Biogeosciences, 16, 2543–2555, 2019 https://doi.org/10.5194/bg-16-2543-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. 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 1Max-Planck Institute for Meteorology, Hamburg, Germany 2GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany 3Meteorological Institute, University of Hamburg, Hamburg, Germany Correspondence: Victor Brovkin ([email protected]) Received: 22 February 2019 – Discussion started: 4 March 2019 Revised: 7 June 2019 – Accepted: 12 June 2019 – Published: 2 July 2019 Abstract. The atmospheric CO2 concentration increased by face alkalinity decrease, for example due to unaccounted for about 20 ppm from 6000 BCE to the pre-industrial period carbonate accumulation processes on shelves, is required for (1850 CE). Several hypotheses have been proposed to ex- consistency with ice-core CO2 data. Consequently, our sim- plain mechanisms of this CO2 growth based on either ocean ulations support the hypothesis that the ocean was a source or land carbon sources. Here, we apply the Earth system of CO2 until the late Holocene when anthropogenic CO2 model MPI-ESM-LR for two transient simulations of climate sources started to affect atmospheric CO2. and carbon cycle dynamics during this period. In the first 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, 1 Introduction stagnates afterwards, and decreases from 1 CE, while the ocean continuously takes CO2 out of the atmosphere after The recent interglacial period, the Holocene, began in 4000 BCE. This leads to a missing source of 166 Pg of carbon about 9700 BCE (Before Common Era) and is character- in the ocean–land–atmosphere system by the end of the simu- ized by a relatively stable climate. In geological archives, lation. In the second experiment, we applied a CO2 nudging the Holocene is the best recorded period, making it pos- technique using surface alkalinity forcing to follow the re- sible to reconstruct changes in climate and vegetation in constructed CO2 concentration while keeping the carbon cy- remarkable detail (e.g. Wanner et al., 2008). Proxy-based cle interactive. In that case the ocean is a source of CO2 from reconstructions suggest a decrease in sea surface tempera- 6000 to 2000 BCE due to a decrease in the surface ocean al- tures in the North Atlantic (Marcott et al., 2013; Kim et al., kalinity. In the prescribed CO2 simulation, surface alkalinity 2004) simultaneous with an increase in land temperature in declines as well. However, it is not sufficient to turn the ocean western Eurasia (Baker et al., 2017), so that net changes in into a CO2 source. The carbonate ion concentration in the the global temperature are small. From Antarctic ice-core deep Atlantic decreases in both the prescribed and the inter- records, we know that the atmospheric CO2 concentration active CO2 simulations, while the magnitude of the decrease increased by about 20 ppm between 5000 BCE and the pre- in the prescribed CO2 experiment is underestimated in com- industrial period (Monnin et al., 2004; Schmitt et al., 2012; parison with available proxies. As the land serves as a carbon Schneider et al., 2013). Hypotheses explaining CO2 growth sink until 2000 BCE due to natural carbon cycle processes in the Holocene could be roughly subdivided into ocean- in both experiments, the missing source of carbon for land and land-based. The ocean mechanisms include changes in and atmosphere can only be attributed to the ocean. Within carbonate chemistry as a result of carbonate compensation our model framework, an additional mechanism, such as sur- to deglaciation processes (Broecker et al., 1999, 2001; Joos et al., 2004), redistribution of carbonate sedimentation from Published by Copernicus Publications on behalf of the European Geosciences Union. 2544 V. Brovkin et al.: What was the atmospheric CO2 source in the Holocene? the deep ocean to shelves, mostly due to coral reef regrowth surface model JSBACH (Raddatz et al., 2007; Reick et al., (Ridgwell et al., 2003; Kleinen et al., 2016), CO2 degassing 2013), and the marine biogeochemistry model HAMOCC5 due to an increase in sea surface temperatures, predominantly (Ilyina et al., 2013). In comparison to the MPI-ESM-LR in the tropics (Indermühle et al., 1999; Brovkin et al., 2008), model used in the Climate Model Intercomparison Project and a decrease in the marine soft tissue pump in response to Phase 5 (CMIP5) simulations (Giorgetta et al., 2013), the circulation changes (Goodwin et al., 2011). Using a deconvo- model has been updated with several new components for 13 lution approach based on ice-core CO2 and δ C data, Elsig the land hydrology and carbon cycle. In addition to the pre- et al. (2009) concluded that a significant fraction of Holocene viously developed dynamic vegetation model (Brovkin et al., CO2 changes were attributed to carbonate compensation ef- 2009), the new JSBACH component includes the soil car- fects during deglaciation. Recent