Marine biologists at UC San Diego’s Scripps Institution of Oceanography in La Jolla are conducting a long-term study of coral reef “bleaching” off the Hawaiian coast in efforts to combat global warming by conserving and restoring precious reef ecosystems.

Using 3D imaging technology, scuba-diving scientists like Scripps’ Orion McCarthy (above), a recent graduate conducting research as a Ph.D. student, have been surveying Maui’s corals annually since 2014, contributing to a unique, long-term dataset that reveals coral survival rates following bleaching events in 2015 and 2019.

It’s invaluable information to know. “It’s complicated telling the story about the dramatic global decline with coral tied to climate change reminding us they are a natural system that goes through ups and downs and that it isn’t a foregone conclusion (that bleached reefs will die), but that there is a middle ground (where they can survive),” McCarthy pointed out. He added one aim of studying bleaching is to devise ways to “locally improve water quality and reduce the impact on coral reefs and their survival during climate change.”

McCarthy said coral surveying has revealed that, while some corals bleached and died during those events, surviving corals showed hopeful signs of resilience. Among coral surviving, researchers found little evidence that bleaching impacted coral growth over time. Therefore, the team’s findings can inform approaches to coral reef conservation and restoration.

Findings also detail exactly how, as oceans warm due to climate change, coral reefs are threatened by bleaching events. These prolonged warming events stress corals and have led to the deaths of many. Yet, some corals have managed to survive. Global researchers are now trying to understand what makes certain corals more resilient than others, which could prove helpful in restoring them.

McCarthy compared tracking coral fates through multiple bleaching events to studying human health outcomes following numerous pandemic-like events. By following the responses of individuals during a single event — for example, the COVID-19 pandemic — researchers can infer whether certain traits or qualities are aligned with better or worse health outcomes. If a second pandemic event occurred a short time later, researchers could then look at those data to better understand who survived the first event, and how those survivors fared during the second event. The same logic applies to corals and bleaching events.

Why is this research important to coastal communities like San Diego? For one, it demonstrates the interconnectedness between coral health and the overall health of the world’s ocean ecosystem, and how they are inextricably tied together.

“This is one of the first studies to use this type of time series to look at multiple coral bleaching events and how the processes of acclimatization and selection play out,” said McCarthy adding. “We found that older corals, which are more likely to have survived multiple bleaching events, could be a good source of outplants for coral restoration.”

Their findings were published July 31 in the journal PLOS ONE. Marine researchers aimed to distinguish between two different processes: natural selection, where only the hardiest corals in a population survive, and acclimatization, where an individual coral becomes more heat-tolerant over time as it is exposed to heat stress.

McCarthy noted there is a distinct connection between marine ecosystems in Hawaii and San Diego. Though San Diego does not have coral reefs, he pointed out there is a lot of cultural connection and interchange between surfers and others who “love to travel and go experience” their sport in both locations.

“There is also a very strong relationship between researchers at Scripps and in Hawaii,” added McCarthy talking about what’s to be done now with information gleaned from the coral bleaching study. “The big thing is the need for climate action, finding ways to reduce, or offset, our carbon footprint to become more eco-friendly,” he said.

The marine biologist concluded coral bleaching studies point the way to potential positive outcomes for helping to find ways for the ocean to adjust to global warming.

“Coral reefs are dynamic and bleaching isn’t necessarily going to kill every coral — at least not in the short term — so there is still cause for hope for these reefs and a need for active conservation,” concluded McCarthy. “Tools like 3D modeling are allowing us to get a more specific understanding of which corals are living and which ones aren’t, and we can use that information to help guide coral restoration efforts.”