The first detailed analysis of a climatic and biological record from the seabed near the North Pole indicates that 55 million years ago the Arctic Ocean was far warmer than scientists imagined - a Floridian year-round average of 74 degrees Fahrenheit.
The findings, published today in the journal Nature, fill in a blank spot in scientists' understanding of climate history. And they suggest that scientists have greatly underestimated the power of greenhouse gases to warm the Arctic.
Previous computer simulations, 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 led 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 2004 expedition that defied the Arctic Ocean ice and pulled the first significant samples from the ancient layered seabed 50 miles from the North Pole: 1,400 feet of slender shafts of muck, 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 that the extraordinary planetwide warm-up called the Paleocene Eocene Thermal Maximum, or PETM, must have been caused by a huge outburst of such greenhouse gases as methane and carbon dioxide. But no one has found a clear cause for the gas discharge.
Most climate experts agree that the present-day greenhouse gas buildup, led by carbon dioxide, 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 past several million years.
Experts not connected with the studies say they support the idea that it is greenhouse gases - not slight variations in Earth's orbit around the sun - that largely determine the extent of warming or cooling.
"In my opinion, 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 Richard B. Alley, a geoscientist at Penn State.
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 for nine days.