A new study finds that scientists have likely underestimated heat stress on coral reefs in the South Atlantic Ocean, further raising concerns for coral bleaching amid climate change.
The study notes that while the Caribbean and Indo-Pacific have well-established long-term ocean temperature and coral monitoring programs, the South Atlantic Ocean has lagged behind, causing gaps in understanding. Many Brazilian reefs, for example, lie in deep, murky waters. Previous studies suggest these cool low-light conditions could make the reefs a refuge for coral as oceans warm.
However, “It’s tricky to say that it is a refuge because we don’t know our history. So, we cannot say that with certainty,” Giovanna Destri, a Ph.D. student with the University of São Paulo in Brazil and study lead author, told Mongabay in a video call.
To better understand that history, Destri and her team looked at 40 years of data from the U.S. National Oceanic and Atmospheric Administration (NOAA) for 33 reef sites across the southern Atlantic Ocean, mostly off the coasts of Africa and Brazil.
They assessed the intensity, duration and frequency of each heating event over the last four decades to create a complete picture of warming trends in the region.
The study found a sharp increase in heat stress episodes over time. From 1985-89, there were 10 significant episodes, two per year on average. From 1990-99, there were 31 episodes, a figure that held steady from 2000-09. However, from 2010-19 there were 81 events. In just the last five years, 2020-24, there have been 75 episodes, roughly 15 per year.
“We had a lot of events that we didn’t even notice,” Destri said.
Many of those heat stress events coincide with global bleaching episodes that happen when ocean waters become too warm and reefs expel their symbiotic algae, becoming brittle and susceptible to disease. Given time, many reefs can recover. But stressful heat events are increasingly frequent, and many corals don’t have enough time to recover before another marine heat wave strikes. Some reefs in the southern part of the study area, near Queimada Grande Island, for instance, faced more than 20 heat stress events in the last 40 years, every two years on average, the study notes.
The researchers found a few bright spots: One site near Rio De Janeiro showed no recorded heat stress at all, perhaps due to upwelling of cool water. Furthermore, at Africa’s São Tomé Island and Libreville, the longest interval between stress events was nearly 29 years.
“The frequency of thermal stress episodes increased over time … across all regions except for Africa,” the researchers note.
Destri said she hopes this study can be used to prioritize protecting the most vulnerable reefs from pollution and physical damage to help make them more resilient to the much harder to control effects of climate change.
Banner photo: a coral reef in Brazil’s Abrolhos Marine National Park by Roberto Costa Pinto via Wikimedia Commons (CC BY-SA 4.0).
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