Loss of vegetation-mediated carbon sequestration during the Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM) around 56 million years ago was a 5-6°C global warming event, representing one of the most important geologic analogues to present-day climate change. Considering that the carbon release rate that triggered the PETM was likely around a magnitude lower than current anthropogenic carbon emissions, it is of major importance to identify the climatic, geologic and biological factors that drove the severity and 200 kyr long duration of the PETM hyperthermal. Based on carbon isotope records of the period, it was suggested that a loss and a 70-100 kyr lagged regrowth of biospheric organic carbon stocks may have contributed to the long duration of the carbon cycle perturbation. In this work, we aim to identify the biological mechanisms that could explain such a sustained loss of vegetation-mediated carbon sequestration on land, and whether these dynamics can be expected under current anthropogenic carbon release. We developed a new, eco-evolutionary vegetation model, grounded in principles of eco-evolutionary optimality, to simulate changes in vegetation structures and traits, organic carbon sequestration and vegetation-mediated silicate weathering enhancement throughout the PETM climatic excursion. By comparing modelled vegetation dynamics with vegetation reconstructions derived from palynofloral records, we show that the PETM warming may have exceeded the capacity of vegetation systems to respond to the environmental changes through evolutionary adaptation of functional traits and climatic tolerances, resulting in reduced fitness and functioning. The magnitude of the warming and the creation of previously non-existent climatic environments during the period further resulted in a limited capacity of plants to avoid the warming-induced stress through dispersal and migration. Our results show that a global warming of similar magnitude as during the PETM could result in a long-lasting loss of vegetation-mediated carbon sequestration and a reduction in the efficiency of the Earth system to regulate perturbations.