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 (nine and 16.2 degrees Fahrenheit) in just a few thousand years.

The Arctic Ocean warmed to 23 C (73 F), 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.

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