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Warming and CO2 increases are often coincident, but it doesn't mean one causes the other.

See what you think about the PETM after reading this:

The article posted says this:

"During the PETM, unknown factors released vast quantities of methane that had been lying frozen in sediment deposits on the ocean floor. After release, most of the methane reacted with dissolved oxygen to form carbon dioxide, which made the seawater more acidic. Acidic seawater corrodes the carbonate shells of microplankton, dissolving them before they can reach the ocean floor and reducing the carbonate content of marine sediment."

I also discovered, for the first time, that the triggering episode is thought to be plate-tectonic changes causing landslides on continental slopes.

Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties (2005), page 69

This page also indicates that there is a competing comet impact explanation for the PETM -- but I don't think Zachos ascribes to it.

Very strange. Is the melting methane ice a theory or a measured fact? It is presented as a little of both (e.g. "what if..."). The paper doesn't explain why ocean levels would have dropped or the bottom of the oceans warmed sufficiently to melt the ice. The most plausible explanation to me would be a rapid ice age with a sudden decrease in plant life and increase in CO2, followed by the methane release due to ocean levels which would be hundreds of feet lower. The warming would be enhanced by the methane itself.

Anyway, this release of methane seems speculative and not a good indicator for what could happen with our current release of CO2.