For 421 straight days, a marine heat wave in the North Atlantic broke — and sometimes shattered — daily temperature records.

The hot streak finally ended April 29, but scientists say the length of the marine heat wave wasn’t the only unsettling part. Another alarm bell was that daily temperature records often fell by a significant margin — on several occasions by more than 1 degree Fahrenheit.

“It’s not just that it was a consecutive string of 421 days,” said Brian McNoldy, an ocean scientist at the University of Miami. “But for so much of that time, it was breaking the records by a lot — not even close.”

And the North Atlantic is far from an outlier.

The world’s oceans — as a whole — are heating up. According to the European Union’s Copernicus Climate Change Service, monthly global sea surface temperatures have been at their warmest on record for 13 months in a row. Last year set a new annual record for global ocean heat.

The seething waters, month after month, have gobsmacked scientists all over the world. And they’re not just unexpected — they’re also dangerous.

Extreme ocean temperatures are jet fuel for tropical cyclones, driving more active hurricane seasons with bigger, stronger storms. They also can boost heat and humidity over nearby land areas, increasing risks to human health during the warmest parts of the year.

Marine heat waves pose major threats to marine life, too. The National Oceanic and Atmospheric Administration recently confirmed the world is experiencing its fourth global coral bleaching event on record, with mass bleaching events observed everywhere from Florida to Fiji.

The oceans have been gradually warming for decades as humans have poured greenhouse gases into the atmosphere. But experts adknowledge the past year has defied expectations. Last year’s temperatures were decades ahead of where most scientists expected them to be by that point, McNoldy said.

The year “2023 was just like, ‘See ya, guys!’” he said. “At this point a year ago, no one would have guessed what was about to come.”

Experts still aren’t sure why the oceans are suddenly warming so fast. But some scientists are worried it could be a symptom of a serious shift in the global climate.

The year 2023 went down in history as the hottest on record for the North Atlantic, the global oceans as a whole and for the entire planet. Local and regional temperatures broke records left and right around the globe, while extreme weather events wreaked havoc all over the world. Scientists warned that unprecedented extremes are likely accelerating as global temperatures rise.

The record-shattering year could be a warning that the world is moving into “uncharted territory,” said Gavin Schmidt, head of NASA’s Goddard Institute for Space Studies, in a somber warning published earlier this year in the scientific journal Nature. And the record-breaking trend already is bleeding into 2024.

“For the past nine months, mean land and sea surface temperatures have overshot previous records each month by up to 0.2 degrees C — a huge margin at the planetary scale,” he wrote.

These heat spikes have “greatly exceeded” the projections made by many statistical models, he added. Scientists expect global temperatures to continue rising as humans keep pumping greenhouse gases into the atmosphere. But this fast?

Experts have floated a few reasons why this might be happening, Schmidt noted. Continued greenhouse gas emissions account for some of the spike, but they can’t explain all of it. A 2022 eruption from the Hunga Tonga–Hunga Ha‘apai could have affected the atmosphere, and increased solar activity may have played a small role, too.

Scientists also have suggested that new global shipping regulations, requiring cleaner fuel, may have played a part. Shipping emissions can have a slight cooling effect on the atmosphere, meaning temperatures may rise as they’re stripped away.

But experts still aren’t sure that all these theories put together can explain the events of 2023.

And if these anomalies continue past August of this year, Schmidt added, it could suggest that global warming is causing new planetary responses that scientists still can’t fully predict or understand.

“It could imply that a warming planet is already fundamentally altering how the climate system operates, much sooner than scientists had anticipated,” he said.

An eye on Atlantic storms

McNoldy remembers the creeping feeling that something wasn’t right in the North Atlantic last year.

The region broke a daily record on March 5, 2023, but he didn’t immediately think much of it. “It happens now and then — you break a record and things calm back down and life goes on,” he said.

But then the records kept coming, one after the next, like toppling dominoes. Unusual marine heat waves started cropping up throughout the Atlantic — around the United Kingdom, off the coast of Florida, near West Africa, in Newfoundland.

By the beginning of June, McNoldy said he started to feel that “something odd is going on — this is not just a short-term warm blip, this is blowing away previous records by more and more each week.”

A year later, the sustained heat is raising red flags for the coming Atlantic hurricane season. Several top research groups recently dropped their annual projections, and all of them predict an unusually busy summer.

Colorado State University has forecasted an “extremely active” hurricane season, one of the busiest it’s ever predicted this early in the year. The forecast calls for 23 named storms, which include both hurricanes and tropical storms, with five of them transitioning into major hurricanes at Category 3 or higher.

That’s compared with the long-term average of 14 named storms per season and three hurricanes. If this year’s forecast comes true, that would make 2024 the third most active season on record. The year 2020 still holds the title with a staggering 30 named storms.

Accuweather meteorologists, meanwhile, have predicted 20 to 25 named storms, with four to seven major hurricanes. And forecasters at the University of Pennsylvania have released an even more aggressive season outlook, with a record-breaking 33 named storms.

“In general, pretty much everybody’s on board for a very busy season,” said Phil Klotzbach, who leads the Colorado State University hurricane season forecasts.

There are two major reasons, he said.

The first is that the planet is currently transitioning out of El Niño and into La Niña. This cyclical pair of natural climate conditions causes ocean temperatures to temporarily shift in parts of the Pacific Ocean, warming during El Niño and cooling during La Niña. They also affect weather patterns elsewhere around the globe.

La Niña tends to be associated with more favorable wind conditions for hurricane formation in the Atlantic Ocean, meaning it’s often a precursor to an above-average season.

Extreme ocean temperatures are the second reason for the busy forecast. Warm waters are hurricane fuel, often contributing to a greater number of total storms. They also help hurricanes intensify faster, meaning a greater number of them likely will swell into major storms.

Ocean temperatures are currently hovering around the levels they typically reach in late July — and they’re likely to continue rising as the summer goes on. They may have dipped below record level for now, but they’re likely to remain at least well above average for the rest of the season.

“I think the die is cast at this point,” Klotzbach said.

Last year’s extreme temperatures were tempered slightly by El Niño, which tends to dampen the wind conditions that help Atlantic hurricanes form. Yet 2023 still produced 20 named storms, making it the fourth-busiest season on record.

This year, the combination of high heat and La Niña conditions makes for a menacing duo, said McNoldy, the University of Miami scientist.

“We’ve never had La Niña combined with an ocean this warm,” he said. “There’s just no historical year that looks like this, which is a little intimidating.”

Like the global oceans as a whole, scientists are short on answers for what’s happening in the North Atlantic.

Last year saw weaker winds than usual blowing off the coast of Africa over the ocean, which likely helped the waters warm faster, McNoldy noted. And experts have floated the shipping emissions theory as well.

But there’s still no definitive answer as to why the ocean warmed so fast last year — or why it stayed warm straight through the winter and into the following spring.

Have we entered a new climate regime? “That is the mega-question that everyone would love an answer to,” McNoldy said.

In general, scientists prefer to have years or decades of data before they can make a definitive judgment about whether a recent trend is a long-term pattern. After one year, it’s probably too early to tell.

But the events of 2023 — and the early warning signs in 2024 — make a scientist wonder.

“I don’t know that we can say ‘no’ yet,” McNoldy said. “‘I hope not’ would be my answer.”