An expedition to the Artic Ocean in 2004 by a team of scientists aboard a fleet of icebreakers collected samples by drilling into the floor of the ocean. The project was part of an international research effort, the Integrated Ocean Drilling Program, which explores the Earth's history and structure as recorded in seafloor sediments and rocks.
"Remains of ancient plant and animal life found in cylindrical core samples from the ocean floor have given us critical new information about the history of the Arctic Ocean and surrounding region," said Mark Pagani, assistant professor of geology and geophysics at Yale and a co-author on the study.
During the warmest time interval, the Paleocene Eocene Thermal Maximum (PETM), sea surface temperatures at the North Pole reached as high as 23 ºC, or around 73 ºF, as determined by a novel technique the authors used to estimate temperature. Today's mean annual temperature is around -20ºC, or minus-4ºF.
"This project was a technological feat, and all of the findings in these papers are especially new and exciting given the fact that nobody's ever taken core samples like this before from the floor of the Arctic Ocean," said Matthew Huber, an assistant professor of earth and atmospheric sciences in Purdue University's College of Science. "As a climate modeler, gaining access to this data is a once in a lifetime opportunity"
"We discovered the remains of tiny algae called dinoflagellates, belonging to the species Apectodinium, which previously had been restricted to warmer regions of the world," said co-author Henk Brinkhuis, a marine palynologist and biogeologist from Utrecht University in the Netherlands. "The presence of Apectodinium during the Paleocene-Eocene Thermal Maximum confirms subtropical conditions in the Arctic during this time.
"Many argue whether or not rising carbon dioxide concentrations will cause global warming. But, those of us who study Earth history know that there is a direct link between Earth's temperature and carbon dioxide," said Pagani. "The PETM represents the clearest evidence for carbon dioxide-induced global warming in the geologic record."
The PETM was caused by a huge release of a greenhouse gas, possibly carbon dioxide or methane, into the atmosphere. If methane was the source of carbon, global warming was ultimately the result of higher carbon dioxide concentrations, as the methane would have been rapidly converted to carbon dioxide in the atmosphere.
"You can look at the PETM as a natural experiment involving carbon dioxide and climate -- that has already been performed. We have a good sense that global temperatures just before the PETM were very high, much higher than today," Pagani said. "High global temperatures were the result of high carbon dioxide levels"
The concentration of carbon dioxide in today's atmosphere is about 380 parts per million. According to Pagani's work published in the journal Science last summer, the concentration of carbon dioxide 55 million years ago is estimated to have been about 2,000 parts per million.
"What is really important is that even though global temperatures were high before the PETM, temperatures continued to rise as more carbon dioxide was released into the atmosphere," Pagani said. "This should serve as an example and evidence for how temperatures will change as we continue to pump carbon dioxide into the atmosphere."
While the climate models had predicted that researchers would discover the Arctic Ocean's freshwater past, the models have underestimated by at least 10 degrees how hot the Earth would have been during that time. "Current models under-estimate how warm the poles were back then and are not sensitive enough to greenhouse gases," Huber said.
Another important discovery of the study was t that 5 million years later, at around 50 million years ago, the Arctic Ocean was frequently covered with dense mats of the freshwater fern Azolla.. Brinkhuis noted, "What these findings say is that the Arctic Ocean must have been isolated, or nearly cut off, from the Atlantic Ocean by land masses that later shifted into the present continents and prevented salt water from ocean surface currents from reaching there. Imagine that the Arctic Ocean was a giant lake, with this vegetation growing in it,"
"The real point is that climate change is natural, and it's easy to set in motion," said Huber. "All it takes is an increase in the concentration of greenhouse gases."
It is possible that in addition to greenhouse gas concentrations, polar clouds or hurricane-induced ocean heat transport were involved, since the tropics maintained livable conditions during the PETM. Mechanisms that feed back onto global warming also are poorly understood and poorly represented in current models. These concerns will continue to be explored in future research."
The Integrated Ocean Drilling Program was funded by the National Science Foundation; Japan's Ministry of Education, Culture, Sports, Science and Technology; the European Consortium for Ocean Research Drilling; and the People's Republic of China Ministry of Science and Technology.
Citation: Nature (June 1, 2006)