scientists said.
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.
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 theorised sources include volcanic activity and the sudden release of methane hydrates in the ocean.
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.
After the big warm-up, the planet eventually cooled around 100,000 years later, but not before there had been a mass extinction, paving the way to the biodiversity that is familiar to us today."
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