Sensitivity of Holocene atmospheric CO₂ and the modern carbon budget to early human land use: analyses with a process-based model

Abstract. A Dynamic Global Vegetation model coupled to a simplified Earth system model is used to simulate the impact of anthropogenic land cover changes (ALCC) on Holocene atmospheric CO₂ and the contemporary carbon cycle. The model results suggest that early agricultural activities cannot explain the mid to late Holocene CO₂ rise of 20 ppm measured on ice cores and that proposed upward revisions of Holocene ALCC imply a smaller contemporary terrestrial carbon sink. A set of illustrative scenarios is applied to test the robustness of these conclusions and to address the large discrepancies between published ALCC reconstructions. Simulated changes in atmospheric CO₂ due to ALCC are less than 1 ppm before 1000 AD and 30 ppm at 2004 AD when the HYDE 3.1 ALCC reconstruction is prescribed for the past 12 000 years. Cumulative emissions of 69 GtC at 1850 and 233 GtC at 2004 AD are comparable to earlier estimates. CO₂ changes due to ALCC exceed the simulated natural interannual variability only after 1000 AD. To consider evidence that land area used per person was higher before than during early industrialisation, agricultural areas from HYDE 3.1 were increased by a factor of two prior to 1700 AD (scenario H2). For the H2 scenario, the contemporary terrestrial carbon sink required to close the atmospheric CO₂ budget is reduced by 0.5 GtC yr⁻¹. Simulated CO₂ remains small even in scenarios where average land use per person is increased beyond the range of published estimates. Even extreme assumptions for preindustrial land conversion and high per-capita land use do not result in simulated CO₂ emissions that are sufficient to explain the magnitude and the timing of the late Holocene CO₂ increase.