A big expansion of Antarctica's ice almost a million years ago may help scientists predict modern climate change, a study has shown.
The report, examining the earth's orbit around the sun in a 100 000-year cycle of cold and warmth, showed that ice sheets took thousands of years to grow at the start of Ice Ages and surprisingly lagged a quicker cooling of the oceans.
That delay, and the discovery of a large build-up of ice into the southern ocean around Antarctica 900 000 years ago, is a step towards understanding the climate system and its link to changes in the planet's orbit, they said.
That in turn could help predict current global warming.
"When we think of future climate change we think about everything happening together," said Harry Elderfield, a professor at Cambridge University and lead author of the study in Friday's edition of the journal Science.
"We might think that it's warmer so sea level would change at the same time" as ice melts on the land and adds to the oceans. "What we are seeing is that things are changing, not really in concert."
"The deep sea cooled to nearly freezing temperatures early in any given glacial cycle, whereas global ice volume typically increased gradually," Peter Clark of Oregon State University wrote in a separate commentary in Science on the findings.
Elderfield's team found records of both sea temperatures and ice volumes by studying the chemical makeup of tiny marine fossils in the seabed off New Zealand.
The last Ice Age was at a maximum about 20 000 years ago and the Earth is into a natural warmer period in a cycle expected to last about 100 000 years.
But manmade global warming is now the main cause of a rise in temperatures in the past half-century, according to the UN panel of climate scientists which projects ever more floods, droughts and rising sea levels.
Until now, most experts have believed that Ice Ages were seen mainly in changes in the volume of ice around the North Pole – giant ice sheets blanketed much of the Northern Hemisphere at the height of the last Ice Age.
"The assumption had been that the big cycles in the Ice Ages occurred with ice volume in the Arctic. We are finding that is not the case. We are finding that Antarctica is changing," Elderfield said.
And the growth of Antarctic ice 900 000 years ago apparently initiated a previously unexplained shift in the cycle of Ice Ages and warmer periods to the current 100 000 years from 41 000 years, they said. Both periods are linked to Earth's orbit.
Scientists previously have been unable to separate information in fossils about the volume of ice on the planet from information about changing temperatures, Elderfield said.
They found that the amount of magnesium indicated changes in temperatures in the fossils and the exact makeup of oxygen indicated the amount of ice. – Reuters