From December 2023 to March 2024, coral loss on the Northern Great Barrier Reef reached unprecedented levels due to extreme weather events, including two cyclones, flooding, and intense heat waves. Coral cover has decreased on 12 of the 19 researched reefs between Lizard Island and Cardwell, according to new findings from the Australian Institute of Marine Science’s (AIMS) in-water surveys. Between August and October 2024, surveys for the Long-Term Monitoring Program (LTMP) revealed losses that ranged from 11% to 72% of pre-summer levels.

Hard coral cover fell from 31.4% to 19.3% in the Cooktown-Lizard Island area, the worst yearly reduction in 39 years. The Cairns sector had a more than one-third decrease in hard coral cover, quantifying the proportion of living coral on the reef surface, whilst the Innisfail sector’s reefs held steady pre-summer levels.

Impacts of Extreme Weather and Bleaching

The Great Barrier Reef was severely impacted by the fifth mass coral bleaching event, which was caused by climate change. It was the most widespread and deadly global bleaching event, with over 77% of the world’s reefs experiencing heat stress at bleaching levels within the last 20 months. In the Great Barrier Reef Marine Park, 73% of the 1,080 reefs examined from the air had bleaching, with 39% of the reefs showing severe bleaching (>90% coral cover).

The Cooktown-Lizard Island sector’s inner and mid-shelf reefs suffered significant damage, with pre-summer hard coral cover loss of around 75%. However, there was little to no coral damage on outer shelf reefs. Coral varieties were affected differently, with Acropora species seeing the highest mortality rates. Massive corals such as Porites, on the other hand, suffered the least.

Tropical Cyclones Jasper and Kirrily caused additional damage by exposing reefs to waves higher than four meters. The levels of heat stress in February and March 2024 were high enough to cause bleaching and, in certain places, a considerable amount of coral loss.

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Ongoing Assessment and Reef Resilience

The scientists at AIMS are working to finish an in-depth analysis of the Marine Park by the middle of 2025, and they are now surveying further reefs in the Cooktown-Lizard Island area. They underlined that the findings highlight the reef’s susceptibility and waning resilience in the face of rising extreme weather and bleaching event frequency and severity.

The scientists observed that “these disturbances have pushed the reef’s resilience to its limits.” While some recovery has occurred due to fast-growing coral species, the coral loss on the Northern Great Barrier Reef highlights the challenges posed by climate change. Efforts to preserve coral biodiversity include ongoing monitoring, evaluation, and targeted conservation measures.

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