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The world’s oceans are extremely hot. We’re about to find out what happens next.

Unprecedented Atlantic Ocean heating and El Niño in the Pacific are pushing the climate into uncharted territory.

In an aerial view, boats pass through sargassum floating on the surface of the ocean on May 18, 2023 in Marathon, Florida.
Huge blooms of sargassum seaweed have emerged in the Atlantic Ocean amid record-high water temperatures.
Joe Raedle/Getty Images
Umair Irfan is a correspondent at Vox writing about climate change, Covid-19, and energy policy. Irfan is also a regular contributor to the radio program Science Friday. Prior to Vox, he was a reporter for ClimateWire at E&E News.

On Wednesday June 14, the surface of the North Atlantic Ocean reached an average temperature of 73 degrees Fahrenheit.

That may sound like a pleasant day at the pool, but it’s actually a record high, and it will have global consequences. The average for this time of year, over the past three decades, is 71 degrees Fahrenheit. That two-degree difference reflects a gargantuan amount of extra energy stored in the ocean. The Atlantic has been riding a wave of extreme heat since last year. And as summer sets in, the temperature will climb.

“This is an incredibly unusual year,” said Gabriel Vecchi, a climate scientist at Princeton University. “A warm Atlantic tends to have a lot of global influences.”

Atlantic Ocean surface temperatures affect rainfall and storms in Brazil, India, the Sahel region of Africa, and the southwestern United States. Hot water is also the fuel for hurricanes, which need the sea surface to be at least 79 degrees Fahrenheit to form. Higher temperatures boost the octane rating of this fuel, leading to more powerful storms. They can also diminish stocks of fish, which feed 3 billion people.

And it’s not just the Atlantic; oceans all around the world are seeing stunningly high average temperatures right now. On the other side of the globe, the Pacific Ocean surface is also heating up as it enters the El Niño phase of its cycle. Together, these phenomena are poised to push the planet’s temperature to new highs.

“It wouldn’t be surprising if we saw this as one of the warmest years ever,” Vecchi said.

Two-thirds of the planet is covered with water, and all of it is covered by the atmosphere. Sea surface temperature is a critical metric because it explains what’s going on where the water meets the sky. It’s the engine of the global climate, driving trade winds, ocean currents, precipitation, and temperature.

Humans have been mucking with that engine, spewing greenhouse gasses, particles, and aerosols into the atmosphere, while dumping all forms of pollution into the oceans. Though the oceans go through natural cycles of heating and cooling, they’re now occurring on top of a profoundly altered world, creating conditions that humanity has never witnessed before.

What’s making the Atlantic Ocean so warm right now?

One of the most important factors behind the underwater heat wave is that humans have raised Earth’s average temperature by 2.3 degrees Fahrenheit since the dawn of the Industrial Revolution by burning fossil fuels and emitting heat-trapping gasses. The vast majority of the warming — 90 percent — is occurring in the ocean.

Since 1955, the oceans have soaked up 345 zettajoules of energy, according to NASA. One zettajoule is 87 times the amount of energy produced by all of the world’s nuclear power plants in a year.

Graph of ocean heat content over time.
Earth’s oceans have absorbed an enormous amount of thermal energy since the 1950s.
NASA

But this warming trend has been going on for more than a century. Why are temperatures suddenly rising in the North Atlantic Ocean now?

There are several converging factors at work here. One is the Azores high, a semi-permanent region of high atmospheric pressure that sits over the North Atlantic. This creates a sinking column of air that generates trade winds blowing east to west over the water. When wind moves over water, it leads to more evaporation which cools the surface of the water, similar to how a breeze can cool your skin when you sweat.

“It’s been weak for the past month or so and what that will tend to do is it’ll weaken the trade winds in the North Atlantic,” explained Dillon Amaya, a research scientist at the National Oceanic and Atmospheric Administration (NOAA). “For as long as it’s weak, that basically reduces the amount of wind that’s blowing on the surface of the ocean and that reduces evaporation and that causes ocean temperatures to skyrocket.”

