Third-largest global coral bleaching event is underway, scientists say

A massive, global coral bleaching event is underway which could affect 38 per cent of the world’s reefs by year’s end, including the Great Barrier Reef, scientists have revealed.
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A before and after image of coral bleaching in American Samoa, with the right image taken in December 2014 Photo: XL Catlin

The consortium of researchers from the National Oceanic and Atmospheric Administration (NOAA) of the US,the University of Queensland,Reef Check, and XL Catlin Seaview Survey says the mass bleaching – only the third of itskind in recorded history – is being driven by increased ocean temperatures.

NOAAhas estimated the event may kill more than12,000 square kilometres of reef worldwide.

The rise in the ocean temperaturesis being caused by the background warming from climate change made worse by this year’s superEl Nino weather event, and a Pacific warm water mass known as “the Blob”, the researchers say.

The Great Barrier Reef shown in healthy conditions. Photo: Australian Institute of Marine Science

The extent of the damage to Australia’s World Heritage-listed Great Barrier Reef was not yet known, but it will become obvious by early 2016, University of Queensland Global Change Institute Director, Professor Ove Hoegh-Guldberg, said in a statement.

“If conditions continue to worsen, the Great Barrier Reef is set to suffer from widespread coral bleaching and subsequent mortality, the most common effect of rising sea temperatures,” Professor Hoegh-Guldberg said.

Coral bleaching occurs when stressed corals exude an algae,zooxanthellae, which lives inside their tissue. After it is expelled, the bright, white skeleton of the coral is left exposed. They can, but do not always, die as a result of the bleaching.

According to the NOAA-led researchers, coral reefs support one quarter of all marine species and a mass bleaching event can “severely deplete” the ecosystems that rely on them.

Bleaching on reefs in American Samoa. Photo: XL Catlin

In 1998, more than half of the Great Barrier Reef experienced bleaching and up to 10 per cent of its corals died. That was the world’s first, major recorded event of its kind and it killed 16 per cent of the globe’s corals.

The second event, five years ago, did not affect the Great Barrier Reef partly because two local cyclones helped to drive down ocean temperatures.

But this year so far, bleaching has already been recorded across the northern Pacific, Indian, and western Atlantic Oceans. It is expected to become obvious in the Caribbean in the next few weeks.

Bleaching only reaches a “global event” stage when all three major ocean basins are affected across multiple reefs spanning 100 kilometres or more, XL Catlin Seaview Survey said.

“This is only the third time we’ve seen a global-scale bleaching event,”NOAACoral Reef Watch coordinator Dr MarkEakinsaid in a statement.

Dr Tyrone Ridgway, fromUQ’sGlobal Change Institute, said the severity of any impact on Australia’s iconic Great Barrier Reef will depend on how long the higher-than-average ocean temperatures last.

“As we move into summer, these temperatures are expected to rise even more,” he told Fairfax Media.

“If we get coral mortality, the health of the system will decline.”

As corals are the “builders” of the Reef, this would affect fish stocks as well as tourism.

Surface watersof the equatorial central and eastern Pacific – where the El Nino has formed -are as much as 4 degrees warmer than average, while deeper gauges are detectinganomalies of 7 degrees.

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