The record heat over the Great Barrier Reef raised fears of a second summer of coral bleaching

"This is not a good sign," said the coral scientist, as November temperatures were the highest seen since 1985

Ocean temperatures on parts of the Great Barrier Reef have hit record highs this month, sparking fears of a second summer in a spate of mass coral bleaching.

Data from the US government's National Oceanic and Atmospheric Administration (Noaa) shows sea surface temperatures in the northern reaches of the reef have been the highest for any November on record since 1985.

With the peak period of heat accumulation over the reef not expected until February, cooler weather conditions and earlier cyclone activity could prevent a mass bleaching event.

Prof Terry Hughes, a leading expert on coral bleaching at James Cook University, said he had never seen heat stress build up on reefs this early, but "a well-timed cyclone" in December could reduce the risk of bleaching.

“Certainly record temperatures are dropping. The warning signs are clear," he said.

Last summer's mass bleaching, declared by the Great Barrier Reef Marine Park Authority (GBRMPA), was the first outbreak during a La Niña – a climate pattern that has historically kept ocean temperatures cool enough to avoid bleaching.

Hughes said: “According to Noaa's predictions, there is a high probability that we will see successive bleaching events. That shouldn't happen until the middle of the century."

Rising ocean temperatures driven by human emissions of greenhouse gases mostly from the burning of fossil fuels led to six mass bleaching events along coral reefs in 1998, 2002, 2016, 2017, 2020 and 2022.

Last summer's bleaching that affected 91% of all individual reefs came after record sea temperatures over the reef in December.

But according to Noaa's data, there is currently more heat accumulation over the coral reefs in the north than at the same time last year.

Noaa's current forecasts indicate by the end of January most of the northern reefs will likely experience significant bleaching and, in the following weeks, some areas could experience enough heat to cause coral death.

The Bureau of Meteorology's forecast for sea surface temperature also shows heat accumulation in December and January over the coral reefs.

Observations show current temperatures in the central and northern parts of the reef to be about 2C above average.

Corals can recover from bleaching if temperatures are not severe. Scientists have seen an increase in the number of corals on the reef in recent years – a recovery driven by the fast-growing corals that experts say are also the most vulnerable to bleaching.

The GBRMPA chief scientist, Dr David Wachenfeld, said authorities were examining predictions from Noaa and the bureau "to understand what might happen over this summer".

He said conditions leading up to this summer were a concern, but said “local weather conditions will greatly affect sea surface temperatures throughout the summer – for example, if there is rain or cloudy conditions. The temperature also tends to be hottest in February.

“At this time, it is too early to say what this summer will mean for the reefs, although the current La Niña event is expected to increase rainfall along the east and northeast coasts.”

He said authorities would use satellite, aerial and underwater observations to monitor conditions and make forecasts.

Prof Ove Hoegh-Guldberg, a pioneering scientist on coral bleaching at the University of Queensland, said: "This is about a steady but rapid increase in sea temperatures and it is very concerning. This [heat stress] happened a few weeks earlier than usual – back in January. I have to check my watch.

"The fact is it's probably the warmest November ever recorded [on the reef] and given what we know about heat stress on the reef, this doesn't bode well."

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