Abnormally high sea surface temperatures and signs of coral bleaching on Australia's Great Barrier Reef have conservationists and tour operators worried another severe bleaching event could return to the reef in coming months.

The UNESCO world heritage site experienced its worst mass bleaching ever in 2015-2016, an event scientists said was 175 times more likely because of human-caused climate change.

More than 93 percent of the smaller reefs that make up the wider ecosystem were affected by bleaching and surveys have shown widespread reef mortality as a result.

Following twelve months of sustained, above average sea surface temperatures since last year's bleaching event, there are now fears that another mass bleaching could be unfolding.

"I think the next two weeks are going to be absolutely critical to see whether this really becomes a severe event or not," Imogen Zethoven from the Australian Marine Conservation Society said. "Right at the moment there hasn't been much rainfall; there hasn't been a normal wet season. There is not much cloud cover; it's very hot, and there is a lot of sunshine."

Bleaching frequently occurs when heightened sea temperatures cause corals to expel tiny photosynthetic algae — which give them color — from inside their tissue, turning them white.

While corals can recover if the temperature drops and the zooxanthellae algae that live inside them return, frequent bleaching events put enormous stress on a coral reef's ability to survive.

Image via National Oceanic and Atmospheric Administration

NOAA's Coral Reef Watch placed large parts of the Great Barrier Reef on red alert for bleaching this month. The agency said the situation was likely to deteriorate by the end of February and raised the possibility of coral death in the reef's northern and far northern regions — areas that suffered the worst mortality last year.

The Great Barrier Reef Marine Park Authority said spot checks conducted between the cities of Cairns and Townsville along Australia's northeast coast this week have revealed moderate to severe coral bleaching of some reefs.

"Initial survey results showed high levels of bleaching among the most sensitive coral species, with 60 per cent or more affected at some sites," Dr. Russell Reichelt, the chairman of the Great Barrier Reef Marine Park Authority, said in a statement on Friday.

John Rumney, a diver and tour operator on the reef, said the early indications were alarming.

"We are having major bleaching in places that were bleached last year and areas that escaped bleaching last year," he said.

Rumney, who has been diving the reef for 40 years, said the reef in many places had not had the chance to recover from last year's major bleaching.

The Great Barrier Reef is facing a number of threats, including climate change, poor water quality due to agricultural runoff, illegal fishing and coastal development.

The Australian federal government promised an additional $171 million to improving water quality and coastal habitats surrounding the reef in last year's budget, but many scientists and conservationists say little is being done to address climate change, the reef's biggest threat.

"It is an appalling failure by the federal government to grasp what needs to be done," Zethoven said. "There is a very clear solution to this: to embrace the energy of the future, which is renewable technologies."

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Australia is the world's largest coal exporter, according to the International Energy Agency, and one of the planet's biggest carbon emitters on a per capita basis, largely due to its reliance on coal-fired power plants.

Despite acknowledging the threat climate change poses to the reef, both state and federal governments have granted approvals for Australia's largest coal mine project in the Galilee Basin in Central Queensland.

The massive mining project, operated by Indian energy giant Adani Group, includes a railway line and port. It will require dredging of waters around the Great Barrier Reef to allow coal ships to pass.

Carbon emissions from the mine are estimated to be 4.7 billion metric tons of CO2-equivalent over its lifetime. Annual average emissions of 79 million metric tons of carbon equivalent will be 20 per cent more than the average annual emissions of New York City, according to the Australia Institute, a progressive think tank.

Scientists say deep cuts to global greenhouse gas emissions will be needed to save the reef — but even then it might be too late.

"We know the reef is in big trouble. We expect in the current climate to see events like we saw last year about one in every four years," said Andrew King, a climate extremes research fellow at the University of Melbourne.

He said this year's above average sea surface temperatures were a sign of things to come.

"Under any emissions scenario we're seeing increases in the risk of bleaching-type sea surface temperatures," he said. "It paints quite a depressing picture to be honest."

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