‘Unprecedented' ocean heat waves in 2023 suggest climate tipping point
That could mark a turning point in the way the oceans behave, potentially signaling a tipping point after which average sea temperatures will be reset higher and some ecosystems may not recover, say the authors of the study, which was published Thursday in the journal Science.
"The ocean going to a new normal — that controls everything,' said Zhenzhong Zeng, an Earth systems scientist at China's Southern University of Science and Technology, who led the work. "Once we destroy it, then maybe it cannot go back.'
Ninety percent of the 2023 oceanic heating anomalies occurred in the North Atlantic, tropical Eastern Pacific, North Pacific and Southwest Pacific. The average length of the heat waves was 120 days — four times the average length between 1980 and 2023, according to Tianyun Dong, lead author on the paper and an Earth systems scientist at China's Eastern Institute of Technology. The North Atlantic marine heat wave, which began in 2022, stretched for 525 days.
"The ocean has a long memory,' Dong said. It plays a key role regulating global temperatures by storing and gradually releasing large amounts of heat, but because it is slower to react than the atmosphere, the impact of long-term heat changes can be both delayed and enormous.
The researchers used a combination of real-world measurements from the ocean, satellite data and computer modeling to determine the scope and causes of the heating. The highest temperature spike was seen in the Tropical Eastern Pacific, which was 1.63 degrees Celsius hotter than normal. While the onset of the El Nino weather phase likely contributed to that, a key finding of the paper is the extent to which different drivers played a role in marine heat waves in different locations. Other factors include increased solar radiation from reduced cloud cover; weaker winds; and changes in ocean currents — factors which can themselves be caused by global warming. Taken together, they show climate change is having an intensifying impact on the oceans.
What the paper doesn't explain is why so many drivers came together in 2023, resulting in so many smashed records, said Zeng. It may be that these changes are beginning to reinforce each other in ways that are not yet understood. Identifying such feedback loops, and figuring out their mechanics, is critical to understanding future heat events, he said.
In the near term, a marine heat wave can be devastating for humans who depend on the ocean for their livelihood, like fishing communities. "It can also fuel stronger hurricanes and storms along the coast,' said Yuntian Chen, a researcher in mechanics at China's Eastern Institute of Technology and one of the study's authors.
And if ocean heat reaches a tipping point, some species won't recover. There are already concerns this could be the case for some of the world's largest coral reefs. That has other ramifications: The loss of coral and kelp, for example, reduces the ocean's ability to sequester carbon, which leads to more heating.
The hottest year on record was 2023 — until 2024, when the Earth's average temperature reached 1.55 C above the pre-industrial average, according to the World Meteorological Organization, and when the average ocean temperatures hit a new high. This year is unlikely to break the annual record again, but is expected to come close. June 2025 was the third-warmest June in 176 years, after 2024 and 2023, according to a recent report by the U.S. National Oceanic and Atmospheric Administration.
The WMO tracks global temperatures based on six international datasets, including one run by NOAA, whose NCEI Coastal Water Temperature Guide was decommissioned by the U.S. government in May.
Zeng's team relied on NOAA data for its work, some of which is no longer available.
The cutbacks are "a disaster,' he said. "We are all living on the same planet. The climate is really changing.'
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