Buried in a story about recovery from the Palaeocene-Eocene Thermal Maximum is disturbing news that, while the unprecedented volcanic eruptions and possible methane hydrate melt may have jumpstarted the extinction event, "At the beginning of the event we see a shift indicating that a lot of organic-derived carbon dioxide had been added to the atmosphere..." "...our results suggest the troubling possibility that widespread decay or burning of large parts of the continental biosphere may have been involved."

Basically, the carbon isotope data are suggesting there may be a tipping point at which land biomes get caught in a positive feedback cycle, more heat/less plants/more heat.

Plankton is already being lost at 1% a year, so ... not encouraging.

I don't think this is included in climate models.