"This has never happened before," Mark Eakin, coordinator of the National Oceanic and Atmospheric Administration's Coral Reef Watch program in College Park, Maryland, said as the NBC News reported. "We're in completely uncharted territory."

Bleachings don't necessarily kill the corals, but it does leave them extremely vulnerable to disease from bacteria or viruses. Bleaching, which occurs in response to abnormal conditions like heat or increased acidity in the water, forces the corals to release the tiny photosynthetic algae that live in their tissue and are responsible for their color, according to NBC News.

The previous two heat stress related bleaching events were in 2016 and 2017. Scientists say the frequency of the heat-induced bleaching is a direct result of the climate crisis, which spells trouble for the vitality of the reef since the corals do not have enough time to recover and to grow back, according to NBC News.

The scientists conducting the aerial surveys are from the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Queensland, Australia.

Terry Hughes, who runs the center, told The Guardian, that after three days of the planned nine-day survey, "We know this is a mass bleaching event and it's a severe one. We know enough now that [the bleaching] is more severe than in 1998 and 2002. How it sits with 2016 and 2017 we are not sure yet."

Global warming is an enormous threat to the future of coral reefs around the world. As The Guardian reported, The UN's Intergovernmental Panel on Climate Change concluded from published evidence that a majority of tropical coral reefs would disappear even if heating was limited to the Paris agreement's target of 1.5 degrees Celsius and would be "at very high risk" at 1.2 degrees Celsius.

Hughes told NBC News there have been only five recorded Great Barrier Reef bleaching events. The first was in 1998, followed by 2002, then in 2016, 2017 and this year, setting up a disturbing pattern.

"The gap between one event and the next is shrinking, not just for the Great Barrier Reef, but forreefs throughout the tropics," Hughes said to NBC News. "That's important, because it takes a decade or so for a half-decent recovery of even the fastest-growing corals. The slowest ones take several decades."

While past bleaching events, like the ones in 2002 and 2016, were driven by El Niño weather events, this one happened just because the Australian summer was too hot.

"We no longer need an El Niño to trigger a bleaching event — we just need a hot summer," Eakin said to NBC News. "And the summers are getting hotter and hotter because of global warming. That is astounding in itself."