Past warming shows gaps in climate knowledge: study

By David Fogarty, Climate Change Correspondent, Asia

SINGAPORE (Reuters) - 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.

Methane is a potent greenhouse gas but much of it is oxidised into CO2 when it is released from hydrate deposits.

Zeebe and his colleagues estimated the amount of CO2 released during the Palaeocene-Eocene event by studying sediment cores from seabeds around the globe. Their study is published in the latest issue of Nature Geoscience.

FUTURE WARMING

They estimated about 3 trillion tons of carbon (11 trillion tons of CO2) was released over several thousand years from the methane deposits, leading to a 70 percent rise in atmospheric CO2 levels from pre-event levels.