Evidence from Oceanographic Cruises

Sabine and colleagues [1] used inorganic carbon measurements from an international survey effort in the 1990s, consisting of 9618 hydrographic stations collected on 95 cruises in different oceans (pH data is mapped in Fig. 1b). They estimated a global oceanic anthropogenic CO2 sink for the period from 1800 to 1994 of 118 ± 19 Pg of carbon, accounting for about 48% of the total fossil-fuel and cement-manufacturing emissions.

A hydrographic survey along the western coast of North America, from central Canada to northern Mexico, revealed upwelling of seawater undersat-urated with respect to aragonite and with low pH (<7.75) onto large portions kW Vw %

Longitude

FIGURE 4 Distribution of the depths of the undersaturated water (aragonite saturation < 1.0; pH < 7.75) on the continental shelf of western North America from Queen Charlotte Sound, Canada to San Gregorio Baja California Sur, Mexico. On transect line 5 the corrosive water reaches all the way to the surface in the inshore waters near the coast. The black dots represent station locations. From Feely et al. [13].

52°N 50°N 48°N 46°N 44°N 42°N 40°N 38°N 36°N 34°N 32°N 30°N 28°N 26°N

Longitude

FIGURE 4 Distribution of the depths of the undersaturated water (aragonite saturation < 1.0; pH < 7.75) on the continental shelf of western North America from Queen Charlotte Sound, Canada to San Gregorio Baja California Sur, Mexico. On transect line 5 the corrosive water reaches all the way to the surface in the inshore waters near the coast. The black dots represent station locations. From Feely et al. [13].

of the continental shelf [13] (Fig. 4). The areal extent of this natural phenomenon has been increased by the ocean uptake of anthropogenic CO2. They estimated that during pre-industrial days the ASH would have been about 50 m deeper with no undersaturated waters reaching the surface. With the additional anthropogenic CO2 signal the ASH has shoaled by around 1 m a 1 bringing increasingly corrosive conditions with pH as low as 7.6 not just to the deeper benthic communities but also, increasingly, to the productive, shallower continental shelf ecosystems. How these ecosystems respond to this seasonal inflow of undersaturated waters from February to August, during the growing season, has not yet been reported but these coastal ecosystems may well represent the first shallow sea ecosystems that experience rapid and nonlinear undersaturation due to uptake of anthropogenic CO2. In CO2-rich vent waters in the Mediterranean, useful 'hot-spots' of waters with a range of pH that offer a useful natural laboratory to study the response of marine organisms to long term exposure to reduced pH [19]. A seawater pH of around 7.8 seemed a critical threshold to the growth and survival of many of the local calcifiers and should organisms on the west coast of North America exhibit similar vulnerability then an ocean acidification 'tipping point' may well have been or soon be reached in these waters.

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