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.

But Zeebe said this could only explain a 1 to 3.5 degree Celsius rise in temperatures, adding that a commonly accepted scientific range for a doubling of CO2 is between 1.5 and 4.5 degrees Celsius.

This meant other factors must have been at work to drive up temperatures between 5 and 9 degrees Celsius.

If this additional warming which we do not really understand, was caused as a response to the CO2 warming, then there is a chance that also a future warming could be more intense than people anticipate right now, Zeebe said.

He said the study suggested there could be atmospheric or ocean processes as yet unknown or poorly understood that might have accelerated the warming. Possibilities could be changes in ocean currents, a much larger release of methane or even greater impacts from higher CO2 levels than currently thought.

At present, CO2 levels have already risen from 280 parts per million to nearly 390 ppm since the Industrial Revolution and could exceed a 70 percent increase during this century, a rate much faster than the Palaeocene-Eocene event, Zeebe said.

While this would cause initial effects, much worse could follow in the coming decades and centuries as the oceans, land and atmosphere tried to deal with the higher CO2 levels, he said.

The carbon that we put into the atmosphere right now is going to stay there for a very long time. Much of it will stay there for tens of thousands of years.

(Editing by Sugita Katyal)

Climate