Global warming: More bad news for coral reefs

New global assessment predicts significant damage to majority of reef ecosystems unless greenhouse gases are curbed drastically

Staghorn coral. Photo courtesy NOAA.

By Summit Voice

SUMMIT COUNTY — Most coral reefs are likely doomed unless humankind acts quickly to curb greenhouse gas emissions, according to a new global assessment of global warming impacts published last week in Nature Climate Change.

“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, of 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.”

The threshold for protecting at least half the world’s coral reef ecosystems is estimated at 1.5 degrees Celsius, according to the study conducted by scientists from Potsdam, the University of British Columbia in Canada and the Universities of Melbourne and Queensland in Australia.

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.

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.

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 relationship between coral and algae breaks down when stressed by warm water temperatures, making the coral “bleach” or turn pale. Some corals are able to rebound from short episodes, but if the heat stress persists, they die.

“This happened in 1998, when an estimated 16 percent of corals were lost in a single, prolonged period of warmth worldwide,” said Frieler.

To account for the ability of some corals to adapt,the study included some optimistic assumptions.

“However, corals themselves have all the wrong characteristics to be able to rapidly evolve new thermal tolerances,” said co-author Ove Hoegh-Guldberg, a marine biologist at the University of Queensland in Australia. “They have long lifecycles of 5-100 years and they show low levels of diversity due to the fact that corals can reproduce by cloning themselves. They are not like fruit flies which can evolve much faster.”

Previous studies estimated the effect of thermal adaptation on bleaching thresholds, but not the possible opposing effect of ocean acidification. Seawater gets more acidic when taking up CO2 from the atmosphere. This is likely to act to the detriment of the calcification processes crucial for the corals’ growth and might also reduce their thermal resilience.

The researchers said their work highlights how close we are to a world without coral reefs as we know them.

“The window of opportunity to preserve the majority of coral reefs, part of the world’s natural heritage, is small,” said Malte Meinshausen, co-author at the Potsdam Institute for Climate Impact Research and the University of Melbourne. “We close this window if we follow another decade of ballooning global greenhouse-gas emissions.”


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