Terrestrial carbon stock vulnerability during the Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum is a key analog for future climate change. However, how terrestrial ecosystems responded to this extreme warming remains uncertain. We integrate a dynamic global vegetation model with a carbon-isotope mass-balance framework to quantify terrestrial carbon-cycle feedbacks. Our simulations reveal that ancient high-latitude carbon stocks were vast but highly sensitive to extreme warming. We identify a fundamental non-linearity in the terrestrial response: while moderate warming triggers ecosystem shifts, extreme warming drives an abrupt collapse of land carbon, releasing up to similar to 900 petagrams. This decline stems from a bipolar ecological shift where massive tropical forest loss and accelerated soil heterotrophic respiration overwhelm high-latitude greening and carbon dioxide fertilization. Furthermore, hybrid scenarios combining terrestrial release of sedimentary methane and volcanic carbon dioxide provide the best fit to the geological record. We identify critical thermal thresholds, warning that future terrestrial carbon-climate feedbacks could be more severe than currently projected.