Rising sea temperatures, resulting in large-scale coral bleaching, has resulted in the world losing 14% of its coral reefs - more than the size of all Australia's reefs combined - between 2009 and 2018.
The increasing frequency and geographic extent of mass coral bleaching events - corals expel symbiotic algae living in their tissues as a result of changes in conditions such as temperature or nutrients, resulting in them turning completely white - have prevented coral cover from recovering, according to a report published by two reef monitoring organisations (GCRMN and ICRI) on Wednesday.
Researchers and scientists reported 20% more algae on the world’s coral reefs in 2019 than in 2010, which is an indicator of the stress felt by the corals.
Dynamic coral cities support up to 800 different species of hard coral and are home to more than 25% of all marine life, and harbour the highest biodiversity of any of the world’s ecosystems.
A "progressive" shift from coral to algae-dominated reefs reduces "the complex three-dimensional habitat that is essential to support high biodiversity and provide valuable goods and services for reef-dependent human communities," the report read.
Before the first mass coral bleaching event which took place in 1998, the global average cover of hard coral was high (more than 30%) and stable, but approximately 8% of the world’s coral was killed following this mass bleaching.
"To put this into context, this represents more than the total amount of living coral in any one of the Caribbean, Red Sea and Gulf of Aden, South Asia or Western Indian Ocean regions," the report stated.
The global average cover of hard coral recovered to pre-1998 levels (33.3% in 2009), which scientists argued suggests that many of the world’s coral reefs "remain resilient and can recover if conditions permit."
However, in the last decade, mainly as a result of elevated sea surface temperatures (SST), large-scale coral bleaching events recurred more often, giving the corals less time to recover.
Total loss is possible
Over the course of the century, a 1.5°C increase in water temperatures could result in a loss of 70% to 90% of reef areas, while this loss could be almost total with a 2°C increase, according to the scenarios put forward in the Intergovernmental Panel on Climate Change (IPCC)’s special report on the ocean and cryosphere.
The report stressed that both "reducing local pressures coral reefs in order to maintain their resilience until climate change is addressed," as well as monitoring data collected in the field to understand the status of coral reef condition, will be critical.
"If we halt and reverse ocean warming through global cooperation, we give coral reefs a chance to come back from the brink. It will, however, take nothing less than ambitious, immediate and well-funded climate and ocean action to save the world's coral reefs," the report read.
The findings of the research, funded by the UN Environment Programme (UNEP), are based on data gathered by more than 300 scientists from 73 countries, who carried out two million individual observations over a span of four decades.