Delivering yet another "wake-up call" after recent studies have shown that heat stress from anthropogenic global warming has killed half of the Great Barrier Reef's corals since 2016, a new analysis from U.S. scientists warns that the entirety of world's largest coral system is at risk of bleaching and death as Australia enters it summer months.

"This is really the first warning bells going off that we are heading for an extraordinarily warm summer and there's a very good chance that we'll lose parts of the reef that we didn't lose in the past couple of years."

—Ove Hoegh-Guldberg, Global Change Institute

The forecast from U.S. National Oceanographic and Atmospheric Administration (NOAA) for November 2018 to February 2019 indicates that the whole reef has a 60 percent chance of reaching "alert level one," under which bleaching is "likely."

When coral is exposed to warm water or pollution, it expels the algae living in its tissues—its main source of food—and turns completely white. Although bleached coral is still alive, the reaction makes it more susceptible to disease and death.

"This is really the first warning bells going off that we are heading for an extraordinarily warm summer and there's a very good chance that we'll lose parts of the reef that we didn't lose in the past couple of years," marine biologist Ove Hoegh-Guldberg, the director of the Global Change Institute at the University of Queensland in Australia, told the Guardian. "These are not good predictions and this is a wake-up call."

Hoegh-Guldberg expressed concern that the analysis shows bleaching could occur before March, which historically has been the main month for such events. "To really have the full picture we're going to have to wait for those projections that cover the main part of bleaching season," he said. "Given sea temperatures usually increase as we get towards March, this is probably conservative."

New forecast shows mass bleaching on the Great Barrier Reef is likely this summer.

https://t.co/FMyajLyxEp pic.twitter.com/Dd5XkzhgKQ — Michael Slezak (@MikeySlezak) October 25, 2018

While NOAA's predictions provoked alarm, Mark Eakin, head of the agency's Coral Reef Watch, noted that "lots of things, including major weather patterns, can change the probabilities over the next three months."

Although "it's much too early to predict that with any certainty," Eakin told Australia's ABC that "depending on what happens with the El Niño," or the warming of the ocean surface in the central and eastern tropical Pacific Ocean, "we could see another global bleaching event in 2019" that would impact not only the Great Barrier Reef but also other coral systems across the globe.

Regardless of whether an El Niño emerges and triggers another bleaching event, scientists are urging governments to heed the urgent warnings of the recent IPCC report—which detailed what the world could look like if the global temperature rises 1.5°C versus 2°C (2.7°F versus 3.6°F) above pre-industrial levels—and ramp up efforts to avert climate catastrophe.

The IPCC report found that coral reefs "are expected to decline by a further 70 to 90 percent even under 1.5ºC, but that rises to more than 99 percent reef loss as temperature rises hit 2ºC," according to ABC. "Researchers say at current emissions rates, the world will hit that point between 10 and 14 years from now."

NOAA's new forecast for bleaching in the Great Barrier Reef is "very consistent with what the IPCC 1.5 degree report told us," concluded Hoegh-Guldberg. "It's extremely important that politicians and our leaders stand up and make the changes we need to make so we don't tread down an even more dangerous path."