Global warming and bleaching of coral reefs

Global warming and recurrent mass bleaching of corals

Terry P. Hughes, James T. Kerry, Mariana Alvarez-Noriega, Jorge G. Alvarez-Romero, Kristen D. Anderson, Andrew H. Baird, Russel C. Babcock, Maria Beger, David R. Bellwood, Ray Berkelmans, Tom C. Bridge, Ian R. Butler, Maria Byrne, Neal E. Cantin, Steeve Comeau, Sean R. Connolly, Graeme S. Cumming, Steven J. Dalton, Guillermo Diaz-Pulido, C. Mark Eakin, Will F. Figueira, James P. Gilmour, Hugo B. Harrison, Scott F. Heron, Andrew S. Hoey, Jean-Paul A. Hobbs, Mia O. Hoogenboom, Emma V. Kennedy, Chao-yang Kuo, Janice M. Lough, Ryan J. Lowe, Gang Liu, Malcolm T. McCulloch, Hamish A. Malcolm, Michael J. McWilliam, John M. Pandolfi, Rachel J. Pears, Morgan S. Pratchett, Verena Schoepf, Tristan Simpson, William J. Skirving, Brigitte Sommer, Gergely Torda, David R. Wachenfeld, Bette L. Willis, and Shaun K. Wilson

Data: The authors surveyed Australian coral reefs around the Australian coast using aerial photographs and underwater images to assess the amount of bleaching experienced by the reefs. They compared these images and data to sea surface temperature data from the area to determine if there was a correlation between sea surface temperature and coral reef bleaching. Learn about what coral bleaching is by clicking here.

Methods: The authors first took aerial photographs of the reefs from an airplane and helicopter, which flew about 150 meters above sea level, for 10 days in 2016. The researchers then ranked the severity of coral bleaching using a scale from 0 to 4, with 4 being the worse bleaching (over 60% of corals). To check that their scale from the aerial images was correct, the scientists also conducted underwater surveys of the same reefs. The same methods were conducted in 1998 and 2002 by other researchers, so the authors of this study compared their data to previous data. In this way, they have 3 years of coral bleaching data from the years 1998, 2002, and 2016 to see if bleaching events are becoming more common and getting worse. The scientists then compared their bleaching scale to observed sea surface temperatures in the area where the surveys were conducted to observe the relationship between temperature and coral bleaching.

A). The severity of bleaching of coral reefs around the northeast coast of Australia from 1998, 2002, and 2016. Dark green areas indicate <1% of corals bleached, light green indicates 1-10% bleaching, yellow indicates 10-30% bleaching, orange areas indicate 30-60% bleaching, and and red areas are where more than 60% of the corals are bleached. B) Patterns of heat stress during mass bleaching events in 1998, 2002, and 2016. Hotter colors indicate maximum heat exposure.

Results: Coral reef bleaching increased significantly from 1998 to 2016. Associated with the bleaching was an increase in the water temperature around the coasts of Australia where the corals are living.

A) Aerial view of a reef that is close to 100% bleached; B) Severe bleaching of an older coral mass on the northern Great Barrier Reef; C) staghorn corals that were killed by the major bleaching event in 1998; D) the same site in C, but 18 years later and the corals have not recovered; E, F) mature staghorn corals that were killed by heat stress and colonized by algae in just a few weeks time in 2016.

Why is this study important? This study is one of the first to examine a huge amount of coral reefs (1,156 in 2016 alone) to assess the effects of increased water temperature on coral bleaching. The researchers indicate that some coral species can grow back in 10-15 years, but some of the corals that are dying in the reefs are slow growers and very old. It will likely take decades for these corals to return to their former glory. This study indicates that we must take action now to save our coral reefs, not just around Australia, but around the world.

The Big Picture: By using large data sets and looking at trends of corals through time, scientists can concretely state that rising sea surface temperatures due to increased CO2 levels are causing mass coral reef bleaching events. When corals are stressed for too long, they die. Coral reefs are considered the rainforests of the sea because they are home to so many species of marine animals. Once the coral reefs begin to die, other animals will lose habitat to live in, and thus their numbers will, and are, declining. This has huge implications on the fishing industry, as people who rely on the ocean to make a living will no longer be able to catch bountiful amounts of fish that live around the reefs. In short, the effects of dying corals has far-reaching implications that will hurt the marine ecosystem, collapse the marine food chain, and affect economies.

Citation: Hughes, T. P., Kerry, J. T., Alvarez-Noriega, M., Alvarez-Romero, J. G., et al., 2016. Global warming and recurrent mass bleaching of corals. Nature, 543 (7645). DOI: 10.1038/nature21707

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