Global Warming Threatens Great Barrier Reef's Deep-Water Coral Sanctuaries

Warming waters have led to widespread coral 'bleaching' in the Great Barrier Reef during five of the past eight years, with the most recent event occurring in Australia. According to a report by the Sydney Morning Herald on April 6, survey results released by the Australian Great Barrier Reef Marine Park Authority on the evening of the 5th showed that 75% of the 1,001 coral reefs have experienced bleaching.

Climate change projections for coral reefs often rely on sea surface temperatures, disregarding the differential warming experienced by deeper waters.

A recent study, conducted by the University of Exeter and the University of Queensland, examined the impact of temperature changes on mesophotic corals (located at depths of 30 to 50 meters). Although certain deeper areas of the Great Barrier Reef are shielded from harmful heat waves, research suggests that this defense will be lost if global warming continues. The findings of this study were published in the Proceedings of the National Academy of Sciences on April 8.

The study reveals that the separation between warm buoyant surface water and cooler deep water acts as a protective barrier for coral reefs against surface heat waves. However, if global warming surpasses 3°C above pre-industrial levels, this safeguard will be compromised. Similar dynamics may be observed in other coral reefs worldwide, but the presence and insulation of deep-water coral sanctuaries from surface heat waves will vary due to local conditions affecting water movement and mixing.

Moreover, the study highlights that a 3°C increase in global warming would elevate mesophotic temperatures in the Great Barrier Reef above 30°C, which is the recognized threshold for coral death. Although not all corals would necessarily perish, this state of stress would raise mortality rates and potentially lead to the collapse of the reef.

To determine their projections of mesophotic reef warming, the research team considered factors such as wind, tidal water mixing, and local complexity. They estimate that by 2050-2060, under projected lower greenhouse gas emissions, the bottom temperature of the Great Barrier Reef (30-50 meters) will rise by 0.5-1°C. Under projected higher greenhouse gas emissions, the increase is expected to range from 1.2 to 1.7°C.

Dr. Paul Halloran, from the Global Systems Institute at the University of Exeter, emphasized that coral reefs face multiple threats beyond climate change. To safeguard these ecosystems, a better understanding of their intricacies is imperative.