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Andrew C. Revkin, "Studies Portray Tropical Arctic in Distant Past" (2006)
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The first detailed analysis of an extraordinary climatic and biological record from the seabed near the North Pole shows that 55 million years ago the Arctic Ocean was much warmer than scientists imagined — a Floridian year-round average of 74 degrees.
The ship Vidar Viking had to remain stationary for days at a time amid drifting sea ice near the North Pole.
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Video: Tropical Arctic?
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The Cenozoic palaeoenvironment of the Arctic Ocean (Nature.com)
Two scientists cutting a core into sections on the drill ship Vidar Viking during a 2004 mission by the Integrated Ocean Drilling Program.
The findings, published today in three papers in the journal Nature, fill in a blank spot in scientists' understanding of climate history. And while they show that much remains to be learned about climate change, they suggest that scientists have greatly underestimated the power of heat-trapping gases to warm the Arctic.
Previous computer simulations, done without the benefit of seabed sampling, did not suggest an ancient Arctic that was nearly so warm, the authors said. So the simulations must have missed elements that lead to greater warming.
"Something extra happens when you push the world into a warmer world, and we just don't understand what it is," said one lead author, Henk Brinkhuis, an expert on ancient Arctic ecology at the University of Utrecht in the Netherlands.
The studies draw on the work of a pioneering 2004 expedition that defied the Arctic Ocean ice and pulled the first significant samples from the ancient layered seabed 150 miles from the North Pole: 1,400 feet of slender shafts of muck, fossils of ancient organisms and rock representing a climate history that dates back 56 million years.
While there is ample fossil evidence around the edges of the Arctic Ocean showing great past swings in climate, until now the sediment samples from the undersea depths had gone back less than 400,000 years.
The new analysis confirms that the Arctic Ocean warmed remarkably 55 million years ago, which is when many scientists say the extraordinary planetwide warm-up called the Paleocene Eocene Thermal Maximum must have been caused by an enormous outburst of heat-trapping, or greenhouse, gases like methane and carbon dioxide. But no one has found a clear cause for the gas discharge. Almost all climate experts agree that the present-day gas buildup is predominantly a result of emissions from smokestacks, tailpipes and burning forests.
The samples also chronicle the subsequent cooling, with many ups and downs, that the researchers say began about 45 million years ago and led to the cycles of ice ages and brief warm spells of the last several million years.
Experts not connected with the studies say they support the idea that heat-trapping gases — not slight variations in Earth's orbit — largely determine warming and cooling.
"The new research provides additional important evidence that greenhouse-gas changes controlled much of climate history, which strengthens the argument that greenhouse-gas changes are likely to control much of the climate future," said one such expert, Richard B. Alley, a geoscientist at Pennsylvania State University.
The $12.5 million Arctic Coring Expedition, run by a consortium called the International Ocean Drilling Program, was the first to drill deep into the layers of sediment deposited over millions of years in the Arctic. The samples were gathered late in the summer of 2004 as two icebreakers shattered huge drifting floes so that a third ship could hold its position and bore for core samples.
Estimates of the prevailing temperatures in the different eras represented by the sediments were made in part by tracking the comings and goings of certain algae called dinoflagellates that typically indicate subtropical or tropical conditions.
Because the samples lacked remains of shell-bearing plankton that are usually relied on to provide temperature records, the researchers used a newer method for approximating past temperatures: gauging changes in the chemical composition of the remains of a primitive phylum of microbes called Crenarchaeota.
Some scientists familiar with the research said that while there were still questions about the precision of this method at temperatures like those in the ancient Arctic Ocean, it was clear that the area was warm.
Another significant discovery came in layers from 49 million years ago, where conditions suddenly fostered the summertime growth of vast mats of an ancient cousin of the Azolla duckweed that now cloaks suburban ponds. The researchers propose that this occurred when straits closed between the Arctic Ocean and the Pacific and Atlantic Oceans.
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Coarse-grained sediments taken from the core samples of the floor of the Arctic Ocean in 2004 are examined under a light microscope.
The flow of water from precipitation and rivers created a great pool of fresh water, but about 800,000 years after the blossoming of duckweed began, it ended with a sudden warming of a few additional degrees. The researchers suggest that this signaled when shifting land formations reconnected the Arctic with the Atlantic, allowing salty, warmer water to flow in, killing off the weed.
The researchers said the sediments held hints that Earth's long slide to colder conditions, and the recent cycle of ice ages and brief thaws, began quite soon after the hothouse conditions 50 million years ago. A centerpiece of their argument 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 probably embedded in an iceberg or perhaps a plate of sea ice that tore free from a gravelly shore. It 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 began to increase 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 lockstep.
This simultaneity tends to support the idea that the cooling was caused by a drop in concentrations of carbon dioxide and other heat-trapping gases, which mix uniformly in the global atmosphere, said Dr. Moran and other members of the team.
Julie Brigham-Grette of the University of Massachusetts, an expert in past Arctic climates who was not connected with the new studies, cautioned against giving too much significance to the single sample, and particularly the single stone from 45 million years ago.
Dr. Brigham-Grette said it was vital to try to mesh the new core results with data gathered around Arctic coasts, where there is plenty of evidence for warm conditions in at least some places as recently as 2.4 million years ago.
Despite her doubts, she said, the project was a stunning achievement.
"It's all very, very exciting to me, because now we can start to rewrite the history of the Arctic," Dr. Brigham-Grette said. "It's like working a giant landscape puzzle of 500 pieces. For a while we only had 100 pieces. Now we have 100 more, and the picture is getting clearer."
Źródło: New York Times
