By Megan Y. Tan ’19

The Great Barrier Reef is the world’s largest coral system stretching over an area of more than 300,000 square kilometers off the coasts of Australia. Though the Great Barrier Reef Marine Park protects a large part of the reef from fishing and tourism, it is still not safe from environmental factors such as global warming. In fact, climate change has increasingly affected temperatures and the reef ecosystem by disrupting the symbiotic relationship between corals and algae, which discolors the coral in a process known as coral bleaching.

Professor Terry P. Hughes of James Cook University in Queensland and his team of researchers examined the impact of high temperatures over time, typically at least 1°C of heat stress over four weeks, on the Great Barrier Reef, which has triggered recurring coral bleaching events in 1998, 2002, and most recently 2016. They examined the multiple scale severity of those bleaching events by using aerial and underwater surveys of 171 individual coral reefs from different parts of the Great Barrier Reef. Professor Hughes and his team also used sea temperatures derived from satellites to determine the effects of extreme heat on the coral reef. They observed a strong relationship between the amount of bleaching measured under water, and the satellite-based estimates of heat exposure on the individual reefs. Contrary to earlier research, prior exposure to bleaching, which occurred in 1998 and 2002, did not lessen the severity of the bleaching in 2016.  The low levels of bleaching were observed at locations that experienced less local heat stress, which demonstrates that marine heat waves can be ameliorated by local weather during a global heat wave. In less-affected reefs, there were reefs that bleached less and had a higher survivorship, which were known as the winner corals, and others which bleached more and were less likely to survive among the corals, which were known as the loser corals. However, this disparity diminished among the more-affected reefs, which are in the northern regions. For instance, winner corals took about 10 to 15 years to recover from the bleaching, whereas loser corals took several decades.

Professor Hughes concluded that global heating was the main driver of coral bleaching and that the severity of the bleaching in the Great Barrier Reef was worse than previously estimated. Because this rise in ocean temperatures is global, the same can be said for coral reefs around the world, like those in Japan and the Caribbean. Thus, the future for coral reefs rests on the reduction of global warming. Further research can test winner and loser corals so there is a better understanding of how to save coral reefs.

References:

1. T. Hughes et al., Global warming and recurrent mass bleaching of corals. Nature 543, 373-377 (2017). doi: 10.1038/nature21707.
2. Image retrieved from: https://johncarlosbaez.wordpress.com/2016/04/22/bleaching-of-the-great-barrier-reef/