Effects of Acidification and Warming on Long-Term Ocean Carbon Cycling Constrained by Observations

Description

Anthropogenic increase of atmospheric carbon dioxide (CO₂) leads to ocean acidification and global warming. Ocean acidification is the change in pH and carbonate chemistry due to the invasion of CO₂ into the surface ocean. These chemical changes will make it more difficult for marine organisms such as coccolithophores, foraminifera, or pteropods, to build their calcium carbonate (CaCO₃) body parts and existing CaCO₃ will dissolve more easily. Thus acidification will likely decrease the production of CaCO₃ in the future ocean. Warmer temperatures, on the other hand, lead to faster metabolic rates, which will likely increase primary and CaCO₃ production. Faster warming of surface waters than deeper waters leads to increased stratification and less nutrient input into the sunlit surface ocean (photic zone). This may cause shifts in plankton species composition favoring coccolithophores and thus increasing CaCO₃ production. Any change in CaCO₃ production may also affect organic carbon fluxes from the surface to the deep ocean due to the aggregation and association of CaCO₃ with organic particles. Here we will examine the relative importance of these effects on future global CaCO₃ production and carbon cycling on long time scales (hundreds to thousands of years) using an improved model calibrated with existing observations. CaCO₃ production increases atmospheric CO₂, thus its future evolution may be an important feedback on climate. We will estimate the sign and uncertainty of this feedback.

Chemistry of ocean acidification. Image from http://www.oceanacidification.org.uk/.

An existing global model of ocean biogeochemical cycles (MOBI) suitable for millennial time scale simulations will be improved by adding a process based formulation of particle aggregation and sinking. The model will consider two mineral forms of CaCO₃, calcite and aragonite, as well as opal, terrigenous, and organic matter as components of the aggregates. A global dataset of particulate organic carbon (POC) will be created by analyzing and calibrating large volume filtration measurements, bottle, transmissometer and satellite data including error estimates. This dataset, together with a large array of existing other global-scale biogeochemical observations will be used to calibrate the model and estimate uncertain parameters as well as different structural formulations of (a) the effect of OA on the production of CaCO₃ and (b) particle aggregation. A Bayesian data assimilation scheme, designed to quantify three hypothetic mechanisms regarding the control of the rain ratio (CaCO₃ over POC export from the euphotic zone), will be applied. Probabilistic projections will be carried out to quantify the effect of each mechanism on long term ocean carbon cycling and its feedback on atmospheric CO₂ concentrations. The ability of the existing observations to constrain the projections will be evaluated. The project aims to lead to a better, more quantitative understanding of multi-variable processes that control global cycling of CaCO₃ and carbon in the ocean and their coupling. Improving ocean biogeochemical cycling in a widely used Earth System Model of intermediate complexity will enhance infrastructure for research and education. The improved model will be made publicly available and benefit future research of long-term carbon cycle processes such as studies of anthropogenic effects or paleoclimate. The global POC dataset will also be made publicly available. International and interdisciplinary collaborations with climate modelers, statisticians and biologists from Australia, Canada and the US will be fostered. A graduate student will be trained in climate modeling, ocean biogeochemical cycling and data assimilation. In the summer of 2015 a 3-day workshop for K-12 educators will be organized with climate change and ocean acidification as its main topics. An undergraduate student will be involved in the project through a summer research experience. In the fall of 2015 a session on ocean particle dynamics will be organized at the AGU fall meeting. Society may benefit from the outcome of this project through an improved assessment (including uncertainties) of the effects of anthropogenic carbon emissions on ocean biogeochemical cycles.