Coral reefs face desperate struggle

The UNESCO report Impacts of Climate Change on World Heritage Coral Reefs, updated in 2018, warned that without major reductions in greenhouse gas emissions, all 29 World Heritage reefs would "cease to exist as functioning coral reef ecosystems by the end of this century".

The study led by Davis－which complements several others that show net coral reef dissolution between 2030 and 2080-relied on historical data to measure the current rate of decline and what may happen in the future.

According to Davis, the most-studied country was Australia (Great Barrier Reef, Lord Howe Island and reefs in Western Australia). Case studies also included reef sites in Japan, Hawaii, the Red Sea, French Polynesia and Bermuda, among others.

Coral reef calcification, or coral growth, is significantly declining over time with "net-zero growth predicted for around mid-century", Davis said.

"As calcification is a key process in coral reef survival, this is a worrying find," she added.

Davis said that in the past five years, the Great Barrier Reef has experienced three mass bleaching events, impacting more than 90 percent of Australian reefs.

"As coral reefs encapsulate an immense amount of biodiversity, it can be difficult to predict how each species within them will react to change," she said.

"If we consider the 'reef' as a whole, it is already being affected by anthropogenic (man-made) change, and will continue to worsen until serious mitigation efforts come into play like global reduction in greenhouse gas emissions and local changes to improve water quality and stabilize fisheries."

To sustain coral reefs, individual corals need to grow to provide food and habitat for the ecosystem.

Davis said that as corals take up calcium carbonate, or CaCO3, from the water column to produce their skeletons, calcification rates can be determined through water chemistry changes.

"Coral dissolution is the opposite, where the skeletons release CaCO3 back into the water, typically in periods of no productivity or when stressed," she said.

"The higher the rate of net growth (the difference between calcification and dissolution), the more reef-building corals and calcifying algae are produced for the ecosystem," she said.

Declining calcification indicates diminished health of coral populations, according to Davis.

"Coral ecosystems with low or negative calcification tend to relate to reefs with low coral cover, high algal cover, a loss of habitat and biodiversity of reef fish and other marine life, and a lower ability for the reef to become healthy again," she said.

"In the reefs that have been studied multiple times, approximately half of them were largely impacted by coral bleaching events or cyclone activity. In these cases, coral bleaching was caused by temperature stress.

"Stressful events like bleaching can reduce calcification for two reasons－bleaching can cause coral mortality and reduce ecosystem calcification because there is less coral to calcify, but corals can also reduce calcification in times of stress as a response to energetic limitations to increase their chance of survival. Other reefs included in this study have been impacted by smaller-scale stressors like dredging, nutrient pollution, or other human pollution."

Davis said that when coral growth hits net-zero, it means coral dissolution equals coral calcification.