Berlin, Sep 17 (IANS): Unless greenhouse gas emissions are curtailed, there is little hope for the survival of coral reefs, supporting a diversity of life forms, warns a new study.

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.

Global warming and ocean acidification, both driven by human-caused CO2 emissions, pose a major threat to these ecosystems, the journal Nature Climate Change reports.

The study is the first comprehensive global survey of coral bleaching to express results in terms of global mean temperature change, according to a Potsdam statement.

It has been conducted by scientists from the Potsdam Institute for Climate Impact Research, Germany, the University of British Columbia, Canada, and the Australian universities of Melbourne and Queensland.

"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," says Katja Frieler from the Potsdam Institute for Climate Impact Research, Germany, who led the study.

"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," adds Frieler.

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. This allows for a more robust representation of uncertainty than any previous study.

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.

The vital symbiosis between polyps and algae can break down when stressed by warm water temperatures, making the coral "bleach" or turn pale.

Though polyps can survive this, but if the heat stress persists long enough they 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," says Frieler.