The 1998 coral bleaching event

The summer of 1997-1998 was the hottest recorded on the Great Barrier Reef since records began in the late 19th century. Mild bleaching began in late January and intensified by February/ March. Extensive aerial surveys of 654 reefs showed great variation in the extent of bleaching between inshore (<10km from the coast) and offshore reefs; and between reef regions along the coast. While only 14% of offshore reefs surveyed were reported to have high levels of bleaching, 67% of the inshore reefs surveyed had high or extreme levels of bleaching. The most severely affected reefs were inshore reefs in the Cairns to Whitsunday coast, and reefs in the Keppel Island group. Most coral bleaching and mortality was seen in shallow water.

High temperatures will cause corals to bleach. Photo by GBRMPA.

These surveys revealed that high sea temperatures combined with periods of slack winds, calm seas, high light, and in some areas, reduced salinity due to flooding, caused the major bleaching event. During the bleaching period, sea temperatures were 1-2°C higher than long-term average values in the central and southern Great Barrier Reef. Water closer to the Queensland coast was generally warmer.

On the most severely bleached areas of reef, subsequent coral mortality was patchy. On some reefs, such as those in the Palm Island group north of Townsville, up to 80% of corals died in the next few months. On other reefs, mortality was low to negligible, including most reefs in the Keppel Island group.

In 1999, surveys of reefs throughout the Great Barrier Reef showed that coral cover declined as a result of bleaching on only a small percentage of the 48 reefs surveyed. The most severely affected reefs were inshore between latitudes 17-19°S. One year after the event, most offshore reefs had changed very little as a result of bleaching.

In 1998, the bleaching impacts on the Great Barrier Reef were relatively small compared with other reefs around the world. Every coral reef region in the world was affected by bleaching that year, making the 1998 bleaching event the worst ever recorded on a global scale.

In many cases, the bleaching events were predicted by advanced reports of water temperature 'hot-spots' from satellite measurements of water temperature patterns. Reports of major water temperature anomalies (deviations from usual values) were recorded throughout the Indo-Pacific as a result of a very strong El Nino Southern Oscillation (ENSO) event. Many reports assert that the seriousness of the impacts of this ENSO event on coral reefs is strong evidence for global warming. However, while the 1998 coral bleaching event was triggered by severe ENSO conditions, not all bleaching events are associated with ENSO anomalies. However, average water temperatures at coral reefs around the world have warmed in association with global climate change.

CRC Reef Research Centre Ltd was established and is supported under the Australian Government’s Cooperative Research Centres Program. Its mission is to plan, fund and manage world-leading science for the sustainable use of the Great Barrier Reef World Heritage Area. CRC Reef is a joint venture between Association of Marine Park Tourism Operators, Australian Institute of Marine Science, Great Barrier Reef Marine Park Authority, Great Barrier Reef Research Foundation, James Cook University, Queensland Department of Primary Industries and Fisheries, Queensland Seafood Industry Association, Sunfish Queensland Inc. The University of Queensland is an associate member.