Holocene peatland and ice-core data constraints on the timing and magnitude of CO₂ emissions from past land use

Stocker, Benjamin David; Yu, Zicheng; Massa, Charly; Joos, Fortunat (2017). Holocene peatland and ice-core data constraints on the timing and magnitude of CO₂ emissions from past land use. Proceedings of the National Academy of Sciences of the United States of America - PNAS, 114(7), pp. 1492-1497. National Academy of Sciences NAS 10.1073/pnas.1613889114

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CO₂ emissions from preindustrial land-use change (LUC) are subject to large uncertainties. Although atmospheric CO₂ records suggest only a small land carbon (C) source since 5,000 y before present (5 kyBP), the concurrent C sink by peat buildup could mask large early LUC emissions. Here, we combine updated continuous peat C reconstructions with the land C balance inferred from double deconvolution analyses of atmospheric CO₂ and δ¹³C at different temporal scales to investigate the terrestrial C budget of the Holocene and the last millennium and constrain LUC emissions. LUC emissions are estimated with transient model simulations for diverging published scenarios of LU area change and shifting cultivation. Our results reveal a large terrestrial nonpeatland C source after the Mid-Holocene (66 + 25 PgC at 7-5 kyBP and 115 + 27 PgC at 5-3 kyBP). Despite high simulated per-capita CO₂ emissions from LUC in early phases of agricultural development, humans emerge as a driver with dominant global C cycle impacts only in the most recent three millennia. Sole anthropogenic causes for particular variations in the CO₂ record (~20 ppm rise after 7 kyBP and ~10 ppm fall between 1500 CE and 1600 CE) are not supported. This analysis puts a strong constraint on preindustrial vs. industrial-era LUC emissions and suggests that upper-end scenarios for the extent of agricultural expansion before 1850 CE are not compatible with the C budget thereafter.

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