Bleaching primarily occurs when rising seawater temperatures lead the corals to expel the symbiotic algae living within them, draining the corals of their color and eliminating their principal food source. We asked marine geologist Justin Ries, associate professor in the Department of Marine and Environmental Sciences, to explain the significance of the new research and what this level of damage means for the health of our ecosystem as a whole, including ourselves.
Last April, researchers issued findings that 93 percent of the 911 reefs in the Great Barrier Reef they surveyed by air had been bleached to some extent, and that more than one-third of them had experienced severe bleaching. What does the new research add to that report?
The new research links patterns of coral bleaching on the Great Barrier Reef during the 1998, 2002, and 2016 mass bleaching events with patterns of ocean warming in those portions of the Great Barrier Reef. The researchers show that the bleaching in 2016, when thermal stress was greatest, was more than four times higher than the bleaching in 1998 or 2002, when thermal stress was less.
Comparison of geographic patterns of coral bleaching with geographic patterns of thermal stress across the reef system during the three bleaching events also shows that even within a bleaching event, the most extreme bleaching coincides with the most extreme thermal stress. This occurred in the southern portion of the Great Barrier Reef in 1998 and 2002 and in the northern portion in 2016.
These observations show that the occurrence of coral bleaching in both space and time is primarily controlled by thermal stress—yet more evidence of the increasingly catastrophic impacts that global warming is having on marine ecosystems.
Furthermore, good water quality, fishing controls, and prior bleaching conferred no protection to the corals from bleaching (although they may aid in recovery post-bleaching), leading the researchers to conclude that the only way to save the corals is to stop the global warming that is occurring.
Why has the destruction to the Great Barrier Reef been so swift and so severe?
Coral bleaching is caused primarily by seawater spiking above temperatures to which the corals are accustomed. The longer the heating continues, the greater the stress to the coral. The researchers show that corals exposed to temperatures as little as 1 degree Celsius above the mean summertime temperatures for more than six to eight weeks can lead to widespread bleaching—even approaching 100 percent bleaching—and death from thermal stress.
Warm spells in the Great Barrier Reef region seem to be persisting for longer than predicted. This is in part due to the difficulty in predicting how global-scale warming will play out over more localized, spatially complex environments, such as barrier reef systems. These results from the Great Barrier Reef may foretell accelerated warming in other reef systems, such as the Mesoamerican Barrier Reef, in the Caribbean, as well as other coastal ecosystems, such as shallow shelves, carbonate banks, atolls, and estuaries, where spatial complexity varies at a finer scale than that of the model used to predict warming.
How do coral reefs contribute to the health of our oceans, our ecosystem as a whole, and to our own well-being? What are some of the repercussions of this level of damage?
Corals support tourism, provide protective barriers to shorelines, create spatially complex reef systems that support biodiversity, provide nursery grounds for commercial fisheries, and harbor organisms that may provide antibacterial, antiviral, and anticancer compounds in the future. Widespread bleaching events such as these endanger these precious resources, potentially contributing to the extinction of some coral species, and even loss of entire reef systems, if bleaching events become even more pervasive.
Photo via Flickr