The PETM isn't my typical cup of tea. Once you get back to about 12,000 years the uncertainty in dating and magnitude of paleo data is large enough you can draw several "conclusions" from the same data. Since I was drawn into a discussion of ocean dynamics, I mentioned that condition that may have existed during the PETM are pretty much irrelevant to today because of events like the closure of the Isthmus of Panama roughly 3 to 15 million years ago and the opening of the Drake Passage some 15 to 49 million years ago. Since the PETM predated both, there is no way the same ocean dynamics could be repeated in any meaningful time frame. Of course a few hundred million years from now things could drift to a similar configuration, but I really don't care.
The closure of the Isthmus of Panama "most likely occurred" about 3 million years ago based on "climate" data. The closure may have occurred 17 million years ago based on DNA sequencing data. So there is a large range of uncertainty due to competing earth science fields. The dating of the Drake Passage opening is even more uncertain. There is a huge amount of literature speculating on the huge impact that both of these events had on climate. I generally reference Brierley and Fedorov 2010 for the potential impact of the Closure of the Isthmus of Panama.
"During the early Pliocene (roughly 4 Myr ago), the ocean warm water pool extended over most of the
tropics. Subsequently, the warm pool gradually contracted toward the equator, while midlatitudes and
subpolar regions cooled, establishing a meridional sea surface temperature (SST) gradient comparable to the modern about 2 Myr ago (as estimated on the eastern side of the Pacific). The zonal SST gradient along the equator, virtually nonexistent in the early Pliocene, reached modern values between 1 and 2 Myr ago. Here, we use an atmospheric general circulation model to investigate the relative roles of the changes in the meridional and zonal temperature gradients for the onset of glacial cycles and for Pliocene‐Pleistocene climate evolution in general. We show that the increase in the meridional SST gradient reduces air temperature and increases snowfall over most of North America, both factors favorable to ice sheet inception. The impacts of changes in the zonal gradient, while also important over North America, are somewhat weaker than those caused by meridional temperature variations. The establishment of the modern meridional and zonal SST distributions leads to roughly 3.2°C and 0.6°C decreases in global mean temperature, respectively. Changes in the two gradients also have large regional consequences, including aridification of Africa (both gradients) and strengthening of the Indian monsoon (zonal gradient). Ultimately, this study suggests that the growth of Northern Hemisphere ice sheets is a result of the global cooling of Earth’s climate since 4 Myr rather than its
initial cause. Thus, reproducing the correct changes in the SST distribution is critical for a model to simulate the transition from the warm early Pliocene to a colder Pleistocene climate."
Moving from a permanent El Nino state i.e. "the warm ocean water extended over most of the tropics." to the alternating ENSO state should be a pretty big deal "climate change" wise. Roughly 3.2 to 3.8 degrees C cooler could be called a new climate regime. To figure out how much impact CO2 would have in the former regime would require a near perfect knowledge of the location of the land masses so that ocean circulation could even be roughly modeled. Since we are pretty confident that ocean circulation changes can have a huge, greater than 3 C impact, ignoring ocean dynamics is in Redneck terms F'ing stupid. Since it is an impossible task I will avoid it like the plague. Drawing firm conclusions without that knowledge is a bad joke.
During the PETM the Antarctic Circumpolar Current (ACC) most likely did not exist. Toggweiler and Bjornsson 2000 is my go to reference for the ACC and Drake Passage.
"The effect of Drake Passage on the Earth’s climate is examined using an idealised coupled model. It is found that the opening of Drake Passage cools the high latitudes of the southern hemisphere by about 3°C and warms the high latitudes of the northern hemisphere by nearly the same amount. This study also attempts to determine whether the width and depth of the Drake Passage channel is likely to be an important factor in the thermal response. A deeper channel is shown to produce more southern cooling but the magnitude of the effect is not large. Channel geometry is relatively unimportant in the model because of a haline response that develops when the channel is first opened up. Published in 2000 by John Wiley & Sons, Ltd."
Again, ocean dynamics changed with continental drift producing a new climate regime.
The video I have linked is one of several animations of the possible timing of continental drift and any opening or closing of a path for ocean circulation could have similar impacts on climate. When ocean circulation is weak you have more of a solar pond situation where the oceans can get much warmer and increased ocean circulation would reduce the solar pond effect. That is a pretty simple but likely accurate explanation. According to Toggwiler and Bjornsson, it doesn't take much of a opening to have a significant impact.
Since we live on a planet that is mainly water, about 70%, and that water has about 1000 times the heat capacity of the atmosphere, ignoring the impact of changes in "mixing" of the oceans is in Redneckese, F'ing Stupid.
That said, could CO2 during the PETM have had much higher impact that today. F'ing A yes it could, but there is no rational way to compare the PETM sensitivity to today's sensitivity since ocean and atmospheric circulations would respond differently with different configuration of the ocean and continents.
Now if "believers" want to completely ignore the dynamics and the "complexity" they use to sell their agenda, they can go for it, but the over simplifications are starting to bite them in the butt.