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NEW YORK: Driven by climate change and a persistent underwater heat wave, our oceans are now facing the third global coral bleaching event that could impact approximately 38 percent of the world’s coral reefs by the end of this year and kill over 12,000 square kilometres of reefs, scientists have warned.

Coral bleaching occurs when corals are exposed to stressful environmental conditions such as high temperature. Corals expel the symbiotic algae living in their tissues, causing corals to turn white or pale. Without the algae, the coral loses its major source of food and is more susceptible to disease.

"The coral bleaching and disease, brought on by climate change and coupled with events like the current El Nino, are the largest and most pervasive threats to coral reefs around the world," said Mark Eakin, coral reef watch coordinator at US National Oceanic and Atmospheric Administration (NOAA).

The first global bleaching event was in 1998, during a strong El Niño that was followed by an equally very strong La Nina. A second one occurred in 2010.

The current bleaching event, which began in the north Pacific in summer 2014 and expanded to the south Pacific and Indian oceans in 2015, is hitting US coral reefs disproportionately hard.

By the end of 2015, almost 95 percent of US coral reefs will have been exposed to ocean conditions that can cause corals to bleach, NOAA estimated.

"What really has us concerned is this event has been going on for more than a year and our preliminary model projections indicate it is likely to last well into 2016," Eakin said.

Although reefs represent less than 0.1 percent of the world’s ocean floor, they help support approximately 25 percent of all marine species.

As a result, the livelihoods of 500 million people and income worth over $30 billion are at stake, the report said.

This announcement made on Thursday stemmed from the latest NOAA Coral Reef Watch satellite coral bleaching monitoring products, and was confirmed through reports from partner organisations, especially the XL Catlin Seaview Survey and ReefCheck.

While corals can recover from mild bleaching, severe or long term bleaching is often lethal.

After corals die, reefs quickly degrade and the structures corals build erode. This provides less shoreline protection from storms and fewer habitats for fish and other marine life, including ecologically and economically important species.

"We need to act locally and think globally to address these bleaching events. Locally produced threats to coral, such as pollution from the land and unsustainable fishing practices, stress the health of corals and decrease the likelihood that corals can either resist bleaching, or recover from it," Jennifer Koss from NOAA noted.

"To solve the long term, global problem, however, we need to better understand how to reduce the unnatural carbon dioxide levels that are the major driver of the warming," Koss pointed out.