Climate change

14 per cent of corals lost since 2009, report finds

Written by Oceanographic Staff

Rising sea surface temperatures have driven the loss of 14 per cent of corals since 2009, a report found. Some corals show resilience, offering hope for recovery.

Published on 5 October, the report was produced by the Global Coral Reef Monitoring Network (GCRMN), drawing on information gathered across 12,000 sites in 73 countries over the course of 40 years. The data on coral loss has been collected by over 300 scientists and through 2 million observations, making the report one of the most detailed scientific pictures of the toll elevated temperatures have taken on the world’s reefs to date.

Key findings include a 14 per cent coral loss on the world’s coral reefs between 2009 and 2018 which equates to around 11,700 square kilometres of coral, more than all the living coral in Australia. The decline in coral cover corresponds with increasing sea surface temperatures.

The analysis examined 10 coral reef-bearing regions around the world and found that coral bleaching events caused by elevated sea surface temperatures (SSTs) were the main driver of coral loss. In 1998, a severe coral bleaching event killed eight per cent of the world’s coral – the equivalent of about 6,500 square kilometres of coral. The greatest impacts of this mass coral bleaching event were seen in the Indian Ocean, Japan, and the Caribbean, with smaller impacts observed in the Red Sea, The Gulf, the northern Pacific in Hawaii and the Caroline Islands, and the southern Pacific in Samoa and New Caledonia. The longer-term decline seen during the last decade coincided with persistent elevated SSTs.

The analysis also investigated changes in the cover of both live hard coral and algae. Live hard coral cover is a scientifically based indicator of coral reef health, while increases in algae are a widely accepted signal of stress to reefs. Since 1978, when the first data used in the report were collected, there has been a 9 per cent decline in the amount of hard coral worldwide. Between 2010 and 2019, the amount of algae has increased by 20 per cent, corresponding with declines in hard coral cover. This progressive transition from coral to algae-dominated reef communities reduces the complex habitat that is essential to support high levels of biodiversity.

The report highlighted that although during the last decade the interval between mass coral bleaching events has been insufficient to allow coral reefs to fully recover, some recovery was observed in 2019 with coral reefs regaining 2 per cent of the coral cover. This indicates that coral reefs are still resilient and if pressures on these critical ecosystems ease, then they have the capacity to recover, potentially within a decade, to the healthy, flourishing reefs that were prevalent pre-1998. Coral reefs in East Asia’s Coral Triangle, for example, have been less impacted by rising sea surface temperatures. Despite some declines in hard coral during the last decade, on average, these reefs have more coral today than in 1983 when the first data from this region were collected.

Dr Paul Hardisty, CEO of the Australian Institute of Marine Science: “This study is the most detailed analysis to date on the state of the world’s coral reefs, and the news is mixed. There are clearly unsettling trends toward coral loss, and we can expect these to continue as warming persists. Despite this, some reefs have shown a remarkable ability to bounce back, which offers hope for the future recovery of degraded reefs. A clear message from the study is that climate change is the biggest threat to the world’s reefs, and we must all do our part by urgently curbing global greenhouse gas emissions and mitigating local pressures.”

Corals reefs across the world are under relentless stress from warming caused by climate change and other local pressures such as overfishing, unsustainable coastal development and declining water quality. An irrevocable loss of coral reefs would be catastrophic. Although reefs cover only 0.2 per cent of the ocean floor they are home to at least a quarter of all marine species, providing critical habitat and a fundamental source of protein, as well as life-saving medicines. It is estimated that hundreds of millions of people around the world depend on them for food, jobs and protection from storms and erosion.

Inger Andersen, Executive Director of the UN Environment Programme (UNEP), which provided financial, technical and communication support to the report: “Since 2009 we have lost more coral, worldwide, than all the living coral in Australia. We are running out of time: we can reverse losses, but we have to act now. At the upcoming climate conference in Glasgow and biodiversity conference in Kunming, decision-makers have an opportunity to show leadership and save our reefs, but only if they are willing to take bold steps. We must not leave future generations to inherit a world without coral.”

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Photography courtesy of Unsplash.

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