The journal Nature says that a core of sediment pulled from the bottom of the Arctic Ocean has confirmed that, millions of years ago, the North Pole was as warm as a balmy summer day.
Drilling in the Arctic Ocean poses enormous technical difficulties, so relatively little has been discovered about the region's climate history. Now, the spoils of the Arctic Coring Expedition (ACEX), extend our knowledge from about half a million years to 80 million years ago.
"This will boost understanding of climate evolution, in the Arctic and globally," says Ursula Röhl, a marine geologist at the University of Bremen in Germany. "It's really fantastic that such an enormous advance is still possible these days."
Some initial findings from the cruise were released two years ago (see 'North Pole once enjoyed Mediterranean climate'). Now the researchers report several analyses of the sediment core in Nature1.
The results are unexpected. Not only did the Arctic heat up to an extent that is inexplicable by current climate models, say the researchers, it also seems that the North Pole began to cool at about the same time as the Antarctic. This timing suggests that climate was being driven by a global factor, such as atmospheric levels of greenhouse gases, rather than something more local, such as geological upheaval.
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| Photo: National Geographic |
"This is a major, major surprise," says Jan Backman, a marine geologist at Stockholm University in Sweden, who co-led the expedition.
Party of ice-breakers
Part of the Integrated Ocean Drilling Program, the US$10-million ACEX project was an adventurous undertaking.
In August 2004, two ice-breakers cleared the Vidar Viking's path to the Lomonosov ridge, a chain of underwater mountains that lie more than 975 metres below the ocean surface and rise up to 3 kilometres above the sea floor. These ice-breakers then sheltered the ship while it drilled for a record-breaking 430 metres of sediment at a point 250 kilometres from the North Pole. Previous cores were only 10 metres long at most.
The isotopic composition of organic carbon from shells and algae in the sediment reveals information about past temperatures. During a period of pronounced warming 55 million years ago, known as the Palaeocene/Eocene thermal maximum, average summertime temperatures in the Arctic Ocean rose to almost 24°C. This is ten degrees higher than what climate models for the period have come up with.
"The Eocene climate apparently knew surprisingly little differences between the poles and the equator," says Röhl.
Scientists believe that the atmosphere at the time was exceptionally rich in greenhouse gases. But Appy Sluijs, a palaeoclimatologist at Utrecht University in the Netherlands and first author of one of the papers2, says that Arctic warming must have been amplified by an additional factor — possibly heat-trapping stratospheric clouds or hurricane-induced ocean mixing.
The New York Times says that a centerpiece of the discovery is a single pebble, about the size of a chickpea, found in a layer created 45 million years ago.
The stone could have been deposited on the raised undersea ridge only if it had been carried overhead in ice, said Kathryn Moran, a chief scientist on the drilling project, who teaches at the University of Rhode Island.
The stone was most likely embedded in an iceberg or perhaps a plate of sea ice that tore free from a gravelly shore. It then sank as the ice melted or broke apart, Dr. Moran proposed. Such "dropstones" have long been used to date when an oceanic region has been ice covered or ice free.
The amount of ice-carried debris in the sediment layers started increasing about 14 million years ago, the scientists said.
That is also about when the great ice sheet that now weighs down eastern Antarctica originated, Dr. Moran noted. In general, the results from the Arctic drilling project suggest that the cooling and ice buildup at both poles happened in relative lock step.
This simultaneity tends to support the idea that the cooling was caused by a drop in concentrations of carbon dioxide and other greenhouse gases, which mix uniformly in the atmosphere around the world, said Dr. Moran and other members of the team.
Impeccable timing
The sediment core also reveals the timing of the north's heating and subsequent cooling. Scientists have found evidence of the Antarctic starting to ice up from 43 million years ago. But evidence of glaciation and sea ice at the North Pole had only been found from about 6 million to 10 million years ago4.
Now geologists have found sand and pebbles in the central Arctic basin that they think can only have been carried there by icebergs 45 million years ago. So it seems that Arctic glaciation began around the same time as the southern freeze. "It revolutionizes our view on how ice and climate developed," says Backman.
"Understanding the geological past is absolutely essential for modellers," adds Röhl. "Only climate simulations that are able to correctly reproduce the past are likely to accurately predict the future."
- Moran K., et al. Nature, 441. 601 - 605 (2006). | Article |
- Brinkhuis H., et al. Nature, 441. 606 - 609 (2006). | Article |
- Sluijs A., et al. Nature, 441. 610 - 613 (2006). | Article |
- Lear C. H., Elderfield P. H. & Wilson P. A. Science, 287. 269 - 272 (2000).
