The Paleocene-Eocene thermal maximum (PETM) represents one of the most prominent and abrupt global warming events in Earth's history. Across the Paleocene-Eocene boundary (PEB; ca. 55 Ma), extreme global warming by 4-8 degrees C coincided with rapid ocean acidification, mass extinction in the deep ocean, and a distinct negative carbon isotope excursion (CIE) in both the marine and terrestrial environments. These lines of evidence strongly suggest a massive and rapid injection of (super 13) C-depleted carbon into the atmosphere and ocean, thereby inducing global warming. However, the origin and magnitude of the massive carbon injection across the PEB remain unresolved. We reconstructed the perturbation of the global carbon cycle across the PEB using an atmosphere-ocean box model, thereby obtaining a precise picture of the PETM event. It is likely that the PETM was triggered by the volcanism that produced the North Atlantic Igneous Province (NAIP). The model results indicate that the CIE of -3 ppm during the PETM was caused by inputs of 2,200 Gt-C of thermogenic methane (triggered by the NAIP volcanism) and 1,300-3,300 Gt-C of biogenic methane following the dissociation of sub-sea-floor gas hydrates. The destabilization of gas hydrate was probably due to a reduction in hydrostatic pressure in the sub-seafloor or the warming of bottom water due to a significant uplift of the seafloor related to eruption of the NAIP. Although the CIE can be readily reconstructed, the calculated temperature rise is 2.6 degrees C at most under the generally accepted range for equilibrium climate sensitivity (rise of 1.5-4.5 degrees C per doubling of the atmospheric CO (sub 2) concentration); hence, we cannot reproduce the global warming of 4-8 degrees C inferred from the geological record during the PETM. Our results indicate that climate sensitivity at this time may have been much higher than previously considered or that unknown feedback and forcings (other than atmospheric CO (sub 2) ) were responsible for a major component of the PETM warming.