Is there a CO2 tipping point for coral reefs?

Only one coral reef in the entire Galápagos Archipelago has recovered and persisted after El Niño Southern Oscillation (ENSO) warming caused large-scale coral bleaching and mortality in 1982-1983 (1). This reef occurs where pH > 8.0 and aragonite saturation state (Ωarag) > 3. Coral reefs that were located where pH < 8.0 and Ωarag < 3 were completely lost within approximately 10 years of this warming event and have not exhibited any recovery (Figs. 1a and 1b). These results suggest that Porites reefs can rebound from significant warming (+3-4oC for > 2 months), but only at acidification levels of pH > 8.0. On the other side of the Pacific, in Palau, high coral cover (Fig. 2) and Porites calcification rates are maintained under chronically high
temperatures (~30°C), across a natural gradient in pH (average pH = 7.8-8.1) and Ωarag (average Ωarag = 2.3-3.7) (2).

The CO2 tipping point in Galápagos, where reefs are lost, generally agrees with initial observations that present-day reefs rarely occur in regions where open ocean surface water Ωarag < 3. However, field evidence from other naturally high CO2 sites have shown reef persistence at higher levels of acidification. At the volcanic CO2 seeps in Papua New Guinea (PNG), Fabricius et al. (3) reported the loss of reef framework structures at a pH of ~7.7. In Palau, reefs with high coral cover and diversity exist at Ωarag <2.7 (2). Among the naturally high-CO2 reef sites currently identified, Palau is unique in supporting high coral cover, diversity, and Porites calcification rates at Ωarag < 2.7. (…)

Manzello D. P. & Shamberger K. E. F., 2015. Is there a CO2 tipping point for coral reefs? OCB News 8(1):1-2. Article.

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