Ocean temperatures over parts of the Great Barrier Reef have reached record levels this month, sparking fears of a second summer in a row of mass coral bleaching.
Data from the US government’s National Oceanic and Atmospheric Administration (Noaa) shows sea surface temperatures over the northern parts of the reef have been the highest for any November on a record going back to 1985.
With the peak period for accumulated heat over the reef not expected until February, cooler weather conditions and cyclone activity before then could stave off a mass bleaching event.
Prof Terry Hughes, a leading expert on coral bleaching at James Cook University, said he had never seen heat stress accumulating on the reef this early, but a “well timed cyclone” in December could reduce the risk of bleaching.
“It is certainly the case that temperature records are tumbling. The warning signs are clear,” he said.
Last summer’s mass bleaching, declared by the Great BarrierReef Marine Park Authority (GBRMPA), was the first outbreak during a La Niña – a climate pattern that historically has kept ocean temperatures cool enough to avoid bleaching.
Hughes said: “According to Noaa’s predictions there’s a good chance we will see another back-to-back bleaching event. That was not supposed to be happening until the middle of this century.”
Rising ocean temperatures driven by human emissions of greenhouse gases mostly from burning fossil fuels caused six mass bleaching events along the reef in 1998, 2002, 2016, 2017, 2020 and 2022.
The bleaching of last summer that affected 91% of all individual reefs came after record ocean temperatures over the reef in December.
But according to Noaa data, there is currently more accumulated heat over the reef in the north than at the same time last year.
A current Noaa forecast suggests by late January large parts of the northern reef will likely see significant bleaching and, in the weeks that follow, some areas could see enough heat to cause coral death.
Bureau of Meteorology forecasts for sea surface temperatures also show heat accumulating in December and January over the reef.
Observations show current temperatures in the central and northern parts of the reef at about 2C above average.
Corals can recover from bleaching if temperatures are not severe. Scientists have seen a rise in the amount of coral over the reef in recent years – a recovery driven by fast-growing corals that experts say are also the most susceptible to bleaching.
GBRMPA’s chief scientist, Dr David Wachenfeld, said the authority was examining predictions from Noaa and the bureau “to understand what might occur over this summer”.
He said the conditions leading in to this summer were a concern, but said “local weather conditions will strongly influence sea surface temperatures throughout the summer – for example, if there is rain or cloudy conditions. Temperatures also tend to be hottest in February.
“Right now, it is too early to say what this summer will mean for the reef, though the current La Niña event is expected to increase rainfall along the east and north-eastern coast.”
He said the authority would be using satellite, aerial and in-water observations to monitor conditions and make forecasts.
Prof Ove Hoegh-Guldberg, a pioneering coral bleaching scientist at the University of Queensland, said: “This is about the steady but rapid rise in ocean temperatures and this is very worrying. This [heat stress] is happening many weeks earlier than usual – in the past it has been in January. I have had to check my watch.
“The fact it’s probably the warmest November on record [over the reef] and given what we know about heat stress on corals, this does not bode well.”