Here's a term I hope catches on: "Earth system sensitivity," which is the expected change in the Earth's global temperature when CO2 doubles, taking into account all accompanying physical changes.
That distinguishes it from "climate sensitivity," which is the temperature change when CO2 doubles but ignoring all other accompanying geological changes (like changes of Arctic albedo from the melting of sea ice, changes in the uptake of CO2 by the oceans, and lots of other factors).
Science knows that latter much better than the former, but, of course, it's ESS we really want to know.
Steve Easterbrook has lots of other good information in this post, written after he attended a talk by Mark Pagani of Yale on the role of methane hydrates in the PETM. He notes another important point Pagani made: unlike climate sensitivity, Earth system sensitivity depends on your starting temperature. So a warming of about 5°C during the PETM does not imply the same warming would take place now if we inject the same amount of carbon into the atmosphere, because whatever caused the PETM started in a world that was already about 5°C warmer than today. Most notably, that starting point 55 M yrs ago was an ice-free planet, so some feedbacks (like the change in Arctic albedo) were then not available.
So be careful with geologic analogies. Interesting.
PS: Here's a 2009 paper in Nature Geosciences by Pagani and colleagues that goes into this further. It concludes that "the Earth-system climate sensitivity has been signiﬁcantly higher over the past ﬁve
million years than estimated from fast feedbacks alone."