Quantifying net carbon cycle feedbacks across the Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM, 56 Ma) is perhaps the most extensively studied paleoclimate event of massive carbon release because the intense global warming and widespread ocean acidification bear resemblance to the predicted worst-case near-future Earth conditions. While emission rate and carbon source were different from today’s perturbation, valuable lessons can be learned from studying the PETM. For instance, whether climate or carbon cycle feedbacks amplify or mitigate the environmental disruption, and what feedback processes contributed to the global climate response. In this study, we quantify the magnitude and sign of ‘net’ carbon cycle feedbacks by integrating: (1) estimates of volcanic carbon emissions from the North Atlantic Igneous Province (active ~56 Ma and considered a major source of carbon release), and (2) the net global environmental response recorded in paleoclimate records such as δ¹⁸O (temperature), δ¹¹B (ocean pH), and δ¹³C (carbon cycle). The difference between the environmental response to volcanic emissions alone and the recorded global response is attributed to feedback processes. Our Earth system model results suggest that carbon release from positive carbon cycle feedbacks (e.g. non-volcanic) likely approached or exceeded volcanic emission rates at the onset of the PETM, raising pCO₂ by 1330 ppm and the global temperature by 4.4°C. The ‘net’ feedback emissions are negative during the PETM recovery. Carbon isotopes indicate that a sustained low emission flux of isotopically light carbon is required to slow down the δ¹³C recovery driven by organic carbon burial, potentially pointing to additional thermogenic or biogenic methane release during the recovery phase.