I THINK IT'S TOTALLY OBVIOUS: it was because the evil Bush didn't sign the Kyoto protocol!

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.