Coral bleaching brought on by record ocean temperatures is hitting reefs around the world, prompting NOAA scientists to issue the third ever global bleaching event.

The alert comes after bleaching first hit Pacific reefs in 2014, moved onto reefs in the Indian Ocean and Hawaii earlier this year and recently started expanding to the Caribbean and threatening coral in Puerto Rico and the U.S. Virgin Islands, NOAA scientists said. Coral bleaching also began in the Florida Keys and South Florida in August and NOAA estimates that by the end of 2015, almost 95 percent of U.S. coral reefs will have been exposed to ocean conditions that can cause corals to bleach.

Globally, NOAA is projecting that 38 percent of the world’s coral reefs will be hit by the end of the year, which could kill over 4,633 square miles of reefs.

“The coral bleaching and disease, brought on by climate change and coupled with events like the current El Niño, are the largest and most pervasive threats to coral reefs around the world,” said Mark Eakin, NOAA’s Coral Reef Watch coordinator. “As a result, we are losing huge areas of coral across the U.S., as well as internationally. What really has us concerned is this event has been going on for more than a year and our preliminary model projections indicate it’s likely to last well into 2016.”

The other concern is the impact of a Pacific warming mass known as “the blob” as well as El Nino, which forecasters are predicting could be one of the strongest on record. Climate models are indicating El Nino – created when the equatorial waters of the Pacific Ocean warm significantly - will cause further bleaching in the Indian and southeastern Pacific Oceans in 2016.

The first ever global bleaching event was in 1998, during a strong El Niño that was followed by an equally very strong La Niña. A second one occurred in 2010. Some 15 percent of reefs around the world were lost following the 1998 event.

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 loss of reefs to bleaching can have dire economic and environmental consequences. Reefs are some of the world's most important ecosystems, supporting more species than any other marine environment, including 4,000 fish species. They are also crucial to sustaining many coastal communities that depend on the $375 billion a year reaped through diving, fishing and tourism.

”Just like in 1998 and 2010, we’re observing bleaching on a global scale, which will cause massive loss of corals. With hundreds of millions of people relying on fisheries and reefs for sustenance, the repercussions of a global coral bleaching event could be potentially disastrous,” said Professor Ove Hoegh-Guldberg, the chief scientist from the Global Change Institute at The University of Queensland.

The current high ocean temperatures in Hawaii come on the heels of bleaching in the Main Hawaiian Islands in 2014 ― only the second bleaching occurrence in the region’s history ― and devastating bleaching and coral death in parts of the remote and well-protected Papahānaumokuākea Marine National Monument in the Northwestern Hawaiian Islands.

“Last year’s bleaching at Lisianski Atoll was the worst our scientists have seen,” said Randy Kosaki, NOAA’s deputy superintendent for the monument. “Almost one and a half square miles of reef bleached last year and are now completely dead.”

Jennifer Koss,NOAA Coral Reef Conservation Program acting program manager, said the latest bleaching reports shows the need to do more to protect reefs.

“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,” Koss said.