Climatologists have long tried to explain why ice-age cycles became longer and more intense some 900,000 years ago, switching from 41,000-year cycles to 100,000-year cycles.
Deep ocean currents that move heat around the globe stalled or may have stopped at that time, possibly due to expanding ice cover in the orthern Hemisphere, says a study.
"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," said Candace Major, programme director in the US National Science Foundation's (NSF) Division of Ocean Sciences.
The slowing currents increased carbon dioxide (CO2) storage in the oceans, leaving less CO2 in the atmosphere. That kept temperatures cold and kicked the climate system into a new phase of colder but less frequent, ice ages, the scientists suggested.
"The oceans started storing more carbon dioxide for a longer period of time," says Leopoldo Pena, a paleoceanographer at Columbia University's Lamont-Doherty Earth Observatory (LDEO) in the US.
"Our evidence shows that the oceans played a major role in slowing the pace of the ice ages and making them more severe," Pena added.
The researchers reconstructed the past strength of earth's system of ocean currents by sampling deep-sea sediments off the coast of South Africa where powerful currents originating in the North Atlantic Ocean pass on their way to Antarctica.
The findings appeared in the journal Science.
