The Great Barrier Reef debate rages on: is it really in danger? David Levell draws up the state of health of the famous Australian marine reserve.
Guesses about the state of the Great Barrier Reef
In recent years, alarming reports have followed one another of coral bleaching in Australia’s Great Barrier Reef, the largest and most famous set of coral reefs in the world. Distressed commentators have declared him dying, if not already dead, from global warming. One news outlet even published his “obituary.”
For their part, climate skeptics consider bleaching exaggerated or as part of a natural cycle – or both. To add to the confusion, the many refutations of the “death” of the Great Barrier have given the impression that the threat to it is not so serious. Who to believe?
What it really is
“Both are far from the truth,” said David Wachenfeld, director of reef recovery for the Great Barrier Reef Marine Park Authority. She’s not dead, but she’s not well. The threats are serious, and more will have to be done than what we are already doing. Worldwide.”
The very dimensions of the Great Barrier Reef are an aspect that is easily overlooked. “It is gigantic, recalls David Wachenfeld. On this scale, it is not easy to understand. For some, it is a tourist destination like the Eiffel Tower. But so far – and I cross my fingers – no cyclone or bleaching or any other event has had a serious impact on the entire Great Barrier Reef.
Visible from Earth orbit
In 1814, explorer Matthew Flinders referred to the Great Barrier Reef in the plural. It has 2,900 distinct reefs scattered over a territory the size of Poland. Most visitors only see a tiny fraction of it. It stretches 2300 kilometers along the eastern coast of Australia and forms the largest living structure in the world. These billions of tiny corals are the only fauna visible from Earth’s orbit.
The coral occupied an area twice as large in the 1980s. Among the causes of its decline, runoff from agricultural land which degrades water quality and the invasion of purple acanthaster, a starfish which feeds of coral and proliferates in runoff water, are at the heart of rescue plans.
A global problem
The Great Barrier Reef faces an existential crisis that Australia cannot face alone. “The biggest threat is global warming,” confirms David Wachenfeld. For the first time, massive coral bleaching affected the Great Barrier Reef in two successive years, in 2016 and 2017.
The phenomenon occurs when the coral in a situation of thermal stress expels the zooxanthellae algae with which it lives in symbiosis and which gives it its color. When zooxanthellae are overexposed to light and heat, they produce too much oxygen, which is toxic to the coral.
But without them, the coral bleaches and, if the water temperature remains too high for too long, it dies, because in addition to being a source of nutrients, these algae recycle waste. After bleaching and if the water temperature drops, the coral will gradually regain its zooxanthellae and color. Otherwise, he will die. With just a 1°C increase in water temperature, some corals bleach in just four weeks and die after eight weeks.
2017: the year of the greatest coral loss
In general, coral reefs recover from a one-time small-scale bleaching event, but extreme events were unknown until 20 years ago. The 2017 one was devastating, although it did not bleach 93% of the total coral as believed. In reality, this is the misinterpreted result of inspecting 911 reefs, 93% of which showed variable bleaching. Follow-up studies showed that the total mortality was instead 22%. It remains the largest coral loss ever recorded.
The damage was hard to see, especially on the northeast coast of Australia. “We flew over the clearest waters for 4,000 kilometers and only four reefs had not suffered from bleaching,” laments coral reef ecologist Professor Terry Hughes, who called his survey in the north “the research trip the saddest in [his] existence”.
Why this gap between north and south?
Do the cooler waters that harbor corals in the south provide better protection against heat-induced bleaching? No. It’s just that coral bleaches at lower temperatures in the south than in the north. The south got a reprieve in 2016 when Cyclone Winston brought tropical rains that acted as a buffer and dropped sea temperatures below local normals while increasing cloud cover.
“We had a narrow escape,” reassures Sara Keltie, naturalist guide at Heron Island where the coral reefs remain vibrant. The island is known for the turtles that come to lay their eggs there and for its eco-responsible hotel.
Will the coral migrate?
El Niño, a climate phenomenon that is characterized by rising ocean temperatures, is no stranger to the massive bleaching of 2016. And since we are already warming the oceans with carbon dioxide, this is accelerating the threshold for bleaching even further. Some predict that the coral will eventually move south.
David Wachenfeld disagrees. “Corals have been around for 400 million years,” he points out. They have experienced many climatic changes. Except that the climate has never changed so quickly. The fact that in the past animals have adapted and survived on a geological time scale does not mean that they will when the situation changes more rapidly.
In addition, hundreds of millions of people rely on coral reefs for food, coastal protection from tidal power, and tourism revenue, among other things. It would be absurd to say to these people: yes, well, the reef will probably die within 20 years because of climate change, but, do not panic, in 5000 years, it will be there again.
Temperature rise is not the only problem
That’s not all. Climate change too rapid for coral to handle poses another threat to reefs: increased ocean acidity. “About 30% of the carbon dioxide that we send into the atmosphere by burning fossil fuels is absorbed by the oceans,” explains David Wachenfeld.
In seawater, carbon dioxide forms carbonic acid. This releases hydrogen ions which bind to the floating carbonate ions that hard coral needs to make its skeleton made of calcium carbonate. The higher the concentration of carbon dioxide in the ocean, the less the hard coral is able to form reefs. If the floating reserve of carbonate runs out, the hydrogen ions will go so far as to dissolve the hard coral and the shells to obtain it.
On Heron Island, an experiment observed the effect of different temperatures on the reproduction of a coral ecosystem. Two projections were tested: the first with a rise of 4°C in global temperature compared to the average in pre-industrial times, a reality expected before 2100 if nothing is done to reduce carbon dioxide emissions; and the second with an increase of 2°C, the maximum targeted by the Paris Agreement. The acidity of the oceans caused by the levels of atmospheric carbon dioxide responsible for these temperature rises has also been studied.
“In these two hypotheses, all the corals bleached, but following different trajectories, says Sara Keltie. In the one where we do nothing, the corals have starved to death and have started to dissolve.”
If the future at +4°C seems fatal, all hope is not lost for coral reefs in a world that respects the objectives set by the Paris Agreement. “In this interventionist projection, some corals survive – especially those that grow more slowly like rock corals, which need less energy, adds Sara Keltie. At the end of the experiment, they continued to develop and reproduce.
Beyond the Paris Agreement
For David Wachenfeld, global efforts should not stop with the Paris Agreement. “None of the forecasts remains below 2°C by 2100,” he points out. Everyone agrees today that in the short term global warming will reach +2°C. But to preserve the health of the coral reefs of tomorrow, it would have to be limited to +1.5°C. And even then, the heat stress will be enormous for the coral reefs – the Great Barrier has indeed seen its worst bleaching episode with around +1°C. The Great Barrier is alive, but is under tremendous pressure and desperately needs more help.”