One of Australia’s greatest natural wonders and a valuable tourism attraction is in grave danger from global warming, experts say.

As much as 93% of the Great Barrier Reef has been damaged by coral bleaching, Australian scientists revealed this week, in a report based on extensive aerial and underwater surveys. The report was published just days after researchers found coral bleaching far south of the Great Barrier Reef in Sydney Harbour and ahead of Earth Day on Friday.

“We’ve never seen anything like this scale of bleaching before,” said Professor Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies, and head of the National Coral Bleaching Task Force that produced the report. “In the northern Great Barrier Reef, it’s like 10 cyclones have come ashore all at once.”

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Coral bleaching occurs when warmer-than-usual water temperatures cause this marine invertebrate to expel a symbiotic algae called zooxanthellae. That algae normally lives mostly inside coral tissue and provides much of its vibrant color, according to the Great Barrier Reef Marine Park Authority.

Without zooxanthellae, the coral’s tissue becomes transparent and its white skeleton is revealed. If the heat stress is sustained, the coral begins to starve and will eventually die. It can be saved if conditions return to normal, and local weather conditions such as cloud or strong wave action can mitigate the effects. But reefs with high levels of bleaching can take many years or decades to recover, and there is concern that the Great Barrier Reef will never be the same again.

The most severe damage has hit the northern sector of the reef, where less than 1% of coral is deemed to have avoided bleaching.

The world is currently in the middle of a prolonged El Niño event, which is warming water in the equatorial Pacific and causing damage to coral reefs worldwide. Australia’s Great Barrier Reef stretches 2,300 kilometers and is one of the worst affected but scientists have also found bleaching on the west coast of Australia.

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“The coastal area that I study north of Broome has huge tides, and we thought the corals there are tough “super corals” because they can normally cope with big swings in temperature,” said Dr. Verena Schoepf from the University of Western Australia in the report. “So, we’re shocked to see up to 80% of them now turning snow-white. Even the tougher species are badly affected”.

Australia’s tourism industry is highly dependent on the Great Barrier Reef, which generates annual income of about $5 billion and employs nearly 70,000 people, the report said. The government and researchers have long fretted about the impact of higher temperatures on the reef.

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“We can’t just ignore coral bleaching and hope for a swift recovery,” said Daniel Gschwind, chief executive of the Queensland Tourism Industry Council. “Short-term development policies have to be weighed up against long-term environmental damage, including impacts on the reef from climate change.”

“ “In the northern Great Barrier Reef, it’s like 10 cyclones have come ashore all at once.” ” — Professor Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies

As the map above shows, the worst damage is being felt in the northern sector of the reef, where less than 1% of coral is deemed to have avoided bleaching. The most remote part of the reef has been insulated from the kind of damage caused by mass tourism, but can do little to fend off climate change, the scientists argue.

The southern sector has fared best thanks to a rainy March with a lot of cloud cover, leaving only 1% of its coral deemed to be severely bleached and 25% not bleached at all. The middle sector has been moderately bleached, and is expected to recover in the coming months.

The Australian government is working with America’s National Oceanic and Atmospheric Administration to monitor the current bleaching event. The local Queensland government is investing in measures to improve water quality, reducing nutrients, pesticides and sediments in farm runoff, in an effort to help the reef cope with heat pressures.

But it’s not just Australia that is suffering. As many as 500 million people worldwide rely on coral reefs for food, protection, income or all three, according to a recent New Yorker article.

The NOAA said last year that record ocean temperatures were creating a third global coral bleaching event and warned that it would take its toll on the Hawaiian islands, and in the Caribbean, putting coral in Puerto Rico and the U.S. Virgin Islands at risk. The event started in the north Pacific in 2014 and expanded to the South Pacific and Indian oceans by 2015.

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The first global bleaching event took place in 1998, during another El Niño that was followed by a La Niña of equal strength. A La Niña is the opposite of El Niño, and occurs when sea surface temperatures in the central Pacific fall to lower-than-normal levels, causing more rain in the Pacific northwest and dry conditions in the south, according to Accuweather.com.

A second global bleaching event took place in 2010.

Not all such events are caused by warm water, according to the NOAA. In January 2010, cold water temperatures in the Florida Keys caused a coral bleaching event that killed off some coral, the agency said on its website.