Another element is the Atlantic Ocean goes through its own temperature cycles on different timescales. The Atlantic Meridional Mode is a shift in the temperature gradient in the water that can swing back and forth between seasons, years, or decades. It can alter wind, heat, and rainfall patterns, and it’s in its warm phase this year. Over longer time scales, the Atlantic Multi-decadal Oscillation plays a role. This is another periodic rise in sea surface temperatures across the Atlantic and it’s also in the warm phase of its cycle right now.

In addition, there’s evidence that a reduction in air pollution over the past half-century had an unexpected side-effect: Since the end of World War II, sulfur and nitrogen aerosols from the tailpipes of cars, smokestacks of ships, and chimneys of factories have been drifting over the North Atlantic from the United States and Europe. That helped keep the Atlantic cool.

“Aerosols shut down sunlight. That sunlight is deflected before it reaches the surface of the ocean,” said Hiroyuki Murakami, a scientist at NOAA studying variations in the Atlantic Ocean. “If we increase aerosols, we expect cooler sea surface conditions.”

But as cities filled with toxic, dirty air, citizens on both sides of the pond clamored for tougher limits on air pollution. This led to regulations that reduced aerosols over the Atlantic, which in turn led to more warming at the surface of the ocean. A new regulation in 2020 that drastically limited sulfur from shipping may also have played a role.

The combination of all of these effects — climate change, the weak Azores high, the Atlantic Ocean temperature cycles, and reduced air pollution — is playing out now. “What we have is a superposition of at least four different things that are pushing the Atlantic into warmth this year,” Vecchi said. “It is not surprising that it is really spectacularly warm.”

The planet’s oceans are in hot water

It’s still too early to see many of the consequences of the record warm sea surface temperatures in the North Atlantic Ocean, but scientists point out that the mechanisms of warmer water are well-established.

Hotter sea surface temperatures can lead to more evaporation and thus more rainfall. It can nudge wind patterns, so some places can get wetter while others get drier. Meanwhile, more heat causes the ocean itself to expand, which already accounts for one-third to one-half of global sea level rise.

Water temperature has huge effects on life in the ocean, too. As water heats up, it holds onto less oxygen and can suffocate fish. Hot water was a factor in thousands of dead Menhaden fish washing up on a beach in Texas earlier this month.

Warm water at the surface also slows upwelling, a critical mechanism that brings nutrients from deep in the ocean to the surface, nourishing life. Half of the world’s fish are caught in upwelling regions, so higher sea surface temperatures could lower the output of fisheries.

Another consequence of higher temperatures is that it makes water more acidic, accelerating ongoing ocean acidification that’s occurring as the seas absorb more carbon dioxide. And along with fertilizer runoff, hotter temperatures in the Atlantic, specifically, are contributing to record blooms of sargassum, a smelly, toxic seaweed that’s now fouling beaches in Florida and in the Caribbean.

This is happening on top of all the other ways humans are stressing the ocean: overfishing, pollution, and habitat destruction.

One of the biggest uncertainties this year is how the hot water in the Atlantic will affect hurricanes. NOAA has forecasted a “near-normal” hurricane season this year. Hotter surface water temperatures tend to charge up hurricanes, but the brewing El Niño in the Pacific stands to destabilize the air above the Atlantic and prevent hurricanes from forming.

“We have this tug of war: El Niño wants to say tropical Atlantic hurricanes should be weaker this year but these really warm ocean [temperatures] say hurricanes should be stronger,” said Amaya. “Only time will tell which process wins out.”

It’s also not clear whether such sudden spikes in temperature in the Atlantic Ocean will become more common. The overall global average water temperature is still poised to rise, however, so many of the consequences of hotter water — changing ocean chemistry, sea level rise, altered weather patterns — will continue to ramp up

“We’ve baked in a certain amount of warming of the planet,” Vecchi said. “But what happens after that will be in many ways strongly influenced by human decisions now.”

Investing in technologies to cut greenhouse gas emissions, pulling carbon dioxide out of the air, and ending the burning of fossil fuels will be critical to limiting heat in the ocean. In the meantime, the world will continue to experience the results of humanity’s unchecked climate experiment, with more alarming records poised to fall.

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