Imminent Onset and Abrupt Increase in Duration of Aragonite Undersaturation Events in the Southern Ocean

Chair: Joellen Russell

Claudine Hauri (1,2)*, Tobias Friedrich (1), Axel Timmermann (1)

1 University of Hawai’i Mānoa, IPRC-SOEST, Honolulu, HI, United States
2 University of Alaska Fairbanks, IARC, Fairbanks, AK, United States

Background
Rapid progression of ocean acidification is a threat to key organisms of the Southern Ocean ecosystem. While the severity of ocean acidification impacts is mainly determined by the duration, intensity, and spatial extent of low aragonite or calcite saturation state events, little is known about the nature of these events, their evolving attributes, and the timing of their onset.

Methods
To study how ocean acidification will affect the Southern Ocean carbon chemistry over the next century we used output of historical and RCP 8.5 simulations from ten Earth System Models from CMIP5.

Findings
The results suggest that aragonite undersaturation, which decreases the calcification rate of pteropods and causes dissolution of their aragonitic shell, will spread rapidly after 2035, covering 70 % of the Southern Ocean surface waters by 2095. Surface aragonite undersaturation events will last for about 5 months in areas south of 60°S by 2055, and for more than 8 months by the end of the century. Overall, the duration of these events increases from 1 month to more than 6 months within fewer than 20 years in >75 % of the affected area. This abrupt change in exposure duration to unfavorable conditions may be too fast for pteropods to adapt, as these chemical changes will occur within just a few generations. As a result of two month-long calcite undersaturation events projected for the end of this century, even organisms built of the more stable calcium carbonate mineral calcite may face prolonged chemical dissolution.

Conclusions
The threat of ocean acidification to the Southern Ocean ecosystem may be more imminent than previously thought, and may spread quickly to the southern tips of New Zealand, South America, and South Africa, with potentially far-reaching consequences to fisheries, local economies, and livelihoods