Understanding the Influence of Physical Dynamics on Biogeochemical Fluxes in Global Oceanic Ecosystems: An Imperative for Earth System Modeling and International Climate Change Policy
Throughout the geological record, changes in oceans’ ecosystem structure have both impacted and been driven by changes in global biogeochemical cycles and carbon storage. The Intergovernmental Panel on Climate Change (IPCC) summarize the risks associated with rising carbon emissions caused by anthropogenic energy demand, but these effects remain to be quantified on multi-millennial timescales. Presently, we are witnessing the effects of increasing global ocean temperatures, and the associated changes on carbonate saturation state, species’ adaptive capacity, biogeochemical cycles, and phytoplankton phenology (Parmesan et al., 2003). However, our mechanistic understanding of how marine ecosystems will respond to future variations in climate is still in its infancy, as this requires a multitude of knowledge on the many processes that occur over a wide range of spatial scales and time scales from 1s to 100 years or more. One major uncertainty in projecting future ecosystem states resides in the uncertainties concerning the rate of lateral mixing and its associated effects on biogeochemical cycles. By evaluating the effects when parameterizing six different values of the turbulent diffusion coefficient, Aredi (Redi, 1982), we are able to study how the uncertainty in the coefficient propagates to uncertainties regarding changes in saturation state, pH, oxygen, phytoplankton community size structure, and export production. We undergo this study by using a suite of coupled atmosphere-ocean-biosphere models in which the only parameter that is changed is the mixing coefficient. We double and quadruple CO2 from pre-industrial values to investigate the delayed response on the climate system. Our results show that the impact of employing ARedi as the turbulent diffusion parameter is shown to alter the initial condition of some biological variables, and thus defining the fluxes taking place on a long-term trajectory. Biochemical reactions are not simply driven by changes in mixing, however, instead the impact of defining a diffusion parameter propagates nonlinearly into biological systems. Lastly, our study shows that the anthropogenic increase of carbon dioxide in the atmosphere will result in a change in convection that will ultimately dictate the availability of heat and nutrients to the surface waters, driving changes in calcification which in turn impacts oxygen availability and ocean sequestration. Because the turbulent diffusion parameter chosen impacts highly nonlinear biogeochemical processes, it governs the effects in a model simulation. Future changes in ocean habitats thus requires further knowledge on the characteristics of ocean overturning to develop a mechanistic understanding for both biogeochemical changes and changes to ventilation at depth. The estimation of this uncertainty provides essential information for assessing climate change risks, impacts, and vulnerabilities, but also for estimating mitigation and adaptation targets for present-day and future international climate change policy agreements.