Modeling the Ocean's Contribution to Past and Future Changes in Global Carbon Cycling

Tschumi, Tobias (2009). Modeling the Ocean's Contribution to Past and Future Changes in Global Carbon Cycling (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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The present thesis investigates the role of the oceans with respect to past and future changes in global carbon cycling on time scales of hundreds to several thousand years. This topic is connected to two specific questions of current carbon cycle research: the projection of atmospheric CO2 levels for the coming centuries to millenia (Plattner et al. [2008], Joos et al. [1999]) and the ocean’s role regarding the CO2 variations with an amplitude of 80-100 ppm occurring in parallel to the glacial cycles (Sigman and Boyle [2000], Archer et al. [2000]).
Why are the oceans important for the regulation of atmospheric CO2? The global oceans are the largest of the three fast-exchanging carbon reservoirs in the earth system, which are the atmosphere, the terrestrial biosphere and the oceans. The equilibration between the oceanic and atmospheric components of the global carbon cycle proceeds on several different time scales ranging from months to millenia. Therefore, oceanic processes have a significant impact on the climatically relevant variations in atmospheric CO2 concentrations on a multitude of different time scales.
The cycling of carbon within the ocean is driven by a complex interplay between physical, biological and chemical processes. The most dominant of these are the exchange of CO2 with the atmosphere, the transport of carbon by ocean currents, the ocean’s carbonate equilibrium chemistry, the marine cycles of organic matter and CaCO3 and the formation of marine sediments. The marine carbon cycle is further coupled to other biogeochemical cycles in the ocean such as the silicate, the iron or the nitrogen cycles (Sarmiento and Gruber [2006]).
The Bern3D+C ocean-carbon cycle model (Müller et al. [2006], Parekh et al. [2008], Tschumi et al. [2008]) is an intermediate-complexity representation of the carbon cycle processes in the ocean. The model’s computational efficiency makes it ideally suited to perform integrations over the time scales of interest, yet allowing for an adequate representation of the relevant processes. All the studies performed in this thesis are based on simulations using the Bern3D+C model.
The introductory chapter ot this thesis gives an overview of the global carbon cycle in general. First, the natural cycling of carbon in the earth system is discussed. Then, it is explained how humans have begun to interfere with the natural carbon cycle and climate since the beginning of the industrial era (∼1750 AD). In a final section, the current understanding of the mechanisms driving the glacial-interglacial CO2 fluctuations is briefly reviewed.
In chapter 2, a study is presented which examines the impact of changes in the zonal wind pattern of the Southern Hemisphere to the large-scale ocean circulation and marine biogeochemical cycling (Tschumi et al. [2008]). The goal of this study is to review an idea put forth by Toggweiler et al. [2006] according to which latitudinal shifts in the Southern Hemisphere westerly belt were responsible for switching the ocean’s deep circulation between two different regimes. According to the hypothesis, this threshold-like behavior is the dominant mechanism causing the glacial-interglacial CO2 variations by bringing the ocean alternately into a state of low and high ventilation.
The main findings of Tschumi et al. [2008] do not lend support for the Toggweiler-hypothesis: In the Bern3D+C model, ocean circulation and atmospheric CO2 show a rather low sensitivity to latitudinal shifts in the Southern Hemisphere westerly belt. The model is more responsive to variations in the winds’ amplitude. However, unrealistically strong reductions in wind strength are required to cause a CO2 drawdown of more than 30 ppm. In the light of these results, it is concluded that changes in the Southern Hemisphere wind pattern are an unlikely candidate to be the dominant driver of low glacial CO2 levels.
A further part of this thesis is the inclusion of a model component for marine sediment diagenesis into the Bern3D+C model (Heinze et al. [1999], Gehlen et al. [2006]). In chapter 3, a description of this sediment module is provided, the coupling to the ocean carbon cycle is discussed and the steady state of the coupled ocean-sediment model for preindustrial boundary conditions is presented. A comparison of the simulated spatial patterns of sediment composition to available sediment data suggests that the model adequately captures the dominant processes responsible for the observed distribution of marine sediments. In a next step, the model response to different atmospheric CO2 pulses is discussed in order to examine the model dynamics with respect to ocean-sediment interactions. Simulated CO2 neutralization time scales are found to be in good agreement with the results of previous studies (Archer et al. [1998], Rigdwell and Hargreaves [2007]).
Chapter 4 presents a study examining the importance of the ocean carbon pumps and ocean-sediment interactions regarding the regulation of atmospheric CO2 in the Bern3D+C model (manuscript in preparation). The analysis confirms the conclusion from other studies with three-dimensional ocean models that changes in the organic matter and CaCO3 pumps alone cannot explain the full glacial CO2 drawdown without conflicting with proxy reconstructions (Kohfeld et al. [2005], Archer et al. [2000]). However, ocean-sediment interactions in response to changes in the export of particulate matter from the surface ocean can potentially have a significant impact on atmospheric CO2 by amplifying the CO2 drawdown by up to a factor of four. The study further suggests that not only imbalances in the ocean budget of CaCO3 but also in the budget of organic matter might have been relevant for past variations in CO2 and for changes in the calcite lysocline depth.
Chapter 5 features a study which aims at assessing uncertainties in projected ocean uptake of anthropogenic CO2 associated with uncertainties in model ocean transport (Cao et al. [2008], submitted). The analysis is based on the comparison of the results from a suite of carbon cycle models including the Bern3D+C model. The study demonstrates that the impact of different ocean transport rates on the ocean uptake of anthropogenic CO2 across the models is of similar magnitude as that of climate-carbon cycle feedbacks in a single model associated with changes in temperature, circulation and marine biology. This finding emphasizes the important role of ocean transport in the fate of anthropogenic CO2.
An outlook regarding possible future model development and research topics is given in chapter 6.

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