Record ocean heat may be sign of irreversible shift – scientists
At this time last year, scientists watched in disbelief as the world’s oceans surged to record levels of warmth, and wondered what could have triggered it. The jump in sea surface temperatures was more dramatic than anything seen before.
The scientists explored a link to El Niño, the climate pattern known for warming up the Pacific Ocean, and potential warming influences from diminished shipping pollution and a major volcanic eruption. But nothing explained the influx of warmth as it held up for months on end and spread heatwaves across nearly all the oceans’ surfaces.
Now, the unprecedented streak of ocean heat is entering a second year. Scientists say it could represent a major change to Earth systems that cannot be reversed on any human time scale.
That’s because what they have seen in the oceans so far “doesn’t add up”, said Gavin Schmidt, director of the Nasa Goddard Institute for Space Studies.
“It could imply that a warming planet is already fundamentally altering how the climate system operates, much sooner than scientists had anticipated,” he wrote in a column in the journal Nature.
The warming has extended far from an El Niño-influenced swath of the Pacific.
Across much of the Atlantic basin, for example, surface temperatures have been running 1 to 2C above a 1971-2000 baseline. The anomaly is 3C or more in some waters off South Africa, Japan and the Netherlands, according to United States National Oceanic and Atmospheric Administration (NOAA) satellite data.
The ocean heatwaves coincide with the warmest conditions ever observed in the atmosphere, too.
Last year, average global air temperatures rose higher than humans have ever known, perhaps bringing the planet to its hottest in more than 100,000 years. Climate scientists predict that 2024 could be even warmer.
But to see such dramatic warming throughout Earth’s oceans was even more alarming, given that it took far more energy to warm water than it did to warm air, said Celeste Saulo, secretary general of the World Meteorological Organisation (WMO).
“The time scale of the oceans is not as fast as the atmosphere,” Saulo said. “Once a change is established, I would say it’s almost irreversible in time scales that go from centennial to millennial.”
In its annual State of the Climate report, issued on Wednesday, the WMO said many climate indicators last year “gave ominous new significance to the phrase ‘off the charts’ ”, including unprecedented glacier melt, Antarctic sea ice loss and sea level rise, as marine heatwaves spread across more than 90% of the oceans’ surfaces at some point during 2023.
The most exceptional warmth hit the eastern North Atlantic, the Gulf of Mexico and the Caribbean, the North Pacific and large areas of the Southern Ocean, the WMO said.
Since last April, global average sea surface temperatures had hit records every month, with records in July, August and September “by a particularly wide margin”, the organisation said.
The warming of the world’s oceans is already having devastating consequences for coral reefs. Fatal levels of heat hit a largely unspoiled section of Australia’s Great Barrier Reef this month, a repeat of the bleaching and mortality of corals around Florida last year.
Other impacts will take more time to detect.
There are worries that the warming and melting is pushing a key Atlantic Ocean current system to collapse, though the tipping point at which that might occur is unknown. It would have massive impacts on underwater ecosystems and weather patterns.
And there are likely to be cascading impacts on marine life.
In the US Gulf of Maine, where waters have been warming much faster than the world’s oceans at large, researchers have already seen important species such as cod and herring struggling to find cool waters within their normal geographic range.
Many fish were growing more rapidly at young ages but then plateauing at smaller sizes, a sign that they weren’t getting enough food or that the heat was stressing their bodies, said Katherine Mills, a senior scientist at the Gulf of Maine Research Institute.
The temperatures observed over the past year were so extreme relative to past conditions, it was becoming more difficult to reliably predict what the consequences could be, Mills said. Existing data on ecosystem changes was becoming too outdated too fast.
“We generally expect that, in the ocean, there will be variability in temperatures. What this has done is send that variability well into a range that we haven’t encountered before.
“I think it’s a real wake-up call,” she added.
Scientists don’t know if or when the extreme ocean warming will subside. So far, none of their theories for what is driving it have answered all questions.
Some warming is likely tied to a decrease in air pollution from shipping, which allows more sunlight to reach the oceans’ surfaces. And the eruption of the underwater volcano Hunga Tonga-Hunga Ha’apai near Tonga in 2022 sent vast amounts of water vapour, a planet-warming greenhouse gas, into the atmosphere. But neither factor explains the drastic surge in ocean heat.
Ocean temperatures surged early last year at the tail end of what had been three straight years of a La Niña global climate pattern, which is the opposite of El Niño and known for suppressing global heating.
The switch from La Niña to what became a historically strong El Niño, known for boosting planetary temperatures, could explain a lot of the jump in ocean warmth, said Boyin Huang, a NOAA oceanographer focused on ocean temperature analysis.
So it is possible that ocean temperatures could moderate later this year, with La Niña conditions forecast to return. But it remains to be seen whether a switch from El Niño back to La Niña would be enough to significantly counteract the warming or the power of greenhouse gases. This could become clearer if ocean temperatures continued setting records, Huang said.
If record warmth persisted even under La Niña conditions, Schmidt wrote, “the world will be in uncharted territory”, with far more uncertainty over its future climate than scientists had previously known.