synthesis of carbon burial bon model YASSO (Goll et al., 2015), a five-layer hydrol- in the ocean during the last glacial cycle suggests excessive ogy scheme (Hagemann and Stacke, 2015), and an interac- accumulation of CaCO3 and organic carbon in the ocean sed- tive albedo scheme (Vamborg et al., 2011). HAMOCC was iments during deglaciation and Holocene (Cartapanis et al., updated with prognostic nitrogen fixers (Paulsen et al., 2017) 2018), implicitly supporting the ocean-based mechanism of and a parameterization for a depth-dependent detritus settling atmospheric CO2 growth in response to a decrease in the velocity and remineralization of organic material (Martin et ocean alkalinity. al., 1987). The land-based explanations suggest a reduction in natural We used a combination of several forcings as boundary vegetation cover, such as decreased boreal forests’ area and conditions for transient simulations (Fig. 1). The orbital forc- increased desert in North Africa (e.g. Foley, 1994; Brovkin ing (Fig. 1a) follows a reconstruction by Berger (1978). We et al., 2002) and anthropogenic land cover changes, mainly use a solar irradiance forcing that was reconstructed from 14C deforestation (Ruddiman, 2003, 2017; Kaplan et al., 2011; tree-ring data that correlate with the past changes in the so- Stocker et al., 2014, 2017). A couple of land processes (CO2 lar open magnetic field (Krivova et al., 2011). CO2,N2O, fertilization, peat accumulation) should have led to terres- and CH4 forcings stem from ice-core reconstructions (Fortu- trial carbon increase in the Holocene (Yu, 2012; Stocker et nat Joos, personal communication, 2016, Fig. 1b,c); see also al., 2017), complicating the land source explanation. The comment by Köhler (2019). We applied a new reconstruc- land-based hypotheses, as well as the ocean soft tissue pump tion of volcanic forcing (Bader et al., 2019) based on the explanation, also have difficulty conforming with the at- GISP2 ice-core volcanic sulfate record (Zielinski et al., 1996) 13 mospheric δ CO2 reconstructions that show no substan- (Fig. 1d). To maintain consistency with the CMIP5 simula- tial changes in the Holocene (Schmitt et al., 2012; Schnei- tions, we included the land use changes based on the LUH1.0 der et al., 2013), contrary to the expectation of a signif- product by Hurrt et al. (2011) and Pongratz et al. (2011), pro- 13 icant atmospheric δ CO2 decrease due to the release of vided for the period after 850 CE. To avoid an abrupt change isotopically light biological carbon. The deconvolution ap- in the land use forcing at 850 CE, we interpolated the land proach by Elsig et al. (2009) suggested a land carbon up- use map from 850 CE linearly backwards to no land use con- take of 290 GtC between 9000 and 3000 BCE. For a de- ditions at 150 BCE (Fig. 1e). The land use areas in 1750 CE tailed overview of process-based hypotheses, see, for exam- are in line with the most recent update of the HYDE dataset ple, Brovkin et al. (2016). (Klein Goldewijk et al., 2017), although our interpolation un- While many numerical experiments have been done to test derestimates crop and pasture areas earlier than 1750 CE. the above-mentioned hypotheses with intermediate complex- The spin-up simulation 8KAF started from initial condi- ity models, this class of models is limited in spatial and tem- tions for the pre-industrial climate and continued with bound- poral resolution and consequently does not resolve well cli- ary conditions for 6000 BCE for more than 1000 years in or- mate patterns and variability. Here, we apply the full-scale der to establish an equilibrium of the climate and carbon cy- Earth system model MPI-ESM-LR for simulations of the cle with the boundary conditions. During the spin-up period coupled climate and carbon cycle during the period from the atmospheric CO2 concentration was kept at a constant 6000 BCE to 1850 CE. The focus of this paper is on the car- level of 260 ppm. The weathering flux was not changed com- bon cycle dynamics for terrestrial and marine components, pared to pre-industrial conditions. The change in the bound- while changes in climate are considered in the companion ary conditions to 6000 BCE led to slightly different sedimen- paper by Bader et al. (2019). tation fluxes, which resulted in a slow decline of alkalinity in 8KAF. Afterwards, the model was run with an interactive carbon cycle to ensure a dynamic equilibrium between land, 2 Methods ocean, and atmospheric carbon cycle components (simula- tion 8KAFc). In the 8KAFc simulation, the equilibration pro- The MPI-ESM-1.2LR used in the Holocene simulations con- cedure in HAMOCC followed the CMIP5 spin-up procedure sists of the coupled general circulation models for the at- (Ilyina et al., 2013): mosphere and the ocean, ECHAM6 (Stevens et al., 2013) and MPIOM (Jungclaus et al., 2013), respectively, the land Biogeosciences, 16, 2543–2555, 2019 www.biogeosciences.net/16/2543/2019/ V.