affects the carbon cycle and climate change.
"The ocean currently contains about 60 times more carbon than the atmosphere - in natural conditions it is the main driver of carbon dioxide variations," said joint lead researcher, Dr Gianluca Marino, from The Australian National University (ANU) Research School of Earth Sciences.
"Carbon can exchange rapidly between the ocean and the atmosphere."
Atmospheric carbon dioxide levels fluctuate from around 185 parts per million (ppm) during the most recent ice age, to 280 ppm during warmer periods such as the last millennium. Since 1850 carbon dioxide levels have risen to nearly 400 ppm.
The team of scientists from UK and Australia reconstructed ancient carbon dioxide levels by studying levels of the element boron in shells of microfossils recovered from the ocean floor, and compared them with the atmospheric carbon dioxide levels previously measured in ice cores from Antarctica.
The team found that at the end of the last ice age, reconstructions of carbon dioxide dissolved in the ocean from tiny organisms that lived at the surface in the South Atlantic Ocean and eastern equatorial Pacific became much higher than the atmospheric levels at the time.
"This suggests the ocean was releasing carbon dioxide into the atmosphere, that had been stored deep in the ocean for the period of the ice age," said joint lead author, Dr Miguel Martínez-Botí from the University of Southampton.
Mixing between the surface and the deep ocean was weaker during the last ice age and this helped carbon remain trapped at the bottom of the ocean for millennia.
The team concluded that at the end of the last ice age, the carbon reserves deep in the Southern Ocean were re-released into the atmosphere.
Other parts of the oceans, such as the Pacific Ocean, may have played a similar role says co-author Dr Gavin Foster, from the University of Southampton.
"Our results support a primary role for the Southern Ocean, but we don't yet know the full story," he said.
"The new data confirms that natural variations in atmospheric carbon dioxide are linked to carbon stored in the oceans.
"We have observed this recently - the oceans have stored more than 30 per cent of humanity's fossil fuel emissions over the last 100 years or so."
The research will be published in the latest edition of Nature.