Heatwave ravages Great Barrier Reef
The Great Barrier Reef’s famed corals are under unprecedented and, in some spots, fatal stress as relentless summer heat in Australia stretches into early autumn.
The bleaching event appears likely to be the worst on record in southern sections of the 2250km reef, and could bring the first significant coral fatalities observed there.
In other sections, what is the fifth major bleaching event in nine years could serve as a test of how resilient the world wonder will be going forward.
Water temperature data suggests that the toll could approach that of 2016, when some 30% of the reef’s corals died after suffering through what were then unprecedented levels of heat stress.
“Those records have now been broken,” said Terry Hughes, director of the Australian Research Council Centre of Excellence for Coral Reef Studies.
While the oceans have steadily absorbed rising levels of global heat for decades, a record-setting spike in global ocean temperatures has persisted for over a year.
There is no evidence of mass bleaching occurring before 1998 but, since an episode that year, marine heatwaves have returned with increasing frequency, in 2002, 2016, 2017, 2020 and 2022.
Australian authorities declared the emergency last week. In a video posted to social media, Tanya Plibersek, minister for the environment and water, called the latest “bad news about the Great Barrier Reef” an alarming sign of the risk climate change poses “to coral reefs around the world”.
But Hughes said the crisis had been building in plain sight all summer, with warmer than normal waters persisting for months.
By February, a measure of cumulative heat stress to corals had shown steady gains throughout the Great Barrier Reef. In its southern region, the heat stress far surpassed record levels by the beginning of this month.
Though past marine heatwaves had delivered more extreme ocean temperatures, this one had lasted much longer, Hughes said. It comes as Australia posted its third-hottest summer on record.
Aerial surveys and dive missions to evaluate the extent of the crisis are still under way but so far show severe bleaching – when corals expel algae that normally live within their tissue – and already some mortality. Bleaching is typically a warning sign rather than a guarantee that corals will die. After a bleaching event, they can bounce back.
But in this case, the heat stress was so extreme that there was little doubt mortality would be widespread, Hughes said. The full death toll wouldn’t be clear for perhaps six months.
David Wachenfeld, research programme director at the Australian Institute of Marine Science, said two-thirds of the reef had been surveyed but rough seas were preventing scientists from inspecting the rest of the vast system, either by air or in dive missions.
He called surveys of the reef’s southern region “really critical”, because temperature data shows that corals there have experienced the greatest amounts of heat stress.
One unusual factor is that the bleaching is so dramatic in the southern sections of the reef, farthest from the equator. In the northern reefs, heat mortality was dramatic in 2016 and 2017. But in the south, corals had been the least exposed to extreme heat during past events.
Elsewhere across the reef, the concern is that heatwaves have been occurring all too often. Most reefs within the Great Barrier have experienced bleaching perhaps two or three times since 2016. But for a smaller number, the latest surge could be a fourth or fifth bleaching in that span, Hughes said.
The frequency of the severe bleaching events is affecting the density and diversity of some reefs. “If you lose a 30-year-old coral, it takes at least that length of time to replace it,” Hughes said. “We just don’t have that kind of time any more.”
What remained to be seen was how reefs that had recovered from past severe bleaching events were faring this time, Wachenfeld said. In those areas, corals were either survivors of past heatwaves, or the offspring of those survivors.
In many cases, the bleaching events coincide with episodes of the El Niño global climate pattern, which tends to increase planetary warmth. –