“It's possible that other greenhouse gases such as methane could have contributed to the [PETM] warming,” Zeebe says. It's also possible that the models are underestimating the climate response to CO2 increases. If that's the case, it “would mean our understanding of the climate system is incomplete,” he says.”
OR maybe they are over estimating the effects of co2, but perish the thought since ignorance only works in one direction.
Bingo!
Rephrased: "- It's also possible that the models are underestimating the climate CO2 response to CO2 climate temperature increases. If that's the case, it “would mean our understanding of the climate system is incomplete,” he says.” -"
If you haven't visited www.CO2science.org, take a look. They have results of studies on the effects of increased CO2 on plant life.