Global predictions of coral reef dissolution in the Anthropocene

Arising from K. Davis et al. Communications Earth & Environment (2021)

Coral reef frameworks are constructed by calcifying organisms and are highly sensitive to ocean acidification. Shifting baselines in seawater chemistry have already had measurable impacts on net ecosystem calcification (Gnet) on coral reefs1, and projections of ocean acidification portray a poor future for reefs in the Anthropocene2. While experimental approaches have revealed much about this trajectory, we lack a clear understanding of: i) the drivers and predictors of net calcification at ecosystem scales, and ii) accurate predictions of when ecosystem calcification will reach net dissolution in the 21st century.

Through a meta-analysis approach, the recent study in Communications Earth & Environment by Davis et al.3 provides important insights into ecosystem-scale calcification on coral reefs. Based upon 53 publications spanning 36 coral reef sites around the world, the study provides a more nuanced understanding of the global drivers of Gnet. Cover of reef calcifiers (predominantly corals) and depth are key predictors of global ecosystem calcification, with evidence of seasonality and wave action as additional factors influencing Gnet3. The meta-analysis outlines important knowledge gaps and research needs and highlights the limited data available for assessing changes in ecosystem calcification at the same reefs through time.

Under future projections, ocean acidification is expected to shift coral reefs from a state of net calcification to net dissolution through reductions in pH and aragonite saturation states (Ωa)4,5. The exact timing of this is unclear, in part due to methodological differences, but estimates of when coral reefs will cross a tipping point to net dissolution vary substantially from 2031 to 20826, 20707, and 2060 to 20804. Through the compilation of Gnet from a subset of sites with repeated measurements (6 of the 36 available coral reefs; n = 29 of the available 116 surveys), Davis et al.3 extrapolate linear predictions of Gnet decline (1975–2017) to conclude that average global net-zero calcification will occur around the year 2054, based on a decline in Gnet of 4.3 ± 1.9% yr−1.

Extrapolating estimates of Gnet into the 21st century based upon the available historical data is complex. We identify four issues with this approach:

Wolfe K. & Roff G., 2022. Global predictions of coral reef dissolution in the Anthropocene. Communications Earth & Environment 3: 42. doi: 10.1038/s43247-022-00363-3. Article.

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