The Weird Reasons Behind the Atlantic Ocean’s Crazy Heat

Lately, the North Atlantic Ocean has been more than just hot. It has been anomalously, weirdly hot. On Sunday, the ocean’s average surface temperature was 74 degrees Fahrenheit, or 23 degrees Celsius — a number normally seen a month from now, in late July. The Atlantic was warmer last month than in any previous May since 1850, according to the Met Office, the United Kingdom’s national weather service. Even more impressively, it beat the previous record by more than any previous record, for any month, has been broken. June seems virtually guaranteed to set another all-time high.

This outrageous warmth is primarily caused by climate change. And in climate science, it is generally not good news when a year’s temperature line is so immediately visible above the pack:

The North Atlantic’s sea surface temperature by date.Courtesy of ClimateReanalyzer.org, Climate Change Institute from the University of Maine

The heat wave is particularly intense in the North Atlantic’s eastern half, which runs from Mauritania to Portugal, France, and the British Isles. According to the National Oceanic and Atmospheric Administration, the marine heat wave around the United Kingdom qualifies as a Level 5, or “beyond extreme,” event.

Such warm water would normally give rise to enormous hurricanes. And the western Atlantic has been off to a productive start, creating Tropical Storms Brett and Cindy earlier this month. But since the western Atlantic, which borders North America and the Caribbean, has been chillier, those storms have been unable to survive the journey across the ocean and have been torn apart by wind shear.

Under other circumstances, a marine heat wave of this magnitude would be dangerous for underwater animals and plants — but perhaps a curiosity for land-dwelling humans. Of course, any anomaly of this magnitude — more than two standard deviations above the trend — is extremely concerning and might raise fears that the planet has entered some kind of new normal. The Atlantic’s outrageous warmth has also attracted wider attention because it raises one of the most controversial questions in climate science: Did we accidentally stop geoengineering the oceans?

Three years ago, the United Nations agency that regulates shipping mandated that cargo ships switch from the high-sulfur form of fuel that they were previously using to a cleaner, lower-sulfur type of fuel. When burned, sulfur creates a pollutant called sulfur dioxide, which causes haze, acid rain, and health problems. The mandate worked: Ships have moved away from high-sulfur fuels, which has significantly cut aerosol emissions.

Which seems like an environmental-policy success story. Except that Leon Simons, a researcher at the Dutch chapter of the Club of Rome, argues that it was a grave mistake. Aerosol pollution reflects the sun’s rays back into space: It’s not wrong to see it as a form of solar-radiation management, or geoengineering. Aerosol emissions cool the planet by about 0.5 degrees Celsius, or about 1 degree Fahrenheit, according to the Intergovernmental Panel on Climate Change. (Aerosol pollution doesn’t just refer to sulfur dioxide, but to any small particle of a solid or liquid that is larger than a molecule but small enough to float in the air.)

When ships began burning low-sulfur fuel, they reduced some of this net cooling effect — even as they kept pouring carbon dioxide and other climate pollution into the atmosphere. Simons asserts that this inadvertent end to geoengineering is partially to blame for the ongoing heat wave afflicting the world’s oceans.

Other researchers are far less certain. Brian McNoldy, a senior research scientist at the University of Miami, told me that the low-sulfur timeline doesn’t add up. Cargo ships had to stop using high-sulfur fuels by January 1, 2020, and sulfur dioxide and aerosols only persist in the atmosphere for a few days or weeks. Those cooling aerosols rained out two and a half years ago. So why did the Atlantic Ocean start cooking in February of this year?

“I don’t totally buy the low-sulfur fuels. It doesn’t explain the past two or three months becoming abruptly record-breaking,” he said. “It might be a driver, but it’s not the reason.”

He explains the North Atlantic heat wave by looking to two other far more weather-related factors. First, he said, the Sahara Desert is generating less dust than it normally does. Every spring and summer, winds moving across northern Africa toss up enormous amounts of sand and dust from the Sahara — so much that it creates a recognizably beige haze over the North Atlantic. Like any other aerosol, that Saharan dust reflects sunlight and cools the Earth’s surface.

In a normal year, so much of that dust would have been kicked up by now that it would have blown all the way to South Florida, according to Michael Lowry, a meteorologist at ABC 10, a Miami news station. But this year, winds haven’t picked up as much dust, and the first major Saharan dust haze only appeared in the past week or so. The satellite DSCOVR picked up the first images of that dust storm on Saturday:

An image from the DSCOVR spacecraft’s Earth Polychromatic Camera, or EPIC, captured on Saturday, June 24, 2023.NASA / Heatmap Illustration

With less dust to reflect the sun’s rays, more have reached the ocean — and warmed its surface.

Second, the weather over the North Atlantic has been unusually stagnant. The wind plays a big role in warming up or cooling down the ocean surface: When winds push the oceans around a lot, surface water tends to mix with deeper water and the air, producing a cooling effect; when winds slacken, the sea sits stagnant and heats up.

The winds have been still lately. There’s a “large-scale blocking pattern” in the jet stream that is preventing storms from moving across the North Atlantic, and generally discouraging winds from pushing around the sea surface, McNoldy said.

The cause of all this stagnation is an atypically weak “Azores High,” a quasi-permanent high pressure system that sits over the North Atlantic throughout the year. It hasn’t drawn in Saharan dust or generated winds to push ocean water around, turning the western Atlantic into the planetary equivalent of a kiddie pool on a hot day. “It’s allowing the ocean to really cook,” McNoldy said.

The warmth is now so pronounced that even a change in weather won’t drive it out for some time. Even if the circumstances causing the warming were to fade now, McNoldy told me, the ocean is “not gonna get back to normal any time soon.”

That could eventually cause problems for folks in the Americas. Right now, the western Atlantic is generating storms like it’s the late summer, while the cooler eastern Atlantic is tearing them apart. Were the eastern Atlantic to get just a little warmer, it might let those storms survive or even strengthen them — leading to an unusually strong hurricane season.