Positive feedback of marine N2O emissions during past extreme warming

The Paleocene–Eocene Thermal Maximum (PETM; 55.9 million years ago) was a rapid, CO2-driven climate polycrisis, with global warming of 5-6°C1,2, ocean acidification3-5, and deoxygenation6-8. These stresses may have affected marine nitrogen cycling, particularly nitrification—a major source of nitrous oxide (N2O) 9 that remains underexplored on geological timescales. By integrating proxy records with Earth System modeling, we show that nitrification globally increased by ~56% during the PETM. Inhibiting effects of ocean deoxygenation10 and acidification11 on nitrification were outweighed by increased ammonium supply and nitrifier niche expansion. Enhanced nitrification led to a ~290% increase in total marine N2O production, contributing 0.3-1.4℃ of warming during the PETM. Our results highlight a strong, positive climatic feedback from nitrogen cycle perturbations under extreme global warming, ocean deoxygenation and acidification.