Arctic ice may have formed 20 million years earlier than previously thought
5 October 2008
Geologists have long debated when the poles first started icing over. Most think that Antarctica saw ice vastly earlier than the Arctic.
Ice sheets in the northern hemisphere may have appeared 20 million years earlier than previously thought.
But, research reported in Nature suggests that previous estimates for exactly when the northern hemisphere first glimpsed ice could be out by around 20 million years.
Scientists usually compare the ratio of the two main oxygen isotopes - heavy oxygen and ordinary oxygen - in the remains of single-celled marine organisms in ocean sediments to find out when ice sheets formed. Geologists think two things determine this ratio: the amount of ice at the poles and temperature. Using this approach to look at the ratio of oxygen isotopes in the shells of marine organisms, scientists can figure out when ice sheets started emerging.
North south divide
Researchers discovered long ago that Antarctica started freezing over around 34 million years ago. And until now most scientists thought that the northern hemisphere started freezing around 31 million years later - just three million years ago.
Recently scientists have questioned this view, because other evidence points to the northern hemisphere icing over much earlier. Some scientists think that the amount of carbon dioxide in the atmosphere holds the key to controlling the timing of ice sheet formation.
Once these climate threshold levels are crossed, the ice sheets grow extremely rapidly, in the twinkling of a geological eye.
So, a team of researchers led by Robert DeConto of the University of Massachusetts and including Professor Paul Wilson and Dr Heiko Pälike of the National Oceanography Centre, Southampton, used a model that included carbon dioxide levels as well as oxygen isotopes to estimate when the poles froze over.
Carbon dioxide levels in the atmosphere gradually reduced from over 1000 parts per million 65 millions years ago to around 280 parts per million, which were the levels we saw at the start of the industrial revolution. Between 55 and 35 millions years ago, Earth was a greenhouse world with no major ice sheets. The researchers' model shows that after this time, a very slow drop in carbon dioxide levels explains how Earth switched rapidly into an icehouse world, when the poles began icing over.
Greenhouse to ice house
At first their results pretty much agreed with accepted wisdom. Antarctica iced over once carbon dioxide levels fell to about 750 parts per million, around 34 to 32 million years ago. But in the model the northern hemisphere started icing over when carbon dioxide levels hit much lower levels - about 280 parts per million. The model suggests this happened around 25 million years ago. At this point glaciers appear relatively rapidly. Ice covers Antarctica sooner than the northern hemisphere, because there is no land mass at the North Pole and the continents surrounding it are at much lower latitudes.
'In the models we drop carbon dioxide levels very slowly in comparison to the rates of change induced by mankind today. But once these climate threshold levels are crossed the ice sheets grow extremely rapidly, in the twinkling of a geological eye,' says Wilson.
The IPCC has predicted that carbon dioxide emissions could rise to between 500 and 900 parts per million by the end of this century. Discovering these climate thresholds in the geological past begs informs scientists about future effects, in particular, as carbon dioxide levels keep rising, what will be the fate of ice in the northern hemisphere?
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