Abstract
Abstract. Due to seasonal upwelling, the upper ocean waters of the California Current System (CCS) have a naturally low pH and aragonite saturation state (Ωarag), making this region particularly prone to the effects of ocean acidification. Here, we use the Regional Oceanic Modeling System (ROMS) to conduct preindustrial and transient (1995–2050) simulations of ocean biogeochemistry in the CCS. The transient simulations were forced with increasing atmospheric pCO2 and increasing oceanic dissolved inorganic carbon concentrations at the lateral boundaries, as projected by the NCAR CSM 1.4 model for the IPCC SRES A2 scenario. Our results show a large seasonal variability in pH (range of ~ 0.14) and Ωarag (~ 0.2) for the nearshore areas (50 km from shore). This variability is created by the interplay of physical and biogeochemical processes. Despite this large variability, we find that present-day pH and Ωarag have already moved outside of their simulated preindustrial variability envelopes (defined by ±1 temporal standard deviation) due to the rapidly increasing concentrations of atmospheric CO2. The nearshore surface pH of the northern and central CCS are simulated to move outside of their present-day variability envelopes by the mid-2040s and late 2030s, respectively. This transition may occur even earlier for nearshore surface Ωarag, which is projected to depart from its present-day variability envelope by the early- to mid-2030s. The aragonite saturation horizon of the central CCS is projected to shoal into the upper 75 m within the next 25 yr, causing near-permanent undersaturation in subsurface waters. Due to the model's overestimation of Ωarag, this transition may occur even earlier than simulated by the model. Overall, our study shows that the CCS joins the Arctic and Southern oceans as one of only a few known ocean regions presently approaching the dual threshold of widespread and near-permanent undersaturation with respect to aragonite and a departure from its variability envelope. In these regions, organisms may be forced to rapidly adjust to conditions that are both inherently chemically challenging and also substantially different from past conditions.
Highlights
Since the onset of the industrial era, the oceans have absorbed about one-third of the anthropogenically emitted carbon dioxide (Sabine et al, 2004)
Hauri et al.: Variability and long-term trends of ocean acidification scientific research (Doney et al, 2012), many results have demonstrated that these changes can have deleterious effects on marine-calcifying invertebrates such as corals, coralline algae, oyster larvae and pteropods (Orr et al, 2005; Kleypas et al, 2006; Martin and Gattuso, 2009)
While a few nearshore areas of the central and northern subregions are presently exposed to temporary undersaturation, our results highlight the fact that the nearshore ecosystems along the US West Coast are already exposed to pH and arag levels outside of the modeled preindustrial variability envelope
Summary
Since the onset of the industrial era, the oceans have absorbed about one-third of the anthropogenically emitted carbon dioxide (Sabine et al, 2004). The uptake of anthropogenic CO2 by the oceans has already reduced the global surface ocean pH by about 0.1 units (Feely et al, 2004) and it is projected to decrease another 0.3 to 0.4 pH units by the end of this century under the IPCC 1992 (IS92a) scenario (788 ppm in 2100, Orr et al, 2005) This chemical change, known as ocean acidification, leads to a decline in the saturation state ( ) of seawater with respect to calcium carbonate minerals, such as calcite or the less stable form aragonite. Hauri et al.: Variability and long-term trends of ocean acidification scientific research (Doney et al, 2012), many ( not all) results have demonstrated that these changes can have deleterious effects on marine-calcifying invertebrates such as corals, coralline algae, oyster larvae and pteropods (Orr et al, 2005; Kleypas et al, 2006; Martin and Gattuso, 2009)
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