Posts Tagged 'regionalmodelling'

Continuous monitoring and future projection of ocean warming, acidification, and deoxygenation on the subarctic coast of Hokkaido, Japan

As the ocean absorbs excessive anthropogenic CO2 and ocean acidification proceeds, it is thought to be harder for marine calcifying organisms, such as shellfish, to form their skeletons and shells made of calcium carbonate. Recent studies have suggested that various marine organisms, both calcifiers and non-calcifiers, will be affected adversely by ocean warming and deoxygenation. However, regardless of their effects on calcifiers, the spatiotemporal variability of parameters affecting ocean acidification and deoxygenation has not been elucidated in the subarctic coasts of Japan. This study conducted the first continuous monitoring and future projection of physical and biogeochemical parameters of the subarctic coast of Hokkaido, Japan. Our results show that the seasonal change in biogeochemical parameters, with higher pH and dissolved oxygen (DO) concentration in winter than in summer, was primarily regulated by water temperature. The daily fluctuations, which were higher in the daytime than at night, were mainly affected by daytime photosynthesis by primary producers and respiration by marine organisms at night. Our projected results suggest that, without ambitious commitment to reducing CO2 and other greenhouse gas emissions, such as by following the Paris Agreement, the impact of ocean warming and acidification on calcifiers along subarctic coasts will become serious, exceeding the critical level of high temperature for 3 months in summer and being close to the critical level of low saturation state of calcium carbonate for 2 months in mid-winter, respectively, by the end of this century. The impact of deoxygenation might often be prominent assuming that the daily fluctuation in DO concentration in the future is similar to that at present. The results also suggest the importance of adaptation strategies by local coastal industries, especially fisheries, such as modifying aquaculture styles.

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The variable circulation and carbonate chemistry of ocean upwelling systems

Ocean upwelling is a process in which winds drive deep waters to the surface ocean. The biogeochemical state of these waters causes upwelling regions to have some of the strongest air-sea fluxes of carbon dioxide (CO2) and most productive fisheries in the global oceans. In this dissertation, I use Earth System models to investigate the variability and projected impacts of climate change on upwelling systems. I first use the Community Earth System Model Large Ensemble (CESM-LE) to project the impacts of climate change on upwelling in the California Current. The CESM-LE provides an ensemble of potential trajectories of the climate system that differ due to internal climate variability. I find that upwelling is expected to weaken over the next century in the summer and intensify poleward in the spring due to anthropogenic climate change. Next, I use the CESM-LE to highlight the role of internal climate variability in modulating air-sea CO2 fluxes in the major Eastern Boundary Upwelling Systems (EBUS). I identify the major mode of internal variability that influences air-sea CO2 flux in each EBUS. I then quantify how the given mode of variability modifies local conditions, which in turn leads to the anomalous air-sea CO2 fluxes. Following this, I use a version of the CESM-LE that is configured for climate prediction to examine predictability of ocean acidification in the California Current. I find that our system makes skillful forecasts of surface pH out to fourteen months relative to observations and has a potential ceiling of skillful prediction out to five years in some regions. Finally, I use the Model for Prediction Across Scales Ocean (MPAS-O) to investigate the pathways over which carbon upwells in the Southern Ocean. I seed a high-resolution version of MPAS-O with 1,000,000 Lagrangian floats and find that regions with complex ocean topography have a disproportionate influence on bringing carbon-rich waters from the deep Southern Ocean to the surface. The results of this dissertation highlight the value of using ensemble methods and the Lagrangian perspective in Earth System models to better understand the dynamic and variable biogeochemistry in ocean upwelling systems.

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Reversing ocean acidification along the Great Barrier Reef using alkalinity injection

The Great Barrier Reef (GBR) is a globally significant coral reef system supporting productive and diverse ecosystems. The GBR is under increasing threat from climate change and local anthropogenic stressors, with its general condition degrading over recent decades. In response to this, a number of techniques have been proposed to offset or ameliorate environmental changes. In this study, we use a coupled hydrodynamic-biogeochemical model of the GBR and surrounding ocean to simulate artificial ocean alkalinisation (AOA) as a means to reverse the impact of global ocean acidification on GBR reefs. Our results demonstrate that a continuous release of 90 000 t of alkalinity every 3 d over one year along the entire length of the GBR, following the Gladstone-Weipa bulk carrier route, increases the mean aragonite saturation state (Ωar) across the GBR’s 3860 reefs by 0.05. This change offsets just over 4 years (∼4.2) of ocean acidification under the present rate of anthropogenic carbon emissions. The injection raises Ωar in the 250 reefs closest to the route by ⩾0.15, reversing further projected Ocean Acidification. Following cessation of alkalinity injection Ωar returns to the value of the waters in the absence of AOA over a 6 month period, primarily due to transport of additional alkalinity into the Coral Sea. Significantly, our study provides for the first time a model of AOA applied along existing shipping infrastructure that has been used to investigate shelf scale impacts. Thus, amelioration of decades of OA on the GBR is feasible using existing infrastructure, but is likely to be extremely expensive, include as yet unquantified risks, and would need to be undertaken continuously until such time, probably centuries in the future, when atmospheric CO2 concentrations have returned to today’s values.

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Projecting ocean acidification impacts for the Gulf of Maine to 2050: new tools and expectations

Ocean acidification (OA) is increasing predictably in the global ocean as rising levels of atmospheric carbon dioxide lead to higher oceanic concentrations of inorganic carbon. The Gulf of Maine (GOM) is a seasonally varying region of confluence for many processes that further affect the carbonate system including freshwater influences and high productivity, particularly near the coast where local processes impart a strong influence. Two main regions within the GOM currently experience carbonate conditions that are suboptimal for many organisms—the nearshore and subsurface deep shelf. OA trends over the past 15 years have been masked in the GOM by recent warming and changes to the regional circulation that locally supply more Gulf Stream waters. The region is home to many commercially important shellfish that are vulnerable to OA conditions, as well as to the human populations whose dependence on shellfish species in the fishery has continued to increase over the past decade. Through a review of the sensitivity of the regional marine ecosystem inhabitants, we identified a critical threshold of 1.5 for the aragonite saturation state (Ωa). A combination of regional high-resolution simulations that include coastal processes were used to project OA conditions for the GOM into 2050. By 2050, the Ωa declines everywhere in the GOM with most pronounced impacts near the coast, in subsurface waters, and associated with freshening. Under the RCP 8.5 projected climate scenario, the entire GOM will experience conditions below the critical Ωa threshold of 1.5 for most of the year by 2050. Despite these declines, the projected warming in the GOM imparts a partial compensatory effect to Ωa by elevating saturation states considerably above what would result from acidification alone and preserving some important fisheries locations, including much of Georges Bank, above the critical threshold.

Continue reading ‘Projecting ocean acidification impacts for the Gulf of Maine to 2050: new tools and expectations’

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Ocean acidification in the IPCC AR5 WG II

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