Global warming has likely caused permanent damage to the world's largest coral reef, according to a new report in the journal Nature.
In the last 20 years, rising temperatures have triggered three severe bleaching events in Australia's Great Barrier Reef. The most recent was in 2016, when more than 90% of the reef's corals were killed or reduced to barren white skeletons.
The result is that almost every corner of the Great Barrier Reef Marine Park has now bleached. To make matters worse, scientists fear another bleaching is sweeping through, just a year after last year's unprecedented event.
Corals are tiny animals that feed on sugars made by algae and calcium carbonate. Working together, coral polyps slowly build large reef structures that attract fish and a variety of other organisms.
Extreme heat disrupts the symbiotic relationship corals maintain with the type of algae that provide the animals with both food and their attractive colors. When the ocean warms beyond a couple of degrees Celsius, the algae die and leave behind a white, bleached coral skeleton.
If the corals don't regain their algae quickly enough, they starve to death.
When the Great Barrier Reef bleached in 1998 and 2002, the Australian government surveyed the damage to about 600 of its individual coral reefs. In 2015, as Australia's Bureau of Meteorology and the U.S. National Oceanic and Atmospheric Administration predicted a new round of bleaching, Hughes and colleagues from 10 research institutions mobilized a small army to document the results.
The following year, Hughes and others performed aerial surveys of more than 1,000 sections of the 1,500-mile reef off the coast of Queensland. Scientists flew as far north as the Torres Strait, between the northeastern tip of Australia and Papua New Guinea, marking the first time that area had been surveyed. One hundred divers then verified the aerial surveys by inspecting reefs up close.
The pattern of bleaching seemed to have spread from south to north between 1996 and 2016, moving from coastal to offshore reefs between the three episodes. In each instance, the most severe bleaching occurred in areas that saw the highest and most prolonged ocean warming.
In 1998, an El Niño year, the bleaching was primarily limited to the coast and was most severe in the southern and central reefs.
Four years later, when there wasn't an El Niño, the phenomenon spread to the offshore reefs that had escaped the 1998 event.
But last year's wave of coral bleaching was unlike anything scientists have seen before.
Back in 1998, one in eight of the reefs surveyed experienced what the study authors considered "extreme bleaching," meaning that more than 60% of the corals in those areas were affected. By 2016, half of the reefs surveyed were victims of extreme bleaching.
Overall, a whopping 1,052 of the 1,156 reefs checked in 2016 experienced bleaching to some extent.
In other words, only 9% of the reefs surveyed escaped the damage last year. Compare that to previous years, in which 45% of studied reefs avoided bleaching in 1998, and 42% in 2002.
Most of the unaffected corals live in a southern offshore end of the Great Barrier Reef that saw relatively low water temperatures while most of Australia — and the rest of the world — broke heat records.
That southern reef was likely "rescued" by the cooling winds, clouds and rain of tropical cyclone Winston, the researchers noted in the study. Reefs in western Australia were also likely saved by tropical cyclone Stan.
Though 1998 and 2016 were both El Niño years, Hughes said the link between bleaching events and such natural cycles is getting weaker. The underlying cause, he said, is global warming.
"The big concern we have is these events are becoming more intense and more frequent," Hughes said. For the corals, that means "the recovery time is now too short for the species to recover."
Many of the shorter-lived coral species would require 10 to 15 years to re-colonize and re-grow. Other species that live up to 50 years would need many more decades to replace themselves.
But that depends on the absence of another big bleaching — which the study suggests is now unrealistic to expect.
This week, Hughes began another seven-day aerial survey to assess the boundaries of a fourth mass bleaching currently underway, which for now seems to be concentrated in the central region of the reef. With this frequency of disturbances, many areas have likely changed permanently and will continue to do so as global warming gets worse.
"We're pretty confident that the mix of species in the northern third of the Great Barrier Reef has changed forever," he said. "We're now in a warmer era where there simply isn't enough time between these recurrent events."
The species most vulnerable to bleaching (and other disturbances) tend to be those that form long branches, such as elkhorn or staghorn corals, as well as the "table-shaped" varieties that look like flat panels sitting on top of a stalk. These are the more abundant species, and also better habitat-builders, Hughes said.
"They make all the nooks and crannies for the fish to live in," Hughes said.
The more resilient corals are the dome- and mound-shaped corals, like the brain coral, that don't provide the same shelter for fish and other marine life. The result is a flatter reef that's less useful to the broader ecosystem.
Despite the no-fishing zones, no-entry zones and many other conservation efforts at play in the Great Barrier Reef, the study found protected areas weren't any less likely to bleach than other areas.
"Why would a dotted line on a map prevent hot water from intruding?" Hughes said.
The authors did note that protecting water quality and local fish populations may aid in the reef recovery in the long term.
While managing the reef is important, Hughes said humans must deal with the "elephant in the room."
These three episodes of severe coral bleaching have occurred with less than 1 degree Celsius of global warming. (A 1-degree temperature change in Celsius is equal to a 1.8-degree change in the Fahrenheit scale.) Within 1 to 2 degrees Celsius of warming, the corals would likely survive, although uncomfortably, Hughes said.
But under a "business as usual" scenario where greenhouse gas emissions continue at their current rate, Earth's temperature will rise to levels that are "incredibly damaging to the world's corals," Hughes said.
"We can't climate-proof a reef," Hughes said. "We have to deal with emissions and global warming."
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