What was the source of the atmospheric CO<sub>2</sub> increase during the Holocene?
<p>The atmospheric <span class="inline-formula">CO<sub>2</sub></span> concentration increased by about 20 ppm from 6000 BCE to the pre-industrial period (1850 CE). Several hypotheses have been proposed to explain mechanisms of...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-07-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/16/2543/2019/bg-16-2543-2019.pdf |
| Summary: | <p>The atmospheric <span class="inline-formula">CO<sub>2</sub></span> concentration increased by about 20 ppm
from 6000 BCE to the pre-industrial period (1850 CE). Several hypotheses have been
proposed to explain mechanisms of this <span class="inline-formula">CO<sub>2</sub></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">CO<sub>2</sub></span> is prescribed
following ice-core <span class="inline-formula">CO<sub>2</sub></span> data. In response to the growing atmospheric
<span class="inline-formula">CO<sub>2</sub></span> concentration, land carbon storage increases until 2000 BCE,
stagnates afterwards, and decreases from 1 CE, while the ocean continuously
takes <span class="inline-formula">CO<sub>2</sub></span> out of the 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 second experiment, we applied a <span class="inline-formula">CO<sub>2</sub></span> nudging
technique using surface alkalinity forcing to follow the reconstructed
<span class="inline-formula">CO<sub>2</sub></span> concentration while keeping the carbon cycle interactive. In that
case the ocean is a source of <span class="inline-formula">CO<sub>2</sub></span> from 6000 to 2000 BCE due to a
decrease in the surface ocean alkalinity. In the prescribed <span class="inline-formula">CO<sub>2</sub></span>
simulation, surface alkalinity declines as well. However, it is not
sufficient to turn the ocean into a <span class="inline-formula">CO<sub>2</sub></span> source. The carbonate ion
concentration in the deep Atlantic decreases in both the prescribed and the
interactive <span class="inline-formula">CO<sub>2</sub></span> simulations, while the magnitude of the decrease in the
prescribed <span class="inline-formula">CO<sub>2</sub></span> 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 for carbonate accumulation processes on
shelves, is required for consistency with ice-core <span class="inline-formula">CO<sub>2</sub></span> data.
Consequently, our simulations support the hypothesis that the ocean was a
source of <span class="inline-formula">CO<sub>2</sub></span> until the late Holocene when anthropogenic <span class="inline-formula">CO<sub>2</sub></span>
sources started to affect atmospheric <span class="inline-formula">CO<sub>2</sub></span>.</p> |
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| ISSN: | 1726-4170 1726-4189 |