March 2024 has been the hottest of any other March on record — just like the nine months preceding it. The confirmation came in early April from Copernicus, an EU-funded Earth observation program. But the air surrounding us does not seem to be the only thing suffering from the record-breaking fever; our oceans do not look well either. Across the globe, sea surface temperatures are following a very different trajectory from those that have been observed so far. Last month, the global ocean reached an average surface temperature of 21.07 degrees Celsius (69.92 degrees Fahrenheit), surpassing any previous record. According to the United States' National Oceanic and Atmospheric Administration (NOAA), this marked the 12th consecutive record-breaking month of the global ocean’s average surface temperature. We spoke with Carlo Buontempo, director of the Copernicus Climate Change Service (C3S), who shared insights on the unusual spikes and anomalies observed in ocean temperatures since May of last year.

In this graph, you can see the trend for sea surface temperature in the global ocean over the course of the year. The dashed line represents the average values for the period 1991 to 2020. The orange line shows the data recorded in 2023, while the red line represents the early months of 2024 (Climate Pulse, European Union).

“In this region, ocean temperatures typically reach their zenith around late March and then drop over the course of the year,” Buontempo explained. “They usually hit an absolute minimum around the end of the year, towards November.”

But last year, in addition to the usual peak in March, the average surface temperature of the ocean reached an absolute peak in August. Since then, temperatures have been continuing to present consistent anomalies to the annual cycle. The highest anomaly was recorded in January of this year (0.73 degrees Celsius or 1.31 degrees Fahrenheit, according to C3S data).

“The situation is highly unusual compared to our existence on this planet as a species,” Buontempo said, referring not only to the oceans but also to air temperatures.

Last June, NOAA announced the arrival of El Niño, a recurring variation in the surface temperature of the tropical Pacific Ocean. This variation alternates between a warm phase (El Niño) and a cold phase (La Niña), which constitute the main drivers of interannual temperature variations. Historical data indicates that during the warm phase, global temperatures usually peak in the months following El Niño's peak, which typically occurs around late December. Therefore, spikes or anomalies during El Niño aren't particularly surprising.

“What is most surprising,” said Buontempo, “is that all oceans contribute to this ocean temperature, not only the Pacific, as would be the case with just El Niño. Particularly, the North Atlantic plays a significant role, despite not having a direct relationship with Pacific events.”

Anomalies in surface temperatures are also being recorded in the Mediterranean, a sea that typically experiences little effect from El Niño variations.

And, since El Niño alone doesn't seem to explain what's happening to our global oceans, other factors could be at play. Among those most frequently mentioned are the effects of the 2022 eruption of the underwater volcano Hunga Tonga in the South Pacific, reductions in the amount of sulfur in the fuel used by some ships because of air quality regulations (which might have led to more energy being absorbed by the sea due to less sun-reflecting air pollution), an approaching peak in the eleven-year solar energy cycle and, above all, global warming. Although there is still much to figure out and understand, according to Buontempo, the effects of greenhouse gas emissions in the atmosphere remain probably the most important contributing factor, especially regarding the Mediterranean.

“We have warmed the climate system. We have warmed the oceans. We have warmed the atmosphere. We have lost polar ice. We have lost ice mass and snow on the Alps, on the Pyrenees, on all the mountain ranges, on the Tibetan Plateau and so on. The climate is heading in a different direction,” said Buontempo. “Thus this warmer Mediterranean is completely in line with what we expect due to the fact that the entire climate system is in a different situation from when we or our parents were born.”

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CARLO BUONTEMPO: Physicist by education, he is the director of the Copernicus Climate Change Service (C3S), an initiative coordinated by the European Centre for Medium-Range Weather Forecasts (ECMWF). Previously, he worked on climate and adaptation for the Met Office, in the U.K., where he was involved in climate projections, forecasts and models used to research global climate change. He is among the scientists who contributed to the volume of the latest Intergovernmental Panel on Climate Change Assessment Report dedicated to climate physics.

This interview has been edited for length and clarity and translated into English.

It seems like the oceans have a fever. What's happening to our seas? 

Last year, we observed three noteworthy occurrences regarding ocean temperatures. The first was a marine heatwave that affected the eastern Atlantic, an area roughly stretching from the Irish Sea to Mauritania. In June, this region experienced a heatwave, with some areas reaching extreme values and anomalies beyond 5 degrees Celsius (9 degrees Fahrenheit). Obviously, 2023 was indeed a year marked by El Niño. But what happened in the Atlantic at that time in June and then throughout the year cannot be directly related to El Niño; it's an additional factor. The second interesting aspect of last year's events concerns the peak in global ocean temperatures. Last year, the temperature peaked as usual in March. It began to decline in April and May, then rised again. It reached an absolute peak in August, which is quite unusual. The third aspect is that since the end of last year, the situation hasn’t changed; we continue to experience unusual and extreme temperatures compared to historical data.

This graph highlights the anomalies for 2023 and the first months of 2024 compared to the reference values of global ocean surface temperatures throughout the year. The highest anomaly was recorded in January of this year (Climate Pulse, European Union).

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