Spatio-temporal dynamics of global peatland extent and carbon stocks as simulated for the past twenty thousand years

Abstract:

Predicting the spatio-temporal dynamics of global peatlands is key to understanding their role under past and future climate change. However, our understanding of peatlands in the Earth System is limited and global modelling studies mostly rely on temporally fixed, prescribed maps of peatland extent, thereby neglecting changes in peatland coverage with climate as evidenced by paleoclimatic proxy information. For global-scale predictions of peatland extent and lateral dynamics, we combine criteria for (i) the ecosystem water balance, (ii) simulated peatland carbon (C) balance conditions, and (iii) the simulated inundation persistency. The latter is based on subgrid-scale topography information and the simulated soil water balance. This model is successful at capturing the spatial distribution and extent of major boreal and tropical peatland complexes and reveals the governing limitations to peatland occurrence across the globe. Peatlands covering large boreal lowlands are reproduced only when accounting for a positive feedback induced by the enhanced mean soil water holding capacity in peatland-dominated regions. Much of our understanding of the role of peatlands in the carbon cycle rests on measured basal dates and C accumulation rates of peat cores from of today's existing peatlands. This data suggest a C sink that has persisted since peat initiation. However, these sink estimates do not include carbon release from peatlands that have disappeared over the past thousands of years. We present results from an application of our model to varying climate boundary conditions as simulated for the transition from the Last Glacial Maximum (LGM) to the present-day. This reveals the spatial dynamics of peatlands in response to climatic shifts, ice sheet retreat, and sea level rise. In line with maps of pollen records of sphagnum cores, a northward shift of peatlands is simulated over the past 20,000 years. A large expansion of the total peatland area is simulated for the Eurasian continent. In South-East Asia, tropical peatlands, wide-spread at the LGM, are simulated to have been submerged by rising sea levels. Our results suggest that C stored in peatlands was around present-day levels at the LGM, dropped during the Deglaciation and accumulated gradually under stable climatic conditions during the Holocene.