The warmth of that period was stabilized by a gradual rise in global carbon dioxide levels, so understanding the reason for that rise is of great interest, said Daniel Sigman, the Dusenbury Professor of Geological and Geophysical Sciences at Princeton.

Scientists have proposed various hypotheses for that carbon dioxide increase, but its ultimate cause has remained unknown. Now, an international collaboration led by scientists from Princeton and the Max Planck Institute for Chemistry point to an increase in Southern Ocean upwelling. Their research appears in the current issue of the journal Nature Geoscience.

“We think we may have found the answer,” said Sigman. “Increased circulation in the Southern Ocean allowed carbon dioxide to leak into the atmosphere, working to warm the planet.”

“Danny brings an entirely new slant to a long-standing question,” said Wallace Broecker, the Newberry Professor of Geology at Columbia University, who was not involved in this research. “As the Southern Ocean acts as a throttle in the transfer of CO2 between ocean and atmosphere, we owe thanks to Danny, for we now know the throttle was at work during what has been viewed as a period of quiescence.”

Their findings about ocean changes could also have implications for predicting how global warming will affect ocean circulation and how much atmospheric carbon dioxide will rise due to fossil fuel burning.

For years, researchers have known that growth and sinking of phytoplankton pumps carbon dioxide deep into the ocean, a process often referred to as the “biological pump.” The biological pump is driven mostly by the low latitude ocean but is undone closer to the poles, where carbon dioxide is vented back to the atmosphere by the rapid exposure of deep waters to the surface, Sigman said. The worst offender is the Southern Ocean, which surrounds Antarctica. “We often refer to the Southern Ocean as a leak in the biological pump,” Sigman said.

Sigman and his colleagues have found that an increase in the Southern Ocean’s upwelling could be responsible for stabilizing the climate of the Holocene, the period reaching more than 10,000 years before the Industrial Revolution.

Most scientists agree that the Holocene’s warmth was critical to the development of human civilization. The Holocene was an “interglacial period,” one of the rare intervals of warm climate that have occurred over the ice age cycles of the last million years. The retreat of the glaciers opened a more expansive landscape for humans, and the higher concentrations of carbon dioxide in the atmosphere made for more productive agriculture, which allowed people to reduce their hunter-gathering activities and build permanent settlements.

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