56 Mio years ago, Global Warming fueled by greenhouse gases in abundance occurred once before! This event is called the Paleocene Eocene Thermal Maximum (PETM).

Due to an initial warming, maybe caused by exceeding volcanic eruptions, clathrate hydrates in oceans became unstable, releasing Methane, a potent greenhouse gas. A positive feedback started!

The global mean temperature on Earth rised from averaged 18°C up to 23°C, causing major changes in marine and terrestrial ecosystems. 

PETM is indicated by three proxies:

Indicators of PETM in the oceans (IPCC, 2007).

Global rise in temperature decreased O18/O16 ratio in oceans through enhanced evaporation of heavier isotope. There is a temperature-sensitive fractioning of oxygene isotopes via differential evaporation from oceans to atmosphere. 

Whereas the release of methane decreased C13/C12 ratio. Carbon isotope C13 is sparse in Methane, as this originates from microbes and life (on Earth) discriminates enzymatically against it. Hence a release of methane means a diminished C13/C12 isotope ratio.

Still in the oceans a part of released Methane was oxidated to CO2 or carbonic acid, dissolving carbonates (CO3) on the seefloor. 

Jens Christian Heuer