North Atlantic 2023 warmer than ever before – these are the reasons

The North Atlantic has never been as warm in summer as it was in 2023. June stood out in particular: Back then, the sea surface temperature of the northeastern Atlantic around Europe was 1.76 degrees Celsius above average. The World Meteorological Organization (WMO) and the Copernicus climate change service described this as the "largest monthly anomaly since records began" in their 2023 "European State of the Climate" report .

The heat in the Atlantic had devastating effects on land: Additional warmth in the oceans can increase air temperatures on land. 2023 was marked by new record temperatures and drought-fueled wildfires. Warmer oceans also release more water vapor into the atmosphere, which favors heavy rain and storms. Devastating floods occurred in Austria and Serbia in 2023, and several hurricanes devastated the United States.

For a long time, it was unclear how the North Atlantic could have warmed so much. Experts suspected, for example, that the marine heatwave was caused by a lack of Saharan dust or by the El Niño weather phenomenon, which cyclically develops in the southeast Pacific. A research team led by Matthew England from the Australian University of New South Wales has now solved the mystery: Unusually weak winds combined with increased solar radiation were apparently the cause.

"The intensity of the warming in that single summer was roughly equivalent to two decades of warming in the North Atlantic," said England. Ironically, he and his team had actually planned to study cooling in the North Atlantic. More specifically, the "cold spot" southeast of Greenland, which has been cooling for decades.

But the researchers discovered exactly the opposite: The "cold spot" had warmed to two degrees Celsius above average in 2023. They realized that the unusually high temperatures were part of an unprecedented marine heatwave in the North Atlantic. They analyzed observational data, reconstructed atmospheric conditions, and used the latest ocean models to determine the cause. They presented their results a few days ago in the journal "Nature."

They found that winds over the North Atlantic were record-breakingly weak in June and July—possibly due to the developing El Niño. This had an impact on the water structure.

One can imagine the ocean divided into two layers, said study co-author Alex Sen Gupta: an upper layer, which is directly influenced by solar radiation and warms in spring and summer, and the cooler deep ocean. How quickly the upper layer heats up depends on its thickness. The thinner it is, the faster it warms.

In summer, the thickness of this surface layer is largely determined by the winds. "Winds churn the ocean surface, and the stronger they are, the deeper the mixing penetrates," England, Gupta, and two other study authors explained in an article for "The Conversation ." "So strong winds create a thicker upper layer, while weak winds create a shallower one."

Sea levels are rising faster than expected, according to a new analysis by the US space agency NASA. And melting ice isn't primarily to blame for this development.

Due to the weak winds, the ocean layer was exceptionally thin in the summer of 2023. According to researchers, it was the shallowest upper layer ever measured. In some areas, it was only ten meters deep, whereas 20 to 40 meters are common. This allowed the North Atlantic to warm even faster.

Climate change is also a contributing factor: "As global warming reduces ocean mixing, the upper ocean layer becomes thinner and more vulnerable to rapid warming. This makes more frequent and intense marine heatwaves increasingly likely," said Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research. Feedback effects could also occur – for example, the Greenland ice sheet could melt faster.

The researchers identified above-average solar radiation on the North Atlantic as a second driver of the marine heat wave. They suspect that shipping may play a role: New international regulations came into force in 2020 aimed at reducing air pollution from ship exhaust, particularly sulfate emissions. Apparently, they were successful.

Sulfate emissions from shipping have decreased, but this has an impact on cloud formation.

Source: Keith Tsuji/ZUMA Press Wire/dpa

However, sulfate aerosols also reflect solar radiation and can lead to cloud formation. Lower sulfate emissions mean clearer skies, allowing more direct sunlight. Consequently, the oceans can warm more quickly.

However, lead study author England emphasizes that this effect is secondary and has only contributed to greater warming in certain regions. He believes the main cause is still the lack of wind.

"Severe marine heatwaves often last only a few weeks or months, but this one in the North Atlantic left traces that lasted for more than a year," England said. Due to the increasing influence of climate change, "the frequency and intensity of marine heatwaves will only worsen in the coming decades and beyond."

The way out of this dilemma is clear to the researcher: "The only way to stop this trend is to phase out the use of fossil fuels. Net zero (meaning removing as many climate-damaging emissions from the Earth's atmosphere as we release into it, ed.) can't come soon enough."