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In a UNSW-led , researchers say that an off-the-scale marine heat wave in the North Atlantic Ocean in 2023 was caused by record-breaking weak winds combined with increased solar radiation—all on the back of ongoing climate change.

From Greenland to the Sahara and across to the Americas, the waters of the North Atlantic Ocean warmed at an unprecedented speed in the summer of 2023.

"The intensity of the warming in that single summer was equivalent to about two decades worth of warming for the North Atlantic," says lead author Professor Matthew England from UNSW Sydney.

"While these extreme temperature events are typically only temporary, we can expect they'll become more frequent in the future."

Ironically, at the time, Prof. England was researching a region of cooling in the North Atlantic. This so-called 'cold blob' southeast of Greenland is one of the more unusual consequences of global warming—a sign of the Atlantic Meridional Overturning Circulation (AMOC) slowing down, which is a scenario made famous by the Hollywood movie, "The Day After Tomorrow."

These waters had been cooling for the past 50–100 years, so when Prof. England and his team saw water temperatures spiking across the region, they realized they were watching something unusual unfolding.

"We even asked ourselves if this was the circulation making a temporary comeback, but the rate of warming was far too rapid for that," Prof. England says.

No chill

The ocean can be thought of as two layers: an upper layer that's directly affected by solar radiation, and the cooler deep ocean. With more exposure to the sun during spring and summer, the upper ocean gradually warms.

Co-author Associate Professor Alex Sen Gupta, also from UNSW, says the rate of warming depends on the thickness of the ocean's upper layer.

"A thin layer will warm faster, much in the same way that a pan of water on a stove with less water will warm faster than a pan with more," A/Prof. Sen Gupta says.

He says the thickness of the layer in summer is set by the winds that churn up the surface waters and mix heat throughout it.

In June and July of 2023, the North Atlantic winds were weaker than ever recorded, "so the upper layer of the ocean was thinner than ever recorded," A/Prof. Sen Gupta says.

In some areas it was only 10 meters deep, compared to the usual 20–40 meters deep, according to calculations made by co-author Dr. Zhi Li, also from UNSW, who led the analyses of ocean observations for the study.

"This meant that the sun heated the ocean's surface more rapidly than normal, which is what led to those record-breaking temperatures," says Dr. Li.

He says the temporary thinning caused by weaker-than-average winds was also bolstered by global warming.

Long-term warming causes the surface ocean to become less dense, suppressing the ability of winds to mix the upper ocean.

"So we were also dealing with a long-term thinning of the upper layer," Dr. Li says.

A sunburned ocean

There was possibly also a further unexpected, localized factor that summer.

In 2020, new international rules were introduced to reduce the sulfur pollution emitted by ships. The aim was to improve air quality around the world's major shipping lanes.

But clearer skies can have an unintended side effect: less aerosol pollution means fewer 'seeds' for clouds. Less cloud cover means more sunlight can reach the sea surface—especially in the North Atlantic, which is a high-traffic shipping area.

However, Prof. England says this effect was secondary, only contributing to localized regions of enhanced warming. Most of the blame, he says, still lies with the lack of wind.

"Reducing sulfate emissions is good for reducing air pollution," he says. "Though it has the unfortunate effect of allowing additional warming of the ocean's surface, because less sunlight is reflected back to space."

The combination of weak winds with reduced mixed layer depths and clearer than average skies meant the rapid warming became a full-basin marine heat wave.

And as the warming waters radiated heat back into the atmosphere, this triggered a series of consequences on land.

Continental scale crises

While a quiet crisis was unfolding at sea, the air masses traveling over the top of the ocean were picking up heat to scorch cities across Europe.

Deadly 40+ degree Celsius heat waves across Germany, France and Italy broke temperature records, while torrential rains devastated parts of Spain and Eastern Europe.

Back underwater, the coral reefs of the Caribbean were bleaching under severe heat stress. Hurricanes, which only occur during summer, fueled by ocean heat, intensified into disasters. That season, Hurricane Idalia hit Florida—causing .

"This wasn't just a small area of warm water off one coast," says Professor Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research (PIK), another co-author of the study.

"This was the entire North Atlantic, with impacts on weather systems, human lives, marine ecosystems and society."

Preparing for the future

As climate change continues to thin the ocean's surface mixed layer, heat from the sun will continue to concentrate and warm it more readily.

Prof. England says this means marine heat waves in the North Atlantic are only set to get worse in the future, with extremely costly consequences for ecosystems and society.

"Severe marine heat waves often only last for a few weeks or months, but this one in the North Atlantic left a legacy that persisted for more than a year," he says.

"Unfortunately, the frequency and intensity of marine heat waves is only set to worsen in the coming decades and beyond.

"The only way to stop this trend is to phase out our use of fossil fuels.

"Net zero cannot come soon enough."