Image description: Bleaching at Lakeside Reef Front, Ningaloo. Photograph: Zoe Richards/Curtin University / The Guardian.

Australia’s two world heritage-listed reefs, Ningaloo on the west coast and the Great Barrier Reef on the east, are experiencing mass coral bleaching events simultaneously. Reef experts are reporting the events as “heartbreaking” and “a profoundly distressing moment”, in what appears to be the first time these two World Heritage-listed reefs have bleached in unison. Bleaching may also hit the World Heritage reef at Shark Bay in Western Australia.

Global heating has triggered heat stress and consequent bleaching across thousands of kilometres of marine habitat, teams of scientists told The Guardian. This summer, an intense marine heatwave impacted northwestern Australia, driving sea surface temperatures up to 4°C above the summer average. The extended marine heatwave has caused highest recorded heat stress, impacting coral reefs all along the vast coastline of western Australia’s famous Ningaloo reef. Bleaching has also been detected from around Townsville to the top of Cape York on the Great Barrier Reef, stretching 1,000 km. Dr Zoe Richards, an associate professor and coral scientist at Curtin University, reported up to 90% of the coral found in shallow areas of the northern lagoon had bleached, while at Ningaloo and further south the heatwave is still unfolding.

Coral bleaching occurs when corals undergo stress and expel the symbiotic zooxanthellae algae. Without these algae, corals fade and turn white, appearing bleached. If high ocean temperatures persist, the corals fail to reabsorb the algae and eventually die. Last summer was the worst bleaching event on record for the reef and the fifth major outbreak in eight years, hitting all across the marine park. Leticia Carvalho, Head of the Marine and Freshwater Branch of the United Nations Environment Programme (UNEP), called the loss of corals from reefs worldwide a “true tragedy”, but emphasised their fate is not yet sealed if action is taken now and with a strong determination.