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Published: April 1, 2009
First published in IGBP's Global Change Newsletter Issue 73, April 2009

High vulnerability of eastern boundary upwelling systems  to ocean acidification

Nicolas Gruber
IMBER SSC Member

Claudine Hauri

Gian-Kasper Plattner

Environmental Physics
Institute of Biogeochemistry  and Pollutant Dynamics
ETH Zurich
Zurich, Switzerland

nicolas.gruber@env.ethz.ch

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The Ocean in a High-CO2 World: Science highlights from the symposium
Eastern boundary upwelling systems, such as the California Current, are particularly sensitive to ocean acidification: the pH of their surface waters is already comparatively low and their change in pH for a given uptake of anthropogenic CO2 is particularly high. Eddy-resolving simulations [1] for the California Current System show that between pre-industrial times and present, the mean pH of the surface ocean has decreased by about 0.1 pH units. As a result, the aragonite saturation horizon has shoaled by ~100 m, bringing waters corrosive to calcifying organisms into the surface (euphotic) zone in a few eddies and in near-shore environments during upwelling (Figure 1). The model data agree with recent observations. Projections for 2050 (IPCC SRES A2-scenario) suggest an additional drop of pH by ~0.2 units and a widespread and year-round shoaling of the saturation horizon into the euphotic zone. Due to the high temporal and spatial variability that characterizes eastern boundary upwelling systems, organisms are exposed to a wide range of pH (variations of up to 0.3 to 0.4 units) and saturation states, making it difficult to define when critical thresholds are crossed. At the same time, these systems may today offer opportunities to study the response to organisms to low and varying pH and saturations states likely to be widely experienced in the future.    
Figure 1: Model simulations of the shoaling of the aragonite saturation horizon (depth in m) in the California Current from 1750 to 2050 (snapshots for the month of August). White areas depict saturated (non-corrosive) waters to the sea floor.
Footnote
1. Produced with the ETH-UCLA Regional Oceanic Modeling System.
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