Opposite latitudinal gradients in projected ocean acidification and bleaching impacts on coral reefs

Abstract

Coral reefs and the services they provide are seriously threatened by ocean acidification and climate change impacts like coral bleaching. Here, we present updated global projections for these key threats to coral reefs based on ensembles of IPCC AR5 climate models using the new Representative Concentration Pathway (RCP) experiments. For all tropical reef locations, we project absolute and percentage changes in aragonite saturation state (Ωarag) for the period between 2006 and the onset of annual severe bleaching (thermal stress >8 degree heating weeks); a point at which it is difficult to believe reefs can persist as we know them. Severe annual bleaching is projected to start 10-15 years later at high-latitude reefs than for reefs in low latitudes under RCP8.5. In these 10-15 years, Ωarag keeps declining and thus any benefits for high-latitude reefs of later onset of annual bleaching may be negated by the effects of acidification. There are no long-term refugia from the effects of both acidification and bleaching. Of all reef locations, 90% are projected to experience severe bleaching annually by 2055. Furthermore, 5% declines in calcification are projected for all reef locations by 2034 under RCP8.5, assuming a 15% decline in calcification per unit of Ωarag. Drastic emissions cuts, such as those represented by RCP6.0, result in an average year for the onset of annual severe bleaching that is ~20 years later (2062 vs. 2044). However, global emissions are tracking above the current worst-case scenario devised by the scientific community, as has happened in previous generations of emission scenarios. The projections here for conditions on coral reefs are dire, but provide the most up-to-date assessment of what the changing climate and ocean acidification mean for the persistence of coral reefs.

Pacific-wide contrast highlights resistance of reef calcifiers to ocean acidification

“…Most empirical studies of the effects of OA on coral reefs have reported a positive correlation between calcification rates and aragonite saturation state (V arag ) [5], supporting pessimistic projections forecasting the disappearance of most coral reefs before the end of the current century [6][7][8]. However, recent studies now indicate more nuanced responses to OA for select reef calcifiers [9], with a compilation of laboratory studies of corals suggesting that coral calcification will decline approximately 10-20% (rather than ceasing) for a doubling of present-day partial pressure of CO 2 (pCO 2 ) [10].…”

Effects of seawater pCO2 and temperature on calcification and productivity in the coral genus Porites spp.: an exploration of potential interaction mechanisms

“…However, in practice, seawater temperature increases are unlikely to enhance calcification at most tropical reef locations where corals already exist close to their upper thermal tolerance limits. Future minor increases in seawater temperature at these sites are likely to induce widespread coral bleaching and thereby suppress coral calcification (Frieler et al 2013;Hooidonk et al 2013). Recent decreases in growth rates of massive Porites spp.…”

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…Accumulation of greenhouse gases in the atmosphere (primarily carbon dioxide, CO 2 ) is resulting in warmer sea surface temperatures (SST) and rising seawater acidity; processes that are predicted to intensify under current climate change projections (van Hooidonk et al, 2014). Even if carbon emissions were to stabilize today, the world's oceans will experience prolonged warming and inevitable change in carbonate chemistry, due to the lag time that oceans reequilibrate with the atmosphere (IPCC, 2014).…”