"SINGAPORE -- A dramatic warming of the planet 55 million years ago cannot be solely explained by a surge in carbon dioxide levels, a study shows, highlighting gaps in scientists' understanding of impacts from rapid climate change.

During an event called the Palaeocene-Eocene Thermal Maximum, global temperatures rose between 5 and 9 degrees Celsius within several thousand years. The world at that time was already warmer than now with no surface ice.

'We now believe that the CO2 did not cause all the warming, that there were additional factors,' said Richard Zeebe, an oceanographer with the University of Hawaii at Manoa.

'There may have been an initial trigger,' he told Reuters on Wednesday from Hawaii. This could be a deep ocean warming that caused a catastrophic release of methane from hydrate deposits under the seabed."

David Fogarty reports for Reuters July 16, 2009. [2]