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