Jonathan Leake, Times Online 15 Nov 09;
Six months is all it took to flip Europe’s climate from warm and sunny into the last ice age, researchers have found.
They have discovered that the northern hemisphere was plunged into a big freeze 12,800 years ago by a sudden slowdown of the Gulf Stream that allowed ice to spread hundreds of miles southwards from the Arctic.
Previous research had suggested the change might have taken place over a longer period — perhaps about 10 years.
The new description, reminiscent of the Hollywood blockbuster The Day After Tomorrow, emerged from one of the most painstaking studies of past climate changes yet attempted.
“It would have been very sudden for those alive at the time,” said William Patterson, a geological sciences professor at the University of Saskatchewan in Saskatoon, Canada, who carried out the research. “It would be the equivalent of taking Britain and moving it to the Arctic over the space of a few months.”
His findings, published at a recent conference, reinforce a series of studies suggesting that the earth’s climate is highly unstable and can flip between warm and cold very rapidly with the right trigger.
Most such research is based on analysing cores drilled from ice or from the sediment found at the bottom of oceans or lakes. In such cores the ice or sediment is found in layers whose composition shows what the climate was like at the time they were laid down.
Ice cores drilled from the Greenland ice cap have already shown that the big freeze of 12,800 years ago — known as the Younger Dryas mini-ice age — happened fast but lacked the detail to pin it down precisely.
Patterson, however, obtained mud deposits from Lough Monreagh, a lake in western Ireland, a region he says has “the best mud in the world in scientific terms”.
Patterson used a precision robotic scalpel to scrape off layers of mud just 0.5mm thick.Each layer represented three months of sediment deposition, so variations between them could be used to measure changes in temperature over very short periods.
Patterson found that temperatures had plummeted, with the lake’s plants and animals rapidly dying over just a few months. The subsequent mini-ice age lasted for 1,300 years.
What caused such a dramatic event? The most likely trigger is the sudden emptying of Lake Agassiz, an inland sea that once covered a swathe of northern Canada.
It is thought to have burst its banks, pouring freezing freshwater into the North Atlantic and Arctic oceans, disrupting the Gulf Stream, whose flows depend on variations in temperature and salinity.
A single year’s disruption in the Gulf Stream could have been enough, said Patterson, to let ice grow far to the south of where it usually formed. Once it had taken over, the Gulf Stream was unable to regain its normal route and the cold took hold for about 1,300 years.
Some scientists have suggested that if the Greenland ice cap melts it could have a similarly dramatic effect by disrupting the world’s ocean currents.
Other research has shown that rapid climate flips are normal. In its 4.5-billion-year history, the earth has experienced at least four main ice ages, of which the last, the Quaternary, is still continuing.
Within each ice age, however, there are periods when ice advances or retreats, and in the past 60,000 years alone the earth is thought to have warmed or cooled by up to 7C at least 20 times. The current interglacial period has lasted about 10,000 years.
“Human civilisation has grown up in a period of remarkable climatic stability,” said Tim Lenton, professor of earth system sciences at the University of East Anglia.
“In the period from 65,000 to 10,000 years ago there were periods of abrupt warming and cooling roughly every 1,500 years, when the temperature in Greenland might fall or rise by 10C in a decade.”
Patterson’s findings are supported by the research of Chris Stringer, professor of human origins at the Natural History Museum in London.
He believes the extinction of Neanderthals roughly 30,000 years ago was linked to a series of rapid climate fluctuations that began more than 40,000 years ago. He said: “Climate is basically unstable, so one of the mysteries is why it has stayed warm for the last 10,000 years.
“Some researchers have suggested this may be linked to the activities of early humans, who started growing crops and clearing forests 8,000 years ago.
“That may have put enough greenhouse gases into the air to stave off another ice age, but the problem now is that we have gone too far the other way.
“The amount of greenhouse gases in the air is greater than at any time in the last million years, so ironically, the threat now is from global warming.”
Patterson is still focusing his efforts on the past. He has built a new robot capable of shaving tiny slivers from the shells of fossilised clams, showing temperature almost day by day from millions of years ago.
