[Wonder Warthog]

"...Zachos estimated that it took 100,000 years after the PETM for carbon dioxide levels in the air and water to return to normal."

Define "normal". At past times, the level of CO2 in the atmosphere has been EIGHT TIMES higher than today's, yet the global temperature wasn't hugely higher than now.

[cogitator]

Define "normal".

Normal for the Eocene. It's not easy to find, but I think a range of 300-450 ppm is generally accepted, and during the PETM it could have gone as high as 2,000 ppm.