The world's third-ever global coral bleaching event is now underway, according to the National Oceanic and Atmospheric Administration (NOAA) along with a consortium of other researchers around the world. Mashable first reported the strong possibility that a global event would be declared in October, pending observations of coral bleaching in the Caribbean and Bahamas, which has now taken place.

This bleaching event, which began in the north Pacific in the summer of 2014 and expanded to the south Pacific and Indian oceans in 2015, is hitting U.S. coral reefs particularly hard. According to a press release, NOAA estimates that by the end of 2015, almost 95% of U.S. coral reefs will have been exposed to ocean conditions that can cause corals to bleach.

Corals are hotbeds for biodiversity and are central to the survival of numerous commercially valuable fish species around the world. Many corals thrive within relatively narrow temperature ranges, which makes them vulnerable to injury and disease when water temperatures stray outside of this range, particularly for long periods of time.

“This is only the third time we’ve seen a global-scale bleaching event,” said Mark Eakin, NOAA coral reef watch coordinator, in a statement. “What really has us concerned is this event has been going on for over a year and is likely to last another year.”

The worst state, when it comes to coral impacts, appears to be Hawaii, where this is just the second bleaching event in two years. Corals can survive bleaching events, but back-to-back events can make them more vulnerable to succumbing to disease, pollution or other ills.

On a global level, this event is expected to affect more than 38% of the world's coral reefs by the end of 2015 and kill more than 4,630 square miles (more than 12,000 square kilometers) of reefs, according to NOAA.

This calculation is based on the bleaching observed from mid-2014 to now, plus the bleaching that is expected to occur according to the four month outlook through January.

“We’re concerned about bleaching kicking in in Puerto Rico and the Virgin Islands,” Eakin told Mashable in an interview. In addition, Hawaii has seen extensive bleaching that is "very severe in some places" Eakin said. Those conditions are not expected to significantly improve anytime soon.

Coral bleaching outlook (areas with 60% chance or greater of bleaching) from October through January 2015-2016. Image: NOAA Coral Reef Watch

Eakin says there is the potential for considerable bleaching in the eastern Pacific as well as the central and southern Pacific, given the mild water temperatures there related to El Niño.

Corals also provide protection for shorelines from storm surge flooding, among other uses. Although reefs represent less than 0.1% of the world’s ocean floor, they help support approximately 25% of all marine species, according to NOAA, putting 500 million people and $30 billion in income at risk.

Coral bleaching occurs when waters reach certain temperatures, causing corals to expel the algae that live in their tissues and give them their vibrant colors, while supporting extraordinarily diverse ecosystems. When a coral bleaches, it exposes its skeleton and turns white. It is then subject to higher mortality rates from further heat stress or the effects of pollution.

Global average surface temperature anomalies during El Nino, La Nina and ENSO-neutral years, showing a long-term warming trend and El Nino-related spikes. Image: Climate Central

The conditions today are similar to conditions going into one of the two past global bleaching events in 1998, when, like today, a strong El Niño event in the tropical Pacific Ocean helped cause ocean temperatures to spike. The difference is that now there is an even larger expanse of warmer than average waters and a bigger jump in ocean temperatures, Eakin said.

“You have an already warm ocean, you toss an El Niño on top of it, plus you toss the warm water mass known as 'The Blob,'” Eakin said. “Really all are conspiring, I don’t have any really good analysis of the percent contribution of each, but all are very important.”

In August, global average ocean temperatures were the warmest they had ever been for any month since record-keeping began in 1880. Part of that is driven by El Niño, while part is due to manmade global warming. While El Niño can account for an unusually mild Pacific Ocean, for example, it cannot account for the fact that parts of every major ocean basin were record warm in August, and for much of 2015.

Sea surface temperature anomalies for mid-September 2015. Image: NOAA/NNVL

The previous two such global events, in 1997-98 and 2010, devastated reefs around the world, some of which have not yet recovered. Global bleaching events are becoming more common because of global warming, with many coral reefs expected to perish by the middle of the century if recent trends continue. Other reefs, which can survive in a broader temperature range, may prove more resilient.

Joining NOAA in declaring the bleaching event is a consortium of ocean scientists, reef mappers and monitoring teams, including the XL Catlin Seaview Survey, the University of Queensland and Reff Check.

The Seaview Survey has set out to document this bleaching event using high-tech camera equipment, targeting areas for observation based on forecasts from NOAA's Coral Reef Watch program.

This year, coral bleaching has already taken place in the northern Pacific, Indian, equatorial Pacific, and western Atlantic Oceans. A global bleaching event is declared when all three major ocean basins — Indian, Pacific and Atlantic —- have recorded widespread bleaching episodes across multiple reefs at least 62 miles wide or more more.

The Great Barrier Reef in Australia, which at about the size of Italy is one of the most iconic reefs in the world, is not yet feeling the effects of this bleaching event, but it may soon.

“A lot of us are feeling it will go the way of the 1998 event,” said Richard Vevers, executive director of XL Catlin Seaview Survey, in an interview. The 1998 event eventually spread to the Great Barrier Reef, the Indian Ocean, Southeast Asia and the Caribbean.

The rapid response team from the University of Queensland and the Catlin Seaview Survey use a camera system attached to an underwater scooter to document their findings. The cameras take about 1,000 high-resolution, 360-degree underwater images for up to 1.2-mile distances in a single dive.

These images are then used to help monitor and document the progression of bleaching events.

One of the more remarkable aspects of this particular bleaching event is that it appears to be circling the globe a second time, with computer models projecting major bleaching in the Indian Ocean starting in early 2016. Similar conditions existed there in 2014.

“You’re seeing the cycle starting all over again,” Eakin said.