Temperature rise, the boon and bane of coral-reef growth

The multicolored coral reefs that harbor fish, protect shorelines, and delight tourists are engineered from calcium carbonate secreted by tiny animals, the corals. Ecologists have long known that the secretion rate of CaCO₃ depends on many environmental factors, including water acidity and salinity, light intensity, food availability, and temperature. But quantifying the relative importance of individual variables has proved difficult because in situ observations are scarce and the variables influence each other; light, for example, increases temperature, which affects acidity and food availability.

Now an international team led by Andreas Andersson and Travis Courtney of the Scripps Institution of Oceanography has combined observations and numerical modeling to tease out the separate contributions of several environmental factors. The researchers found that temperature is the single most important determinant of coral-reef growth and that modest temperature rise leads to a greater rate of CaCO₃ secretion. Their empirical work was conducted over a period of two years at Hog Reef (shown here) and Crescent Reef, both north of Bermuda. The team chose those two reefs in part because their relatively temperate latitudes lead to large seasonal variations in environmental factors and growth rates.

From a Bermudan coral’s perspective, the rising temperature predicted by climate modelers is a boon to home construction. At least for a while. If the temperature rises too high, corals will bleach—that is, they will perish after expelling the symbiotic algae that live in their tissues and give the reefs their color. Thus Courtney and colleagues turned to climate simulations and evaluated several climate scenarios with an eye to their effect on coral bleaching. They found that in a scenario called RCP 4.5, which posits moderate reductions in carbon dioxide emissions, the Bermuda corals should escape catastrophic bleaching until at least 2070. The goals of the Paris climate accord, if met, will probably lead to warming similar to that anticipated by RCP 4.5. The corals, of course, get no say as to whether the global community will honor the agreement. (T. A. Courtney et al., Sci. Adv. 3, e1701356, 2017 .)