The Great Barrier Reef likely just experienced its most widespread bleaching event on record

(CNN)Australia's Great Barrier Reef has likely experienced its most widespread bleaching event on record, according to a US government scientist who monitors the world's coral reefs. This marks the third mass bleaching event on the reef in just the last five years. And scientists say that the rapid warming of the planet due to human emissions of heat-trapping gases are to blame. On the heels of severe bleaching events in 2016 and 2017 that left half of the coral on the Great Barrier Reef dead, scientists fear this one could be a devastating blow.

"If we do not deal with climate change quickly ... we are going to continue to see more severe and more frequent bleaching, and we are going to see the loss of coral reefs in much of the world," said Dr. C. Mark Eakin, coordinator of the National Oceanic and Atmospheric Administration's (NOAA) Coral Reef Watch.

The mass bleaching conditions were observed by Coral Reef Watch, which uses remote sensing and modeling to predict and monitor for signs of bleaching. Eakin says that the bleaching in 2016 and 2017 was extremely intense, but severe damage was concentrated in a few hotspots in the northern and central parts of the reef. Early indications show that this latest event was not as damaging, but that a much larger area of reef experienced at least some bleaching. Past bleaching events have typically occurred in years with a strong El Niño-Southern Oscillation, a climate phenomena that can increase the odds of a host of extreme weather events around the globe.

El Niño is characterized by warmer waters in the Pacific ocean, which makes bleaching events in the region more likely. But there is no El Niño currently, which Eakin says makes this bleaching that much more surprising -- and frightening. "The upper ocean has absorbed a tremendous amount of heat in recent years, and it has really put coral reefs around the globe much closer to their upper thermal limits. " Why the Great Barrier Reef is so critical Coral reefs are some of the most vibrant marine ecosystems on the planet -- between a quarter and one-third of all marine species rely on them at some point in their life cycle.

And none is more vital than the Great Barrier Reef. Covering nearly 133,000 square miles, it is the world's largest coral reef and is home to more than 1,500 species of fish, 411 species of hard corals and dozens of other species. It's also a vital resource to Australia's economy, contributing more than $5.