The Receding Great Barrier Reef


The ocean’s condition is typically one of the last items to be considered when thinking about climate change. However, it is currently the primary source of food for nearly 3.5 billion people and home to a multitude of ecosystems, making its fate just as important as that of Earth’s terrestrial areas. Among the most archetypal marine ecosystems in the world, the Great Barrier Reef located off of the coast of Australia provides an alarming look into the severe effects of coral bleaching caused by drastically rising ocean temperatures.

The Great Barrier Reef is consists of thousands of smaller reefs that support nearby ecosystems with their vast ecological diversity. Reefs are formed by coral polyps which attach to rocks and secrete calcium carbonate, a compound that houses the organism and gives the reef its structure. Photosynthetic organisms symbiotically interact with the coral polyps and function as the primary producers of the ecosystem, the foundation of the food web within the environment. By killing these photosynthetic organisms, rising ocean temperatures indirectly threaten higher levels of ecosystem interactions that would otherwise not be affected.

Coral bleaching is the result of the death of the coral’s overheated symbionts revealing the white skeleton left by the coral polyps. Once bleached, these coral cannot survive for long but could potentially recover if sea temperatures return to lower levels. As rising ocean temperatures become a more prevalent issue, there have been significantly higher incidences of coral reef bleaching from 2015 to 2016. In a study published by Nature in March 2017, Australian researchers studied portions of the Great Barrier Reef over the course of 18 years in order to determine why warming ocean temperatures have produced such drastically different degrees of damage to certain reefs.

In the study, the researchers documented three years of significant bleaching activity: 1998, 2002, and 2016. In 1998, bleaching was mainly found in coastal areas concentrated in the southern and central regions. However, in 2002, the bleaching continued, affecting many offshore areas and damaged reefs that had survived in 1998. By 2016, the reef had witnessed an incidence of “extreme bleaching” (more than 60% of corals were bleached) that was four times higher than in previous years—there were only 8.6% of reefs out of the 1,156-reef sample size that were unharmed.

This healthy staghorn coral bed (c) located on Orpheus Island in Queensland, Australia was killed in the 1998 bleaching event. Researchers visited the site again in May 2016 (d) and discovered that the existing coral population area had never recovered nor had been able to accommodate colonization by other coral larvae.

The researchers determined that the variance in event severity and spatial distribution between bleached areas of the reef is predominantly related to rising sea-surface temperatures. In certain cases, some reefs were spared as a result of extreme weather phenomena, such as El Niño. Additionally, they discovered that areas that are subjected to frequent heat stresses are most prone to future bleaching events since the events are independent of one another. Even if there are no stresses in the area, temperature-related or otherwise, recovery may take up to 15 years for resilient species. Given the damage that is done to the oceans everyday aside from global-warming-related causes, natural recovery will essentially never happen.

Well-intended ecosystem-protection policies, such as those relating to commercial fishing, will be unable to prevent bleaching but may help in the reefs’ recoveries. Researchers predict that instances of coral reef bleaching will continue into the foreseeable future until direct action is taken to ensure lower ocean temperatures. Until then, coral reef bleaching continues to pose a major threat to the biodiversity of the Great Barrier Reef and may ultimately become yet another symbol of the global climate change tragedy.