Seven months after Florida corals faced what scientists called their worst bleaching event ever, a similar emergency grips the Great Barrier Reef: Its famed corals are under unprecedented and, in some spots, fatal stress as relentless summer heat in Australia stretches into early fall.
The bleaching event appears likely to be the worst on record in southern sections of the 1,400-mile-long 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 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 Center of Excellence for Coral Reef Studies.
A coral head along the Great Barrier Reef in 2022 on Hastings Reef, Australia. Bleaching events and global warming have done significant damage to the Great Barrier Reef. Michael Robinson Chávez/The Washington Post
It is the latest symptom of off-the-charts ocean heat around the world. 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.
On the Great Barrier Reef, the relatively rapid succession of such extreme bleaching events is a clear sign of human-caused climate change, said David Wachenfeld, research program director at the Australian Institute of Marine Science. There is no evidence of mass bleaching occurring before 1998 but, since an episode that year, marine heat waves have returned with increasing frequency, in 2002, 2016, 2017, 2020 and 2022.
How those events compare with this latest one will become clear in coming months as scientists survey the famous formation of some 2,900 coral reefs spread across an area the size of Italy.
“This is an anxious time as we continue to collect information,” Wachenfeld said.
A BLEAK PICTURE
Australian authorities declared the emergency last week. In a video posted to social media Thursday, 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, the final month of Southern Hemisphere summer, 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 heat waves have delivered more extreme ocean temperatures, this one has lasted much longer, Hughes said. It comes as Australia posted its third-hottest summer on record, with little respite from the heat on land. In Brisbane, the closest major city to the reef, overnight low temperatures failed to drop below 68 degrees Fahrenheit for more than two months straight.
Aerial surveys and dive missions to evaluate the extent of the crisis are still underway 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 is so extreme that Hughes said there is little doubt mortality will be widespread. The full death toll won’t be clear for perhaps six months, he said.
Wachenfeld said that two-thirds of the reef had been surveyed but that rough seas are 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.
In the meantime, the bathwater-like conditions are not going away any time soon, unless weather patterns bring clouds, rain or waves to ease them. That means the corals’ heat exposure has not yet peaked, Hughes said.
It puts reef lovers in a position of begging for a different sort of dangerous weather.
“We are hoping for an extreme cyclone to reduce the impacts of another extreme weather event,” said Anna Marsden, managing director of the Great Barrier Reef Foundation
WHY THIS BLEACHING IS SO ALARMING
Reef watchers say no two bleaching events occur in the same way. They tend to be patchy, and can affect some regions of the Great Barrier Reef more than others. This time, the pattern and prevalence are particularly alarming, scientists said.
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 have been the least exposed to extreme heat during past events, Marsden said.
“There’s concern corals there won’t make it through,” she said.
Elsewhere across the reef, the concern is that heat waves 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 anymore.”
What remains to be seen is how reefs that have recovered from past severe bleaching events are faring this time, Wachenfeld said. In those areas, corals are either survivors of past heat waves, or the offspring of those survivors.
“Are they perhaps a little tougher?” he asked. “That’s important for understanding how the impacts of climate change are likely to unfold.”
RISK OF MORE-FREQUENT BLEACHINGS
And then there is the likelihood that severe bleaching events will become more common as the planet warms.
In many cases, the events coincide with episodes of the El Niño global climate pattern, which tends to increase planetary warmth. That was the case in the first major bleaching in 1998, in the devastating bleaching event of 2016, and again this year. Though an El Niño that began last year is waning, its effects are still expected through the Northern Hemisphere spring.
But rising global temperatures, the result of humans burning fossil fuels, meant that severe bleaching even occurred in 2022 during La Niña conditions – which tend to have a slight cooling effect around the world. La Niña is likely to return this year.
“We no longer need an El Niño to make a hot summer dangerous,” Hughes said.
Scientists have been fearing and anticipating what could happen on the Great Barrier Reef since marine heat waves hit the Atlantic basin during this past Northern Hemisphere summer, bringing some surface temperature readings into triple digits off the Florida coast. The heat stress was so high, Hughes said, some corals “died more or less instantly. They didn’t have time to bleach.”
He said he fears some of the same consequences for the world’s most famous corals.
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