What caused global warming 55 million years ago?
A runaway spurt of global warming 55 million years ago turned Earth into a hothouse but how this happened remains worryingly unclear, scientists said on Monday.
Previous research into this period, called the Palaeocene-Eocene Thermal Maximum, or PETM, estimates the planet's surface temperature blasted upwards by between five and nine degrees Celsius in just a few thousand years.
The Arctic Ocean warmed to 23 C, or about the temperature of a lukewarm bath.
How PETM happened is unclear but climatologists are eager to find out, as this could shed light on aspects of global warming today.
What seems clear is that a huge amount of heat-trapping "greenhouse" gases — natural, as opposed to man-made — were disgorged in a very short time.
The theorized sources include volcanic activity and the sudden release of methane hydrates in the ocean.
But all this CO2 can only account for between one and 3.5 C of PETM's warming if the models for climate sensitivity are right, the team found.
There must have been some other factor that stoked temperatures higher.
Even though there are big differences between Earth's geology and ice cover then and now, the findings are relevant as they highlight the risk of hidden mechanisms that add dramatically to warming, says the paper.