96% of oceans worldwide experienced extreme heatwaves in 2023, new study finds

In 2023, global marine heatwaves were the biggest, most intense and most persistent on record, a new study reveals. The researchers suggest that these heat waves were driven by climate change and may signal a climate tipping point.

Global marine heatwaves (MHWs) are prolonged periods of unexpectedly warm ocean temperatures. These warm periods can critically threaten marine ecosystems, for instance by leading to coral bleaching and mass marine die offs, and can cause economic challenges by disrupting fisheries and aquaculture. While it's widely accepted that human-driven climate change is making MHWs more destructive, little is known about the ocean dynamics behind the phenomenon.

"Marine heatwaves have emerged globally as one of the most severe threats to marine ecosystems," Ryan Walter, a marine scientist at the California Polytechnic State University who was not involved in the study, told Live Science.

A climate tipping point?

In a study published Thursday (July 24) in the journal Science, the researchers used satellite observations and ocean circulation data to evaluate the MHWs of 2023. They found that the year set new records for MHW temperatures, duration and geographic range — some of which have been measured since the 1950s — with these events lasting four times longer than the historical average and covering 96% of oceans worldwide.

The most intense warming, which occurred in the North Atlantic, tropical Pacific, South Pacific and North Pacific, accounted for 90% of unexpected oceanic heating during 2023. The North Atlantic MHW lasted for 525 days, and the Southwest Pacific MHW broke records for geographic extent and duration.

Related: A Hot Blob in the Pacific Ocean Caused 1 Million Seabirds to Die

The scientists identified several drivers behind the extreme MHWs, including rising solar radiation due to reduced cloud cover, weakened winds and changes in ocean currents.

They suggest that the 2023 MHWs may indicate a fundamental shift in ocean dynamics — which could be early warnings of a climate tipping point. Though there's not a singular definition of a tipping point, most researchers use it to mean the threshold at which certain effects of climate change are irreversible.

It's still uncertain whether or not oceans have reached a crucial tipping point just yet. "Tipping points are difficult to quantify," Walter said. Because the ocean and atmosphere contain many feedback loops, "if you change one thing, it changes another," so making exact predictions of where climate tipping points occur is tricky.

Other factors may also have influenced 2023's record-breaking ocean heat waves. A large El Niño event — a climate cycle in which waters off the eastern Pacific are warmer than usual — in the summer of that year meant "a lot of heat was released from the deeper waters of the ocean into the atmosphere, helping to fuel a lot of these heat waves that the authors write about," Michael McPhaden, a senior scientist at the National Oceanic and Atmospheric Administration who was not involved with the study, told Live Science. For example, in the Tropical Eastern Pacific, temperature anomalies peaked at 34.9 degrees Fahrenheit (1.6 degrees Celsius) during the onset of El Niño, the new paper found.

McPhaden agreed that 2023 was a remarkable year for MHWs and other climate extremes, but said, "I don't consider 2023 to be a tipping point." Though extreme temperature events are on the rise due to climate change, the natural variability that comes with El Niños also affects year-to-year oceanic measurements.

"There are going to be years when things go off the charts, and those are going to be the years when we have big El Niños," McPhaden said.

Marine ecosystems and human livelihoods

Regardless of whether or not 2023 represented a tipping point, extreme MHWs across the globe emphasized the vulnerability of marine ecosystems and human livelihoods that depend on them. MHWs "not only have impacts on foundational ecosystems like kelp forests, seagrasses and coral reefs, all of which provide many valuable ecosystem services and support other species, but they also impact many economies," Walter said.

These extreme events can also lead to the expansion of certain species' habitats — potentially further destabilizing battered ecosystems. Warmer waters off the coast of California, for example, drew equatorial venomous sea snakes to the state. "These sea snakes that typically live in the equatorial Pacific can follow warm waters as far north as Southern and even parts of central California," Walter said.

These extreme MHWs won't be the last. "What you're seeing is a consequence of climate change," McPhaden said. "We're just going to see more temperature extremes in the ocean and in the atmosphere."