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Online DEposition over OceaNs: Modeling the effect of air pollution on ocean bio-geochemistry in an Earth System Model

Periodic Reporting for period 1 - ODEON (Online DEposition over OceaNs: Modeling the effect of air pollution on ocean bio-geochemistryin an Earth System Model)

Reporting period: 2016-10-01 to 2018-09-30

Oceans play an essential role in the adjustment of climate by absorbing part of the emitted CO2 (roughly 30%) from human activities (i.e. fossil-fuel burning, cement production, and land-use change), through the marine ecology and biogeochemistry. Indeed, ocean ecosystems keep CO2 concentrations about 150-200 ppmv lower that would have been without marine phytoplankton. The ocean sequestration of CO2 by biological processes (the biological pump) is directly linked however to the marine productivity and the biomass resources, helping sustain life on our planet. The surface ocean ecosystem comprises, thus, sources and sinks of atmospheric carbon into the deep ocean, with its functioning being controlled by the micro- and macro-nutrients availability which significantly impacted by the atmospheric deposition. Iron (Fe) is a key element for marine life and is required for photosynthesis and respiration. Micronutrient Fe delivered via atmospheric pathways may influence the primary and export production of carbon over the high-nutrient low-chlorophyll (HNLC) oceanic regions (i.e. the oceanic regions where Fe is the limiting factor for phytoplankton productivity). An understanding of the impact of Fe on global marine productivity requires a knowledge of the rates and locations of Fe supply to the ocean, and of the physicochemical forms of Fe that can be utilized by marine biota (i.e. those that are bioavailable). Note, that atmospheric deposition, and in particular that of dust particles, is highly episodic and the annual/monthly time-resolution fluxes of nutrient deposition usually used in state-of-the-art models cannot capture the observed episodic transport. The atmospheric deposition of nutrients may be thus delocalized compared to its impact, since even a non-growth- limiting nutrient deposited over the surface ocean, they may be transported at long distances before directly impacting productivity. The overall objective of ODEON was to address the long-standing shortcoming regarding the global air-quality effect on marine limitations and the ocean carbon cycle, by performing state-of-the-art atmosphere-ocean climate simulations. The main goal of the ODEON project was, eventually, to remedy the aforementioned uncertainties concerning the effects of air-quality and episodic dust deposition into the global ocean using the state-of-the-art ESM.
During the ODEON project, beyond-state-of-the-art modelling tools were developed, able to simulate the atmospheric Fe-cycle in an ESM, taking also into account the past, present-day and the projected socioeconomic impacts in our calculations. For this, the European state-of-the-art Earth System Model (ESM) EC-Earth was here extended with a detailed atmospheric Fe cycle: the atmospheric Fe deposition fields from the atmospheric chemistry transport model of the EC-Earth were coupled to its ocean biogeochemistry model, by replacing its simplistic Fe input configuration (which was based only on the atmospheric mineral dust deposition). In our calculations, we considered the up-to-date understanding of the effects of air quality on the atmospheric Fe-cycle in order to investigate the ocean biogeochemistry perturbations. The scientifically high risk/high gain element of the proposal was, however, the incorporation of an on-line Fe-scheme in the atmosphere and couple it in the ocean. For this, each module was carefully evaluated after each phase of model development, and present-day simulations were thoroughly compared to observations. Each successful evaluation of the project was an intermediate target (milestone) that allowed moving to the next phase of the project. Thus, the couplings of air-quality and atmospheric Fe input to the oceanic biogeochemistry was activated in a stepwise approach via Uncoupled (U) and Coupled (C) configurations of the Fe input. For the U-runs, a Fe- containing aerosol atmospheric processing scheme was developed and it implemented, for the first time, in the atmospheric chemistry transport model of EC-Earth. For the C-runs, the ocean biogeochemistry model’s current Fe input parameterization into the ocean was replaced by the new ODEON’s Fe deposition fields. Note that the U-runs provided us reference for C-runs’ evaluations of Fe deposition fluxes as well as for the novel estimation of the marine primary productivity, i.e. under historical (i.e. preindustrial-to-present) and future changes.
ODEON’s results and code developments were disseminated to the scientific-groups and science-users of the EC-Earth consortium, who will benefit mainly from its innovated methodology of potential air-quality impacts on marine primary production. In more details, the project provided a new code for calculating the atmospheric chemistry in EC-Earth, as well as a set of atmospheric concentrations and deposition fields for carbon-cycle simulations, improving overall the knowledge chain and delivering reliable Earth system change information to policy/decision makers. Note that all project’s results are open-access available, with no IPR protection issues and links to all publication and are available online via the social networks and through digital repositories.
For the first time, the deposition fields of a state-of-the-art atmospheric Fe-cycle scheme coupled to the ocean biogeochemistry code of EC-Earth. Up to now, the model considered atmospheric Fe supply to depend only on dust deposition, with a constant Fe-content (3.5%) and a soluble Fe-fraction (1%). For this project, Fe deposition fields were coupled to the biogeochemistry model, but as a result of a detailed dust mineralogy, dust atmospheric processing, and direct Fe emissions from combustion processes. The novel ESM aspects of this project led to beyond-state-of-the-art descriptions of the ocean fertilization and the marine primary productivity. Changes in atmospheric nutrient deposition fluxes impacted on the surface nutrient limitation, resulting in small differences in simulated surface Chl-a concentrations. However, strong regional changes on oceanic productivity and carbon uptake were calculated, especially in the subtropical gyres. This work indicated that changes in nutrient deposition affect the marine ecosystems as well as the simulated biological pump and, overall, may significantly impact on the global carbon cycle and thus the climate. ESMs’ simplifications conceal large uncertainties on climate-carbon cycle projections emphasizing the need of more complex ocean ecosystem, as well as a more detailed and systematic model evaluation during the development process. Thus, the ODEON’s innovative concept (and evaluation plan) is expected to bring EC-Earth to the forefront of future carbon-cycle studies.