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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-64
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-64
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 04 Mar 2019

Research article | 04 Mar 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

What was the source of the atmospheric CO2 increase during the Holocene?

Victor Brovkin1, Stephan Lorenz1, Thomas Raddatz1, Tatiana Ilyina1, Irene Stemmler1, Matthew Toohey2, and Martin Claussen1,3 Victor Brovkin et al.
  • 1Max-Planck Institute for Meteorology, Hamburg, Germany
  • 2GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany
  • 3Meteorological Institute, University of Hamburg, Germany

Abstract. The atmospheric CO2 concentration increased by about 20 ppm from 6000 BCE to pre-industrial (1850 CE). Several hypotheses have been proposed to explain mechanisms of this CO2 growth based on either ocean or land carbon sources. Here, we apply the Earth System model MPI-ESM-LR for two transient simulations of climate and carbon cycle dynamics during this period. In the 1st simulation, atmospheric CO2 is prescribed following ice-core CO2 data. In response to the growing atmospheric CO2 concentration, land carbon storage increases until 2000 BCE, stagnates afterwards, and decreases from 1 CE, while the ocean continuously takes CO2 out of atmosphere after 4000 BCE. This leads to a missing source of 166 Pg of carbon in the ocean-land-atmosphere system by the end of the simulation. In the 2nd experiment, we applied a CO2-nudging technique using surface alkalinity forcing to follow the reconstructed CO2 concentration while keeping the carbon cycle interactive. In that case the ocean is a source of CO2 from 6000 to 2000 BCE due to a decrease in the surface ocean alkalinity. In the prescribed CO2 simulation, surface alkalinity declines as well. However, it is not sufficient to turn the ocean into a CO2 source. The carbonate ion concentration in the deep Atlantic decreases in both the prescribed and the interactive CO2 simulations, while the magnitude of the decrease in the prescribed CO2 experiment is underestimated in comparison with available proxies. As the land serves as a carbon sink until 2000 BCE due to natural carbon cycle processes in both experiments, the missing source of carbon for land and atmosphere can only be attributed to the ocean. Within our model framework, an additional mechanism, such as surface alkalinity decrease, for example due to unaccounted carbonate accumulation processes on shelves, is required for consistency with ice-core CO2 data. Consequently, our simulations support the hypothesis that the ocean was a source of CO2 until the late Holocene when anthropogenic CO2 sources started to affect atmospheric CO2.

Victor Brovkin et al.
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Victor Brovkin et al.
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Short summary
Mechanisms of atmospheric CO2 growth by 20 ppm from 6000 BCE to pre-industrial are still uncertain. We apply the Earth System model MPI-ESM-LR for two transient simulations of climate and carbon cycle. We show that an additional process, such as carbonate accumulation on shelves, is required for consistency with ice-core CO2 data. Our simulations support the hypothesis that the ocean was a source of CO2 until the late Holocene when anthropogenic CO2 sources started to affect atmospheric CO2.
Mechanisms of atmospheric CO2 growth by 20 ppm from 6000 BCE to pre-industrial are still...
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