The global ocean hit a new record temperature of 21.1 ?C in early April, 0.1 ?C higher than the last record in March 2016. Although striking, the figure (see 'How the ocean is warming') is in line with the ocean warming anticipated from climate change. What is remarkable is its occurrence ahead of — rather than during — the El Ni?o climate event that is expected to bring warmer, wetter weather to the eastern Pacific region later this year.

That means warmer-than-average ocean temperatures are likely to persist or even intensify, bringing with them more-extreme weather and marine heatwaves, which spell problems for marine life from corals to whales.

“We are probably looking at a string of record highs over the next year or so,” says Josh Willis, an oceanographer at NASA’s Jet Propulsion Laboratory in Pasadena, California. “This coming year is gonna be a wild ride if the El Ni?o really takes off.”

The El Ni?o Southern Oscillation (ENSO) is a natural, cyclical climate pattern. During the El Ni?o phase, winds over the Pacific are weakened or reversed, allowing warm waters to slosh eastwards in the Pacific. El Ni?o tends to coincide with warmer years both in the ocean and on land. The previous record of 21.0 ?C, for example, occurred during a very strong El Ni?o event.

ENSO is currently in a neutral phase, coming out of a rare extended three-year period of La Ni?a (the opposite phase to El Ni?o). But El Ni?o is expected to kick in this year: according to the World Meteorological Organization, there is a 60% chance of it developing between May and July, and an up to 80% chance of it happening by October.

Return of ‘the Blob’

Andrew Leising, an oceanographer at the Southwest Fisheries Science Center of the US National Oceanographic and Atmospheric Administration (NOAA) in La Jolla, California, expects to see unusually warm waters in the Pacific off the west coast of the United States during the summer and autumn. If the El Ni?o develops as expected, he adds, “this could create a situation like 2014 to 2015, when we got smacked by the Blob heatwave”, a particularly big and damaging marine heatwave.

Marine heatwaves can be devastating for wildlife, and fisheries. Large heatwaves on the US Pacific coast tend to compress the habitable zone for many species into a narrow strip along the coast, Leising says. That can bring whales closer to shore as they chase food, which can increase ship strikes and entanglements in fishing gear. When warm waters butt up against the shore, he adds, they can host harmful algal blooms that close crab and mussel fisheries. But at the moment, Leising says, there is some unusually strong upwelling of cold water occurring along the US west coast, which could protect against some warming this year.

In the lead-up to April’s record ocean temperature, some regions in the Southern Hemisphere experienced marine heatwaves, starting in February — among them, waters off Peru’s coast and in the Southern Ocean, says Boyin Huang, an oceanographer who works on sea surface temperature data for NOAA’s National Centers for Environmental Information in Asheville, North Carolina.

Unusually warm waters bring particular stress for corals. Almost all coral regions are currently experiencing remarkably high temperatures, says Matthew England, a physical oceanographer at the University of New South Wales in Sydney, Australia. “What we’re seeing now for coral reefs is they’re getting pushed to extreme temps, and they don’t get to regrow because it doesn’t come back to cooler temperatures.”

The last record-breaking ocean temperature year of 2016 coincided with an unusual global bleaching event for corals, only the third ever known to have happened. Bleached corals — which have expelled the algae that give them their colour — have poorer health, and many die.

“It’s fairly likely that we can expect another global bleaching event this year,” says Christian Voolstra, who studies corals at the University of Konstanz in Germany. Even if an El Ni?o doesn’t settle in this year, he adds, it will come soon enough. “It’s bad news no matter what.”

Heating planet

Warm waters are also physically less capable of holding dissolved oxygen, adding to the stress for marine life. “With ocean warming and deoxygenation, the available habitats for many species are decreasing,” says William Cheung, a marine biologist at the University of British Columbia in Vancouver.

And high ocean temperatures can trigger extreme weather. The unusually warm waters off Peru this year have helped to feed intense rainfall and Tropical Cyclone Yaku — the first such storm to hit the area in decades.

The ocean temperature spike — recorded by NOAA and likely the highest in more than 100,000 years — coincides with other warming trends. For example, in the southern hemisphere, the sea ice extent hit a new all-time low in February 2023. The ocean absorbs about 90% of the extra heat in the climate system resulting from global warming. But because it takes more energy to heat water than air, the surface water temperature is rising more slowly than the surface air temperature is.

“This wouldn’t have happened without climate change,” tweeted Jens Terhaar, an ocean biogeochemical modeller at the Woods Hole Oceanographic Institution in Massachusetts, in response to the news of the new temperature record. “We are in a new climate state, extremes are the new normal.