Climate scientists found that ocean currents that move heat around the globe may have stopped at one time, possibly due to expanding ice cover in the Northern Hemisphere.
In a paper published in the journal Science researchers worked to explain why ice cycles got longer and more intense about 900,000 years ago and switched from 41,000-year cycles to 100,000-year cycles, a National Science Foundation news release reported.
"The research is a breakthrough in understanding a major change in the rhythm of Earth's climate, and shows that the ocean played a central role," says Candace Major, program director in the National Science Foundation (NSF)'s Division of Ocean Sciences, which funded the research.
The team found slowing currents increased the carbon dioxide (CO2) storage in the ocean, leaving less CO2 in the atmosphere. This may have kept temperatures colder than usual, leading to less frequent ice ages.
"The oceans started storing more carbon dioxide for a longer period of time," Leopoldo Pena, the paper's lead author and a paleoceanographer at Columbia University's Lamont-Doherty Earth Observatory (LDEO), said in the news release. "Our evidence shows that the oceans played a major role in slowing the pace of the ice ages and making them more severe."
To make their findings the researchers reconstructed the strength of the Earth's oceans at the time of the supposed change by looking at deep-sea sediments off the coast of South Africa. How strong the currents were can be determined through an isotope reading that shows how much North Atlantic water made it to the region.
The team determined that over the past 2.1 million years the "conveyer-like" currents strengthened during warmer periods and lessened during ice ages. About 950,000 years ago the circulation slowed down and stayed that way for 100,000 years. When this occurred the Earth skipped its interglacial (the warm period between ice ages) phase and launched into a period of longer 100,000-year ice age cycles. After this point ocean currents remained weak.
"Our discovery of such a major breakdown in the ocean circulation system was a big surprise," paper co-author Steven Goldstein, a geochemist at LDEO, said. "It allowed the ice sheets to grow when they should have melted, triggering the first 100,000-year cycle."
Ice ages are often triggered by variations in the Earth's orbit, but these changes alone are not enough to explain the switch to longer ice intervals.
One theory for the change is that advancing glaciers in North America stripped away Canadian soils, causing more ice to build up on the bedrock. In this case the moving ice would could have triggered a slow-down in deep ocean currents; this phenomenon would have caused the oceans to vent less carbon dioxide, resulting in the skipped interglacial period.
