Over the past 8 years, five instances of ocean warming have led to widespread coral bleaching in the Great Barrier Reef, with the latest occurring in Australia. According to a report by The Sydney Morning Herald on April 6, survey results released by the Australian Marine Park Authority on the 5th revealed that 75% of the 1001 coral reefs surveyed exhibited bleaching.
Climate change forecasts for coral reefs typically rely on sea surface temperatures, but this overlooks the fact that deeper waters may not necessarily experience the same warming as the surface.
A new study led by the University of Exeter and the University of Queensland investigated the impact of temperature changes on mesophotic corals (depths of 30-50 meters). The research indicates that some deeper areas of the Great Barrier Reef are insulated from harmful heatwaves, but this resilience will be lost if global warming continues. On April 8, the paper was published in the Proceedings of the National Academy of Sciences.
Photos taken before and after the "bleaching" event near Lizard Island on the Great Barrier Reef. Image source: George Roff
Research has found that the separation between warm buoyant surface waters and cooler deep waters can protect coral reefs from the impacts of surface heatwaves. However, if global warming exceeds 3°C above pre-industrial levels, this protective mechanism will be lost.
Similar patterns may occur on other coral reefs globally, but local conditions affecting how water moves and mixes will mean the extent of deep-water coral refuges and their insulation from surface heatwaves will vary.
"Coral bleaching is a prominent indicator of human impact on the planet," says lead author Dr. Jennifer McWhorter of the US National Oceanic and Atmospheric Administration's Atlantic Oceanographic and Meteorological Laboratory. "Our research provides both hope—that some reefs may withstand current levels of climate change—and a warning—that this resilience is finite."
The study found that a 3°C global warming would push the mid-light temperatures of the Great Barrier Reef beyond 30°C, a widely recognized threshold for coral mortality.
This doesn't necessarily mean all corals will die, but it would stress them, increasing mortality rates and possibly leading to reef collapse.
To calculate their predictions for warming of mid-light coral reefs, the research team considered factors such as wind and tidal mixing of waters, as well as local complexities.
They estimate that by 2050-2060, bottom temperatures of the Great Barrier Reef (at 30-50 meters) would rise by 0.5-1°C under low greenhouse gas emissions scenarios and by 1.2-1.7°C under high emissions scenarios.
"Corals face multiple threats, not just climate change. To protect coral reefs, we need a better understanding of them," says Dr. Paul Halloran of the University of Exeter's Global Systems Institute.
Related paper: https://doi.org/10.1073/pnas.2303336121
