An Ominous Heating Event Is Unfolding in the Oceans

Average sea surface temperatures have soared to record highs, and stayed there. It’s a worrying signal of an ocean in crisis.

To call what’s happening in the oceans right now an anomaly is a bit of an understatement. Since March, average sea surface temperatures have been climbing to record highs, as shown in the dark line in the graph below.

Illustration: Sean Birkel/University of Maine

Since this record-keeping began in the early 1980s—the other squiggly lines are previous years—the global average for the world’s ocean surfaces has oscillated seasonally between 19.7 and 21 degrees Celsius (67.5 and 69.8 Fahrenheit). Toward the end of March, the average shot above the 21-degree mark and stayed there for a month. (The most recent reading, for April 26, was just a hair under 21 degrees.) This temperature spike is not just unprecedented, but extreme.

“It’s surprising to me that we’re this far off the trajectory,” says Robert Rohde, lead scientist at Berkeley Earth, a nonprofit that gathers climate data. “Usually when you have a particular warming event, we’re beating the previous record by a little bit. Right now we’re sitting well above the past records for this time of year, for a considerable period of time.”

Rhode points out that temperatures this week were just under two-tenths of a degree warmer than the previous record. “Two-tenths doesn’t sound like a lot—but in ocean terms two-tenths is actually a lot because it doesn’t warm as quickly as the land,” he says.

As you can see from the chart’s record of past years, March is normally when average sea surface temperatures start declining. That’s because the Southern Hemisphere has transitioned from summer to autumn—and that hemisphere has more ocean covering it than the Northern Hemisphere, which has more bulky land masses. As southern oceans cool, they bring down the average global sea surface temperature.

But at the moment, temperature anomalies are widespread around the world’s oceans. (That near-real-time data comes from a network of satellites, buoys, and other ocean instruments.) “It’s above-average temperatures nearly everywhere,” says Rohde. “And there’s a significant heat wave in the North Pacific, which has been going on for many months.”

Warming in the Atlantic may be contributing to the extreme heat that’s hitting Spain right now, and it shows the broader problem caused by high ocean temperatures: What happens in the sea doesn’t stay in the sea. The oceans have absorbed something like 90 percent of the excess heat humans have put into the atmosphere, but the oceans are also capable of handing that heat back to the atmosphere, which in turn heats the land. “Both the atmosphere and oceans are becoming warmer and warmer,” says Boyin Huang, a physical scientist and oceanographer at the National Oceanic and Atmospheric Administration. “If the atmosphere pushes the ocean, then the ocean will push back into the atmosphere.”

Last year, researchers reported that climate change has made extreme heat events in the ocean the new normal. Thanks to historical data collected from ships all over the world, they determined the highest surface temperatures between the years 1870 to 1919—essentially setting a baseline for extremes. They found that in the 19th century, 2 percent of the ocean was hitting these extremes, but because of climate change it’s now 57 percent. In other words, extreme heat events in the ocean are now typical. (These differ from an overall increase in heat, in that temperatures come down from extreme peaks, but the general upward trend isn’t reversing itself.)

Illustration: MBARI

Scientists haven’t yet worked out what contribution climate change has made to the current surface temperature anomaly. But they can say that the longer-term trend since the early 1900s, averaged globally, shows a rise in the intensity of sea surface temperature anomalies, as you can see in the graph above.

That warmer water is already causing problems across the world’s oceans. Not only are higher ocean temperatures rapidly eating away at Antarctica’s massive ice shelves, but hotter water actually expands to take up more space, raising sea levels.

Illustration: MBARI

The dark red areas on the map above show that the Pacific waters off of South America are currently very warm. This is an unusual “coastal El Niño” that is not linked to the larger El Niño with global climate implications, says biological oceanographer Francisco Chavez of the Monterey Bay Aquarium Research Institute. A classical El Niño is a band of warm water that develops across the Pacific. That’s in contrast to the La Niña we’ve had the past few years, which is a band of cold water in the Pacific.

Models suggest there’s a 62 percent chance of a classical El Niño developing by June or July, with a four in 10 chance of a strong El Niño. But it’s not a sure thing because El Niño is a consequence of complex atmospheric dynamics—basically, wind blowing warm water over from Asia. “There’s still a lot of uncertainty,” says Chavez. “Forecasting the real El Niño is difficult because the atmosphere is chaotic.”

Whenever El Niño does arrive, it’ll have consequences. On the upside, there tends to be less hurricane activity over in the Atlantic when El Niño is active in the Pacific. But the outcomes for precipitation are mixed: For Peru, El Niño tends to create more rainfall, but to the east in the Amazon rainforest, it can lead to devastating drought. And all that extra heat in the Pacific could significantly raise global temperatures. “There’s a chance for 2023 to be the record warmest year,” says Rohde. “If an El Niño develops, as we now think is likely, 2024 will probably be warmer than 2023.”

In the ocean itself, warmer waters—due to El Niño or just overall long-term heating—can become less biologically productive. Some organisms that reach their thermal limit can migrate to colder waters, transforming both the ecosystems they leave and the new ones where they take shelter. But others, like corals, are stuck in place. These animals are particularly sensitive to heat, and bleach in response, releasing their symbiotic algae that provide them energy.

The ocean food chain also depends on the natural circulation of water, which is influenced in part by temperature. When cold water in the depths upwells to the surface, it brings up nutrients that fertilize phytoplankton. These microscopic plants grow in the sunlight, becoming a critical food source for tiny animals called zooplankton. But when water heats up at the surface, it stratifies, turning into a sort of cap that sits on top of colder waters below. “The bigger the cap, the harder it is to break. By heating the ocean, you’re going to basically decrease the amount of nutrients that come up,” says Chavez. “A longer-term concern is: How much is this overall heating going to change the natural fertilization processes, like upwelling? Will the ocean become more of a desert over time?”

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