Representations of biological calcification in two climate models

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Copyright: Kvale, Karin
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Abstract
Present and future changes to the ocean organic carbon and carbonate pumps are and will continue to affect ocean ecosystem dynamics and biogeochemical cycles. In this thesis, the ocean organic and carbonate cycles are modelled with a variety of parameterisations and complexity and used to simulate both pre-industrial as well as future hypothetical states. The models produce similar pre-industrial states but very different transient behaviours. A simplistic Biogeochemistry-Ocean Model, the CSIRO-Mk3L, demonstrates a large future ocean saturation state sensitivity to enhanced organic carbon export and reduced carbonate export, though sedimentary carbonate compensation is not considered. A more complex Earth System Model, the University of Victoria Earth System Climate Model (UVic ESCM), is improved with the inclusion of a calcifying phytoplankton functional type, a full calcite tracer, carbonate chemistry dependent calcite dissolution rates, and a ballasting scheme. These modifications slightly improve UVic ESCM performance with respect to observed carbon and nutrient fluxes and greatly improve the mechanistic realism of the model. Addition of a carbonate chemistry dependency on calcifier microbial recycling rates improves calcifier biogeography, but produces mixed results with respect to observed carbon and nutrient fluxes. Greenhouse gas forcing of four variations of the UVic ESCM (1: without calcifiers, 2: with calcifiers, 3: with calcifiers and prognostic CaCO3, and 4: with calcifiers, prognostic CaCO3, and chemistry-dependent calcifier microbial recycling) show a common response of a transition between two globally dominant biogeochemical feedbacks. The first is a physically driven one that dominates at lower changes in sea surface temperatures (SSTs) and is sensitive to phytoplankton community composition. The second dominant feedback is biologically driven and overtakes the first at higher SST changes. The deep ocean carbon export and oxygen response is found to be highly sensitive to the inclusion of a prognostic calcite tracer for higher SST changes under this second dominant feedback. This thesis underscores the importance of accurate representation of phytoplankton physiology and ecology in climate models, as well as inclusion of prognostic calcite that accounts for organic carbon ballasting. It also highlights the potentially under-appreciated role of microbial processes under both ocean warming and acidification.
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Author(s)
Kvale, Karin
Supervisor(s)
Meissner, Katrin
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Publication Year
2014
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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