Two thirds of corals could be saved only under a scenario with strong action on mitigating greenhouse-gas emissions and the assumption that corals can adapt at extremely rapid rates, suggests a study.
Otherwise all coral reefs are expected to be subject to severe degradation.
Coral reefs house almost a quarter of the species in the oceans and provide critical services including coastal protection, tourism and fishing to millions of people worldwide. Global warming and ocean acidification, both driven by human-caused CO2 emissions, pose a major threat to these ecosystems.
Our findings show that under current assumptions regarding thermal sensitivity, coral reefs might no longer be prominent coastal ecosystems if global mean temperatures actually exceed 2 degrees Celsius above the pre-industrial level, said lead author Katja Frieler from the Potsdam Institute for Climate Impact Research.
Without a yet uncertain process of adaptation or acclimation, however, already about 70 percent of corals are projected to suffer from long-term degradation by 2030 even under an ambitious mitigation scenario, Frieler noted.
Thus, the threshold to protect at least half of the coral reefs worldwide is estimated to be below 1.5 degrees Celsius mean temperature increase.
A more comprehensive and robust representation than in previous studies
This study has been conducted by scientists from Potsdam, the University of British Columbia in Canada and the Universities of Melbourne and Queensland in Australia.
It is the first comprehensive global survey of coral bleaching to express results in terms of global mean temperature change.
To project the cumulative heat stress at 2160 reef locations worldwide, they used an extensive set of 19 global climate models. By applying different emission scenarios covering the 21st century and multiple climate model simulations, a total of more than 32,000 simulation years was diagnosed. This allows for a more robust representation of uncertainty than any previous study.
Corals derive most of their energy, as well as most of their famous color, from a close symbiotic relationship with a special type of microalgae. The vital symbiosis between coral and algae can break down when stressed by warm water temperatures, making the coral bleach or turn pale. Though corals can survive this, if the heat stress persists long enough the corals can die in great numbers.
This happened in 1998, when an estimated 16 percent of corals were lost in a single, prolonged period of warmth worldwide, said Frieler.
Adaptation is uncertain and ocean acidification means even more stress
To account for a possible acclimation or