Bleached Acropora coral (foreground) and normal colony (background), Keppel Islands, Great Barrier Reef. Credit: Wikimedia Commons.

Recent, major bleaching events in the Great Barrier Reef — the largest living structure on the planet — has dramatically compromised the recruitment of new corals. According to researchers, the number of juvenile corals that settled in the reef was 89% lower in 2018 than the historical average.

A bleak future

Australia’s Great Barrier Reef has been hampered by four mass coral bleaching events since 1998, the most recent one lasting from June 2014 to May 2017. This was the longest, most damaging coral bleaching event on record killing 30% of the reef. An estimated half billion people around the world directly depend on reefs for income from fishing and tourism. Economic activity derived from the Great Barrer Reef alone is thought to be worth $4.5 billion annually.

Bleaching occurs when the ocean’s waters become too warm and expel the photosynthetic algae, called zooxanthellae, which live in a symbiotic relationship with the coral. Without the algae, the coral dies and seaweeds take over. The main culprit is man-made climate change, which warms and increases the acidity of the waters. Although some think the effects of climate change are hazy and yet to rear their head, it has actually been affecting the reef for at least 20 years. A 2018 study found that the number of ocean heatwaves has risen by more than 50% since 1925, threatening to collapse marine ecosystems all over the world, coral reefs being no exception.

Scientists believe that under normal conditions, the coral would need 10 years to bounce back. But a new study led by researchers at ARC Centre of Excellence for Coral Reef Studies suggests conditions are anything but normal.

The rate of new coral recruitment is abysmally low. Researchers measured how many adult corals along the reef had survived following the mass bleaching events, as well as the number of new corals that had been produced in 2018. Compared to 1990-levels, a period where there were no bleaching events, there was an average 90% decline in coral recruitment across the whole length of the Great Barrier Reef.

Typically, when one reef is destroyed, it can be replenished by babies from another reef. However, the 2016 an 2017 bleaching was so severe that in many parts of the reef there were no longer any adjacent reefs to provide offspring.

Not only does the Great Barrier Reef’s future hang by a thread, what remains of it is also morphing dramatically. Some corals are more resilient than others, which means that they now breed more, altering the coral composition. For instance, the hardest hit species is Acropora, which saw a 93% decline.

Coral reefs are complex ecosystems, so when a coral species disappears, so does the habitat for countless other species of marine wildlife.

“The collapse in stock–recruitment relationships indicates that the low resistance of adult brood stocks to repeated episodes of coral bleaching is inexorably tied to an impaired capacity for recovery, which highlights the multifaceted processes that underlie the global decline of coral reefs. The extent to which the Great Barrier Reef will be able to recover from the collapse in stock–recruitment relationships remains uncertain, given the projected increased frequency of extreme climate events over the next two decades,” the authors wrote in their study.

If current trends continue unabated, coral bleaching might affect 99% of the world’s reefs within this century, the United Nations warns. Previously, the U.N. Intergovernmental Panel on Climate Change warned that tropical reefs could decline by 70% to 90%, if the planet warms by 1.5ºC compared to preindustrial average temperatures — the upper limit set by the Paris Agreement. At 2ºC of warming, 99% of the world’s reefs could perish.

“Going to 2C and above gets to a point where corals can no longer grow back, or you have annual bleaching events. On the other hand, at 1.5C there’s still significant areas which are not heating up or not exposed to the same levels of stress such that they would lose coral, and so we’re fairly confident that we would have parts of those ecosystems remaining,” said Professor Ove Hoegh-Guldberg, a coral reefs expert with the University of Queensland.

Last year, Australian scientists bred baby corals in an artificial environment and later moved them to some of the most damaged parts of the reef. Eight months later, the juvenile coral had survived and grown, lending hope that coral transplants can restore similarly damaged ecosystems, not just in the Great Barrier Reef, but around the world as well. However, this is just patchwork. The only viable long-term solution is cutting global greenhouse emission. But even if we manage to avert 1.5ºC of warming, the Great Barrier Reef will never be the same.

The findings appeared in the journal Nature.