File:Heatwave Impacts Great Barrier Reef (MODIS 2022-04-04).jpg

English: When viewed from space, Australia’s Great Barrier Reef appears to be a string of precious gems strung across the Coral Sea just off the Queensland coast. When viewed from Earth, the uniqueness of this precious ecosystem becomes even more obvious. According to the United Nations Education, Science and Cultural Organization (UNESCO) World Heritage Convention, the Great Barrier Reef is a site of remarkable variety and beauty. It contains the world’s largest collection of coral reefs, with 400 types of coral, 1,500 species of fish, and 4,000 types of mollusk. It also holds great scientific interest as the habitat of species such as the dugong (‘sea cow’) and the large green turtle, which are threatened with extinction. Because the diversity of species and habitats—and their unique interconnectivity—make the Great Barrier Reef one of the richest and most complex natural ecosystems on Earth, this natural wonder was inscribed on the World Heritage List in October 1981.

Despite the fact that no other World Heritage property contains such biodiversity, the Great Barrier Reef is in trouble. In early March 2022, sea temperatures across the reef spiked, creating heatwave conditions. On April 1, the Australian Government’s Great Barrier Reef Marine Park Authority confirmed that these warm temperatures had created a mass bleaching event of the reef, with bleaching observed at multiple reefs in all four management areas of the Reef. This was considered to be largely consistent with the spatial distribution of the heat stress experienced over summer. This is the fourth time in six years that the Great Barrier Reef has been impacted by bleaching, and it the first time it has occurred during La Niña conditions. Typically, La Niña brings cooler temperatures to local waters.

Bleaching is a stress response by coral and does not necessarily mean that the coral has died. Corals have a symbiotic relationship with a type of microscopic algae known as zooxanthellae. The colorful zooxanthellae live within the coral, providing oxygen and helping the coral to remove wastes. In turn, the coral provides the algae with a protected environment and the compounds they need for photosynthesis. The zooxanthellae also provide the glorious colors that we associate with coral. When coral become stressed—most often by a rise in ocean temperature of as little as 1˚C—the corals expel the algae, exposing the pale white ‘skeleton’ of the coral. This bleaching isn’t immediately fatal to the coral, but it weakens them, opening them to disease, starvation, and death. If conditions return to normal and the symbiotic relationship is repaired, coral may recover from a bleaching event.

The Great Barrier Reef Marine Park’s website stated on April 1 that water temperatures had cooled since early March but noted that most of the Marine Park was continuing to experience the effects of significant heat stress. Localized weather patterns will remain critical in determining the overall extent and severity of coral bleaching. The website also noted that the Reef Authority is now working with its partners to conduct in-water surveys to complement aerial surveys of the reef. This will provide more detailed information about bleaching intensity, mortality and depth range of bleaching impacts.

On March 31, 2022, the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite acquired a true-color image of the Great Barrier Reef. The reef remains sparkling with blue tones throughout the length visible in this image. The MODIS instrument is quite useful for measuring Sea Surface Temperatures (SSTs), which are correlated with coral bleaching events, but true-color images do not usually show substantial color changes in bleaching events, as coral sits underneath water and thus reflectivity may not change substantially. Currently, airplane flights or inspection by diving are more sensitive for estimating the amount of bleaching on any given coral reef than most satellites.