Three reasons why the ocean’s record-breaking hot streak is devastating

Global sea surface temperatures have hit record highs every day for more than 12 months

A scuba diver swims over a coral reef. Many of the individuals corals are pale white, from bleaching.

Bleaching is afflicting many of the corals in Australia’s Great Barrier Reef, some of which can be seen in this photo taken on April 5, 2024.

David Gray/AFP/Getty Images

Earth’s largest ecosystem is broiling. Every day for the last 12 months, the average temperature of most of the sea’s surface has been the highest ever recorded on that calendar date, preliminary data from the National Oceanic and Atmospheric Administration show.

“And we’re currently outpacing last year,” says Robert West, a NOAA meteorologist in Miami. “We’re continuing to set records, even now over last year’s records.”

One of the primary reasons that global sea surface temperatures are so high is El Niño, a natural climate phenomenon that involves warm surface waters spreading across the tropical Pacific Ocean. El Niño is a recurring event, and this one emerged late last spring (SN: 7/13/23).

But natural climate cycles can’t explain what’s growing beneath the sea’s surface. The amount of heat stored within the sea’s top 2 kilometers has been increasing for decades, says NOAA oceanographer Hosmay Lopez, also in Miami. And the rate of that growth is accelerating.

Human-caused climate change has done this (SN: 3/10/22). Since 1971, the ocean has absorbed more than 90 percent of the excess heat trapped in the atmosphere by greenhouse gases, or more than 380 zettajoules of heat. For comparison, that’s about 1.5 million times as much energy as was released during the Hunga Tonga-Hunga Ha’apai volcanic eruption in 2022, or 25 billion times as much energy as was released by the atomic bomb dropped on Hiroshima, Japan, in 1945.

Charging the ocean with all that heat has countless consequences. Here is a look at just a few.

This year’s Atlantic hurricane season may be hyperactive

Hurricanes feed on water vapor and heat from the surface of the Atlantic Ocean. And right now, the Atlantic is very hot (SN: 6/15/23). Researchers are forecasting an extremely active hurricane season.

The 2024 seasonal outlook report from researchers at Colorado State University in Fort Collins, released on April 4, forecasts 23 named storms this upcoming season, of which five will be major hurricanes, meaning Category 3 or greater. When storms that powerful make landfall, they can be destructive and even deadly. And there’s a 62 percent chance of a major hurricane hitting the United States, the CSU team notes. A more recent outlook report from researchers at the University of Pennsylvania, released April 24, forecasts around 33 named storms this season.

Most hurricanes form in a stretch of the Atlantic Ocean between the Caribbean Sea and west Africa. This region is known as the main development region, or MDR, and sea surface temperatures there have been abnormally high. Right now, they’re more than 1.5 degrees Celsius above the long-term average for late April, which is about 25.5° C, data from NOAA’s Coral Reef Watch show.

Since 1981, there have been only 10 months for which the MDR’s surface has been that anomalously warm, West says. “Eight of those months, not yet including April 2024, have occurred in the last year.”

The likely emergence of La Niña, the counterpart to El Niño in which relatively cool surface waters return to much of the tropical Pacific, is also contributing to the forecasted hurricane activity. That’s because during La Niña, winds over the Atlantic that tear apart developing hurricanes become weaker (SN: 9/13/23). As of April 11, NOAA reports that there is an 80 percent chance that La Niña will emerge by August to October, around the peak of the hurricane season.  

“It only takes one hurricane making landfall to make it an active season,” the CSU report reminds coastal residents. “Thorough preparations should be made every season, regardless of predicted activity.” NOAA will be releasing its own early seasonal hurricane outlook in late May.

Corals are undergoing a mass bleaching

The sweltering seas are proving perilous for the world’s corals, living structures that support roughly 25 percent of all known marine species. When stressed by rising temperatures, corals expel the vibrant photosynthetic algae that live in their tissues and provide them with food, laying bare their white skeletons. This algal evacuation is known as bleaching, and it can be fatal for corals (SN: 8/9/23).

Since early 2023, coral bleaching has become so widespread that NOAA has confirmed it’s a global coral bleaching event, the fourth such event since mass bleaching was discovered in the 1980s. “From February 2023 to April 2024, significant coral bleaching has been documented in both the Northern and Southern hemispheres of each major ocean basin,” said NOAA coral reef ecologist Derek Manzello in College Park, Md., said in a statement released on April 15.

Ultimately, the coral death toll from this bleaching is something we won’t know until months or years after the event is over, says marine ecologist Carly Kenkel of the University of Southern California in Los Angeles. “But I can say that this is the worst bleaching that we’ve ever seen for the Caribbean, and it’s certainly looking like that for the Great Barrier Reef as well.”

Antarctic sea ice keeps reaching new lows

The Southern Ocean has absorbed almost as much heat from human-caused climate change as the Atlantic, Pacific and Indian Oceans combined. That’s partly because strong winds circulating over the Southern Ocean continuously draw cold, heat-sapping waters to its surface. And over the last year, Antarctic sea ice has fared terribly (SN: 7/5/23).

In a typical February, Antarctic sea ice dwindles to an annual minimum of roughly 3 million square kilometers, according to data from the National Snow and Ice Data Center in Boulder, Colo. This February, it retreated to just 2 million square kilometers, tying for the second lowest annual minimum extent on record. And just five months before that, in September 2023, the ice reached a paltry annual maximum extent of about 17 million square kilometers, a new record low.

Ocean warming and changes in atmospheric circulation were probably the main factors driving these lows, says climatologist Monica Ionita of Alfred Wegner Institute Helmholtz Center for Polar and Marine Research in Bremerhaven, Germany. “It was too warm above the ice, and too warm below.”

Up until around 2015, Antarctic sea ice was more or less stable over the long term, contrary to the predictions of climate models. Subsurface temperatures in the Southern Ocean began climbing around that time, and since then there have been three Antarctic summers during which sea ice hit record lows. As a result, some researchers have suggested that the heating of the Southern Ocean has helped push Antarctic sea ice into a new, diminished state.

Statistically, a shift appears to have occurred or at least started, Ionita says, though having only four decades of satellite data make it hard to say for sure.

On the other side of the planet, Arctic sea ice extent has steadily declined by about 12 percent each decade (SN: 11/15/21). But the Arctic’s sea ice has not reached record lows in recent years. That could be because it has already settled into a new low state, where, at least for now, it can no longer break record after record, Ionita speculates. If a similar transition is under way in Antarctica, sea ice decline might eventually stabilize there, even if only momentarily, she adds. “We’ll have to see.”

As for when sea surface temperatures will stop breaking records, scientists remain unsure. The departure of El Niño and the emergence of La Niña may help bring sea surface temperatures down, Lopez says, as cool surface waters sweep across the tropical Pacific.

However, record-breaking sea surface temperatures occurred during the most recent La Niña, which stretched from 2020 to 2023. What that shows, West says, is that “even if you have a relatively cooler area in the equatorial Pacific, it doesn’t necessarily mean that you stop breaking records everywhere.”

Nikk Ogasa is a staff writer who focuses on the physical sciences for Science News. He has a master's degree in geology from McGill University, and a master's degree in science communication from the University of California, Santa Cruz.

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