Coral bleaching is now a well-known phenomenon. When corals are exposed to heat stress, they expel the symbiotic algae that give them their colour and provide them with a large proportion of their energy needs. In 1998, vast tracts of the Great Barrier Reef were bleached ghostly white. A second recent bleaching event occurred in 2010–2011, when record high rainfalls in eastern Australia caused discharges of freshwater to reduce the salinity of coastal waters. Then, in 2015–16, elevated sea temperatures, combined with a strong El Niño event, caused 93 per cent of coral on the northern section of the Great Barrier Reef to bleach.
Coral scientists have the diffi cult job of predicting the extent of coral damage under future climate scenarios. A recent study from NOAA’s Atlantic Oceanographic and Meteorological Laboratory predicted that, by 2034, bleaching will occur in 100 per cent of corals if the world at large adopts a fossil-fuel-aggressive climate path. On the other hand, the authors predict that we can delay severe bleaching of corals by 11 years if we instead adopt an ambitious but feasible course of action in which warming is limited to 3.8–4.2°C by the end of the century.
However, not all scientists are convinced that coral’s adaptability to thermal stress has been modelled accurately. Forecasts such as NOAA’s typically rely on satellite data to measure projected sea-surface temperatures, setting a ‘bleaching threshold’ beyond which corals in a given area will bleach. Some adaptability to thermal stress is usually modelled mathematically. While these models certainly tick the box for mass coverage, they may miss out subtle differences that cause variability in coral’s resilience.
In the aftermath of the bleaching events of 1998 and 2016, coral scientist Tim McClanahan noticed that colleagues at different locations reported varying levels of bleaching. ‘The stuff coming out in the literature didn’t align with what my colleagues in different locations were reporting,’ he says. ‘There were some areas really feeling the effects, but others that weren’t. It was assumed that corals acclimate to thermal stresses at the same rate, but that’s without accounting for the fact that there are many types of corals in many different regions.’
In 2019, a separate team of scientists reported that corals that were predicted to exceed the bleaching threshold, defined by previous climate models, were actually showing a greater ability to adapt to thermal stresses. To investigate, McClanahan launched a study with researchers from 19 tropical research institutions to assess the sensitivities of 226 reefs in 12 countries across 2016, one of the Earth’s warmest years on record. Field observation data of bleaching events were collected and compared with satellite data of coral exposure to high sea temperaturesThe team found that past climate warming models overestimated coral destruction in the Coral Triangle – the region that spans Indonesia, Malaysia, Papua New Guinea, the Philippines and the Solomon Islands, where three quarters of the world’s coral species live. In particular, reefs around the Australian, Indonesian and Fiji-Caroline regions were better able to adapt to thermal stress than was previously thought.
According to McClanahan, one reason might be that those corals that routinely experience heat oscillations during El Niño events may adapt more quickly to thermal stress. ‘Corals have been growing in these environments for millennia, evolving with the exposure to thermal stresses,’ he points out. With resources for marine conservation constrained, the research suggests that treating the Coral Triangle as a ‘climate refuge’ could be the best bet for targeted conservation action.