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

• Main Authors: V. Brovkin, S. Lorenz, T. Raddatz, T. Ilyina, I. Stemmler, M. Toohey, M. Claussen
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-07-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/16/2543/2019/bg-16-2543-2019.pdf

<p>The atmospheric <span class="inline-formula">CO₂</span> concentration increased by about 20&thinsp;ppm from 6000&thinsp;BCE to the pre-industrial period (1850&thinsp;CE). Several hypotheses have been proposed to explain mechanisms of this <span class="inline-formula">CO₂</span> 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 first simulation, atmospheric <span class="inline-formula">CO₂</span> is prescribed following ice-core <span class="inline-formula">CO₂</span> data. In response to the growing atmospheric <span class="inline-formula">CO₂</span> concentration, land carbon storage increases until 2000&thinsp;BCE, stagnates afterwards, and decreases from 1&thinsp;CE, while the ocean continuously takes <span class="inline-formula">CO₂</span> out of the atmosphere after 4000&thinsp;BCE. This leads to a missing source of 166&thinsp;Pg of carbon in the ocean–land–atmosphere system by the end of the simulation. In the second experiment, we applied a <span class="inline-formula">CO₂</span> nudging technique using surface alkalinity forcing to follow the reconstructed <span class="inline-formula">CO₂</span> concentration while keeping the carbon cycle interactive. In that case the ocean is a source of <span class="inline-formula">CO₂</span> from 6000 to 2000&thinsp;BCE due to a decrease in the surface ocean alkalinity. In the prescribed <span class="inline-formula">CO₂</span> simulation, surface alkalinity declines as well. However, it is not sufficient to turn the ocean into a <span class="inline-formula">CO₂</span> source. The carbonate ion concentration in the deep Atlantic decreases in both the prescribed and the interactive <span class="inline-formula">CO₂</span> simulations, while the magnitude of the decrease in the prescribed <span class="inline-formula">CO₂</span> experiment is underestimated in comparison with available proxies. As the land serves as a carbon sink until 2000&thinsp;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 for carbonate accumulation processes on shelves, is required for consistency with ice-core <span class="inline-formula">CO₂</span> data. Consequently, our simulations support the hypothesis that the ocean was a source of <span class="inline-formula">CO₂</span> until the late Holocene when anthropogenic <span class="inline-formula">CO₂</span> sources started to affect atmospheric <span class="inline-formula">CO₂</span>.</p>