Past warming shows gaps in climate knowledge - study

They estimated about 3 trillion tonnes of carbon (11 trillion tonnes 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."