A team of scientists led by a University of Hawaii oceanographer had a big surprise when they looked to the ancient past for clues to global warming.

Atmospheric carbon increased 70 percent during the period known as Paleocene-Eocene Thermal Maximum 55 million years ago, said Richard Zeebe. Yet it was less than expected to explain a rapid increase in temperatures, he said in an interview.

“;This constitutes an enigma because carbon dioxide released cannot account for the entire warming. This means something else contributed significantly to the warming,”; he said.

“;We're not saying carbon dioxide is not important,”; he emphasized. “;It is very important. Current and future warming is almost entirely due to carbon emissions. There is no doubt about this.”;

However, his team believes other mechanisms could have contributed to the rapid heating during the Paleocene-Eocene Thermal Maximum period, which they said represents “;a possible analogue for the future.”;

“;The question is still open by how much the global temperature will increase until the end of the century, depending how much carbon dioxide we put into the atmosphere,”; Zeebe said.

However, based on estimates of CO2 emissions into the atmosphere within this century, he added, “;We will more than double it.”;

Zeebe's study, with colleagues Gerald Dickens, Rice University professor of earth science, and James Zachos, University of California, Santa Cruz, professor of earth and planetary sciences, appears in Nature Geoscience.

They examined data from deep-sea sediment cores collected during ocean drilling expeditions around the world to look at the impacts of global warming. Changes in the carbon cycle are embedded in the cores.

Global surface temperatures rose 9 to 16 degrees Fahrenheit in just a few thousand years in the period studied, Zeebe said. “;It was pretty remarkable warming within a short period of time.”;

The scientists expected a three- to eightfold increase in carbon dioxide to explain the fast-rising temperatures, but it was less than a twofold increase, Zeebe said.

Other greenhouse gases such as methane could have contributed to the warming in response to the increased CO2, called a “;feedback,”; he said. The warming could have enhanced the cycle of methane gas from the earth's surface and increased the concentration in the atmosphere for a stronger greenhouse effect, he explained.

“;The important message is we believe there are other mechanisms that contribute significantly to warming when we release CO2,”; he said. “;That's why we're concerned about future warming. If we do not precisely understand those mechanisms, warming could be stronger than people believe now.”;