Scientists have warned that there is little hope for the survival of coral reefs unless greenhouse gas emissions are curtailed. Global warming and ocean acidification, both driven by human-caused carbon dioxide emissions, pose a major threat to these ecosystems, they said.

“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 two degrees Celsius above the pre-industrial level,” said researcher Katja Frieler from the Potsdam Institute for Climate Impact Research, Germany.

“Without a yet uncertain process of adaptation or acclimation, however, already about 70 per cent of corals are projected to suffer from long-term degradation by 2030 even under an ambitious mitigation scenario,” Frieler said in a statement.

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.

To project the cumulative heat stress at 2,160 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.

Polyps, the builders of coral reefs, derive most of their energy, as well as most of their colour, from a close symbiotic relationship with a special type of microalgae. Though polyps can survive this, but if the heat stress persists long enough they can die in great numbers.

Coral reefs are home to almost a quarter of the species in the oceans and provide critical services — including coastal protection, tourism and fishing — to millions of people world-wide.

The findings are published in the journal Nature Climate Change.