“what caused the permafrost to thaw?”
Apparently you are unaware of man’s massive burning of carbon-based fuels 55 million years ago....
More seriously, this I believe is the explanation:
“PETM and other hyperthermals occurred during periods when Earth’s orbit around the sun was both highly eccentric (non-circular) and oblique (tilted)”
I gather that this is a “feedback” explanation: initial warming caused by variations in Earth’s orbit that placed it closer to the sun triggers a release of carbon which in turn creates a global warming effect that amplifies this initial warming, so then the patterns keeps repeating/amplifying. Of course, if it keeps amplifying, it’s not obvious to me (I’m no climate scientist) what makes the cycle stop and reverse. Nevertheless, as explained, the cycle sounds plausible, and note that there is ZERO contribution of humans to make it happen!
Tilted? in relation to what? And why would that matter?
However, let's go with that theory.
initial warming caused by variations in Earth’s orbit that placed it closer to the sun triggers a release of carbon which in turn creates a global warming effect that amplifies this initial warming, so then the patterns keeps repeating/amplifying.
If the Earth occupies a HIGHLY ECCENTRIC orbit, then not only is it CLOSER to the SUN for a short amount of time but it is also FURTHER from the SUN for a much longer amount of time.
This would make it an iceball that gets ICE burnt off it's equator once a 'year'. (Wonder how long a 'year' was back then?)
And since neither of these is true of the Earth's orbit currently, the basis for their model is false.