Heat stress more dangerous to corals than ocean acidification. Here‘s why

Global warming poses a more significant threat to coral growth and reef accretion than ocean acidification (OA), according to a new study. 

As temperatures rise, mass coral bleaching events and infectious disease outbreaks are becoming more frequent. The study published in the Communications Biology journal explained the susceptibility of corals to heat-stress. 

There has been enough evidence to show that ocean acidification has been affecting the corals. Carbon dioxide absorbed into the ocean from the atmosphere has already begun to reduce calcification rates in reef-building and reef-associated organisms by altering seawater chemistry through decreases in pH. 

But by applying a thermal anomaly of +2 degrees Celsius for 10 days, the researchers from Universidad Nacional Autónoma de México, Quintana Roo, Mexico observed severe disturbances on coral photosynthesis and calcification

In contrast, the experimental simulation of the expected OA conditions by 2100 caused moderate changes in coral performance.

"Heat-stress directly affects coral performance through in hospite exacerbation of light-stress in the symbionts, whereas ocean acidification induces moderate effects on coral metabolism, some of them even positive," said the authors of the study. 

The new study is in conformity with the August 2013 study The effects of thermal and high-CO2 stresses on the metabolism and surrounding microenvironment of the coral Galaxea fascicularis. The 2013 study found that the effects of temperature on the metabolism of corals were stronger than the effects of increased CO2.

Corals are animals. Coral reefs boost biodiversity, buffer storms and support livelihoods for over one billion people.

In the last decade, the world lost about 14 per cent of its coral reefs, according to a report by Global Coral Reef Monitoring Network. This report also stressed on the catastrophic consequences of global warming but said that some coral reefs can be saved by arresting greenhouse gases.

A December 21, 2022 study by Hanny E Rivera et al found that ocean warming is killing corals, but heat tolerant populations exist and if protected, they could replenish affected reefs naturally or through restoration. 

A case in point is Palau’s Rock Islands, which experience consistently higher temperatures and extreme heatwaves. Yet their diverse coral communities bleach less than those on Palau’s cooler outer reefs.

A better understanding of the underlying mechanisms that enable these corals to tolerate heat as well as the larval corals’ ability to disperse will significantly improve coral reef conservation and restoration efforts in the ocean, researchers at the Woods Hole Oceanographic Institution (WHOI) said.

Coral reefs do not absorb carbon and do not play a direct role in climate change mitigation. However, they are important for climate adaptation.