The incidence of coral disease has tripled in the past 25 years, and it is projected that almost 75% of coral will be affected by disease by the end of the next century.

A deadly coral disease is spreading as global temperatures rise, and will likely be endemic to coral reefs around the world by the next century, according to new research.

The study published in Environmental Science Lettershow much coral reef health will suffer from climate change, which threatens to wipe out entire coral reef habitats and destroy coastal communities.

For the meta-analysis, researchers from the University of New South Wales in Sydney analyzed 108 coral health studies in which corals were scanned for symptoms of disease. They then linked disease surveys to sea surface temperature records to understand how climate change – specifically ocean warming – affects the spread of coral diseases worldwide and performed disease prediction modeling under future warming scenarios.

They found that coral diseases increase with ocean temperatures over time, tripling in the past 25 years to 9.92% globally. Their models also predict that the disease’s prevalence could increase to 76.8% in 2100 if temperatures continue to rise on the same trajectory – the most conservative scenario for the worst.

Samantha Burke, lead author of the study and Ph.D. Candidate in the School of Biological, Terrestrial and Environmental Sciences, he says the findings highlight the devastating effects of rising temperatures on coral reefs and the urgent need for swift action to mitigate climate change.

“Coral disease is a serious cause of coral mortality globally and coral decline, and our modeling predicts that it will continue to worsen – even if ocean temperatures remain conservative,” Burke says.

The study also indicates that coral reef diseases are more likely to worsen in the Pacific Ocean than in the Atlantic or Indian Oceans based on current data.

“Certain oceans are more at risk, but it’s hard for us to tell if that’s just from rising ocean temperatures or combined with the many other stressors facing coral reefs,” Burke says. “But what is clear is that the prevalence of coral diseases is on the rise around the world, and without urgent action to address rising temperatures, more corals will develop disease.”

An ecosystem on the brink

Coral reefs play an important role in the marine ecosystem, supporting about a quarter of the world’s fish. They are also vital to coastal communities that depend on the reef for fisheries and tourism, as well as the protection it provides from storm surges and coastal erosion.

“They are habitat builders. Without coral, there is no coral ecology and no coastal industry,” Burke says.

Coral disease occurs when a coral’s immune system is weakened, usually after it has been infected with a disease — such as a bacteria or fungus — that causes the disease in the animal. It differs from coral bleaching, in which the coral turns white under stress by expelling the zooxanthellae algae that live within their tissues which are responsible for the colouration.

“There are certain diseases that act more quickly than others, but most corals end up getting sick,” Burke says. “Because it takes so long for corals to form, corals may not recover, and entire sections of reefs can be lost.”

Corals are delicate organisms and require a precise set of environmental conditions to survive, including water temperature, salinity, and quality. Living outside this natural range can make corals “stressed” – unable to grow, reproduce and ultimately survive.

Although infectious pathogens such as bacteria and fungi ultimately cause coral disease, stressed corals are more susceptible to infection. Higher water temperatures may also increase the virulence or growth rate of disease-causing organisms.

“As the ocean warms, it increases stress on the coral reefs which lowers their immune response,” Burke says. “Higher temperatures can also create conditions more favorable for disease-causing pathogens.”

Many diseases affecting corals are known by their appearance, such as black band disease or yellow band disease. But scientists have not yet identified many disease-causing pathogens.

“It is still relatively unknown whether the microbes associated with diseased corals are the cause or symptom of the disease, only that the coral is sick, and the tissues are dying,” Burke says. “It is not clear whether the fungi or bacteria present cause disease or simply feed on dying tissue, so researchers need to study further.”

Ms. Burke says more research into coral diseases will also help scientists develop effective disease interventions and show the complexity of the threats coral reef ecosystems now face.

“The solution to coral disease is likely to be complex and needs action on a large and small scale. We can’t wait and hope for a silver bullet like a universal antibiotic,” says Ms. Burke.

“Given what is at stake, we need to take many steps forward to develop effective mitigation strategies, and tackling warming would be a great place to start.”