2023’s marine heatwaves unprecedented, signals early stages of irreversible changes to coral reefs, ecosystems: Study

Marine heatwaves—prolonged periods of raised ocean temperatures than what are typical for that time of year—experienced in 2023 were not only unprecedented, but may have also signalled a potential climate tipping point, with irreversible, negative impacts to coral reefs and ecosystems, according to a new study.

Researchers from China and the US found that regions in oceans across the globe, including the North Atlantic, and the tropical, south and north Pacific experienced extreme marine heatwaves—the longest in duration, widest in extent and highest in intensity on record.

The findings, published in the journal Science, show that the most intense warming occurred in the North Atlantic, tropical eastern Pacific, north Pacific, and southwest Pacific, which “collectively accounted for 90 per cent of the global ocean warming”.

Heatwaves in the North Atlantic which began mid-2022 persisted for 525 days with an intensity four times the typical, making it “the longest recorded marine heatwave in the region”, the authors said.

The southwest Pacific heat event broke records for its expanse and prolonged duration, while unusual temperatures in the tropical eastern Pacific peaked at 1.63 degrees Celsius during the onset of El Nino, they said.

‘El Nino’ is the warm phase of the ‘El Nino-Southern Oscillation’ (ENSO) natural climate pattern, which involves changes in temperatures and atmospheric pressures in the Pacific Ocean. El Nino is linked with warmer ocean temperatures.

The authors, including those from the US’ Scripps Institution of Oceanography at the University of California, said the marine heatwaves of 2023 were “a global event with a 50-year return period, (with a less than two per cent chance of occurrence)”.

First author Tianyun Dong, a joint postdoctoral researcher at the Eastern Institute of Technology, Ningbo, and Southern University of Science and Technology in China, told PTI in an email, “Global warming, observed over a long-term and primarily driven by greenhouse gas emissions, has raised the ocean’s baseline state, making marine heatwaves increasingly frequent and intense.”    Further, the trends observed also suggest a “possible indication of an approaching climatic tipping point”, the study said.

A climate tipping point, potentially a ‘point of no return’, is related with irreversible, disproportionate health and economic consequences for the world’s most vulnerable, such as tropical coral reefs, and ice sheets of Greenland and Antarctica.

“While a full (ocean-climate) system collapse has not occurred, irreversible impacts—mass coral bleaching in tropical reefs, collapse of key habitats—are already emerging,” Dong said.

Coral bleaching in reefs, or the loss of algae in reefs causing a ‘whitening’ and vulnerability to disease and death, are showing few signs of recovering, while the functional collapse of key habitats in some of the world’s regions undermine biodiversity and the role of ecosystems, Dong explained.

“These impacts reflect more than a short-term stress, suggesting a shift towards a permanent ecological change. The 2023 marine heatwaves may, therefore, mark a dangerous step closer to tipping points in the ocean-climate system,” the first author said.

Arpita Mondal, associate professor at the department of civil engineering and centre for climate studies, IIT Bombay, and not involved with the study, told PTI, “Based on a primary reading of the paper, I’m quite convinced of the scientific robustness.”     Mondal explained that the ocean and atmosphere “talk to each other through processes—called the ‘teleconnections’—through which far-away processes can influence local weather”.

“For example, processes in the southern and equatorial Pacific Ocean can influence the Indian monsoon,” she said.

“Of course, El Nino is the most significant phenomenon which affects not only monsoons in South Asia, but ENSO-neutral conditions—where neither El Nino nor La Nina dominate—have been linked with the heat waves in the pre-monsoon season too,” Mondal explained.

“Similarly, oscillations and circulation of currents in oceans and atmosphere can impact weather worldwide,” she added.

The study team analysed satellite observations and temperature data of the world’s oceans. Data from the NASA-funded ‘ECCO2’ project, focussed on producing a high-resolution estimate of the state of the world’s oceans and sea ice, was also analysed.

First author Dong said that despite a relatively weak El Nino—studies show its growth rate unexpectedly decelerated in mid-2023 -- the unprecedented nature of 2023’s marine heatwaves cannot be explained by natural climate variability alone.

“A continued warming of the planet—especially when combined with El Nino—could produce even more extreme marine heatwaves, pointing toward a potential new normal of amplified ocean heat extremes and a higher chance of crossing physical and ecological tipping points,” the first author said.

Mondal, who is not involved in the study, pointed to the concerning lack of reliable, long-term ocean data.

“My concern is more about the lack of long-term, reliable ocean data globally. We have only been monitoring oceans in the recent years. We do not have say, 150 to 200 years of ocean surface temperatures recorded. However, satellites developed in the last 50 years or so have definitely helped in complementing the buoys which have been in place for monitoring the ocean,” she said.