The Great Barrier Reef is unlikely to recover fully from the huge bleaching event that has killed off more than half its corals in some northern reefs as temperatures rise, scientists say.
Research, including by Tracy Ainsworth from James Cook University, has found that corals have natural mechanisms helping them to acclimatise to rising sea-level temperatures and avoid bleaching.
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| Coral reefs may be losing their natural ability to acclimatise. Photo: Zoe Richards |
However, the ability to cope with heat stress will be overwhelmed by the expected increase in frequency and temperature extremes, according to research published last month in the journal Science by a team led by Dr Ainsworth.
Tom Di Liberto, a meteorologist with the US National Oceanic and Atmospheric Administration, used the findings to argue this week that the rising greenhouse gases are likely to reduce corals' use of "practice runs" in the future.
"[If] ocean waters warm by as little as 0.5C overall, as predicted for the near future, there won't be a pre-stress practice run," Mr Di Liberto wrote in a NOAA blog." Without it, corals are at a greater risk of dying during bleaching, which means reefs are more likely to see a faster decline in coral cover."
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| Coral reefs have been hit from Australia to the Maldives and beyond by mass bleaching. Photo: James Cook University |
"While some recovery will occur over time, the sad truth is that ongoing ocean warming may keep some reefs from ever recovering their previous level of health, diversity, and productivity," he wrote.
In the charts below, NOAA adapted Dr Ainsworth's research to show how the frequency of so-called "protective" moderate heat stress periods on the Great Barrier Reef would decrease as global temperatures increased.
By contrast, the chart below shows how the number of single bleaching events without a "trial run and recovery" period in advance would increase as global temperatures rose.
"The bar charts show how today (at 0C), roughly 80 per cent of bleaching events are preceded by a protective trial run, while only about 20 per cent are not," the blog post said. "As waters warm, that balance is projected to switch."
Ove Hoegh-Guldberg, director of the Global Change Institute at
the University of Queensland, said the fact that bleaching events are
happening is an indication of the limits to corals' ability to
acclimatise to warmer waters.On Monday, Queensland scientists said about 35 per cent of the corals in the central and northern Great Barrier Reef were dead or dying, in the reef's third big bleaching event since 1998.
"If you gave corals exposure to heat a couple of weeks before then they were less susceptible when exposed to high temperatures [soon after]. It's a very transitory phenomenon."
"As temperatures rise, you get less and less time at moderate temperatures at which [the corals] can acclimate," said Professor Hoegh-Guldberg, who has also published on this ability. "This would only work at the initial rise in temperature but this disappears later on when it gets more extreme."
Corals evolved to cope with rising temperatures, such as the 5-degree warming during 10,000-15,000 years after the last ice age.
"We're doing something similar but over 50-100 years," he said, adding: "It's really a bit of a hopeful pipe-dream" that corals will be able to evolve fast enough to cope.
The fact oceans are becoming more acidic as they absorb more carbon-dioxide from the atmosphere would also weaken coral skeletons further and their ability to grow back, Professor Hoegh-Guldberg said.
"We're increasing the number of underwater heatwaves and cyclones, and reducing the ability of corals to grow back," he said. "We're tipping the balance towards reefs that don't have corals on them."
He welcomed Labor's pledge of $500 million during five years to improve water quality in the reef region. The total compares with the $171 million pledged in the May budget by the Turnbull government.
"This isn't going to be solved by a bunch of Band-Aids," he said. "If they are polluted and lying under sediment, they won't grow back."
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