Scientists who track the tropical Pacific had a number to explain in early November: 3.21°C. That is how far the sea surface in the Niño 3.4 box, the patch of equatorial ocean that defines El Niño, ran above its normal in October 2026. It is the highest monthly reading in the record, and it landed on top of a planet that had just logged its warmest October on record.
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Samantha Burgess, the strategic lead for climate at the European Centre for Medium-Range Weather Forecasts, said an exceptionally strong El Niño is adding further heat to a climate already warmed by human activities. ECMWF runs the Copernicus climate service for the European Union, and its October bulletin was the third in a row to report global temperatures above the 1.5°C line against pre-industrial levels.
The 3.21°C reading and the events it passed
The Copernicus Climate Change Service, run by ECMWF for the European Union, published the figure in its October 2026 climate summary on Nov. 5. The Niño 3.4 monthly anomaly of 3.21°C was above the peaks of the three strong El Niño events that anchor the modern record: 1982-83, 1997-98 and 2015-16. The monthly average sea surface temperature in the region, 29.94°C, was also an all-time high for that part of the ocean.
Copernicus builds its numbers from ERA5, the reanalysis that blends observations with a weather model, and publishes them as monthly climate bulletins. That matters for comparison, because other agencies measure the same ocean with different datasets and different baselines. The NOAA Climate Prediction Center’s Oct. 8 discussion put its own Niño 3.4 index at +2.1°C for the previous month, and the two numbers should not be set side by side as if one corrected the other.
The “record-strong” claim, then, belongs to a specific measurement: Copernicus’s monthly anomaly in its own dataset, for October alone. A reader comparing agencies will find different values for the same ocean region, depending on the dataset, the baseline period and whether the figure is weekly, monthly or a three-month average, so the record is best cited with its source attached.
A hot ocean on a hot planet
The same Copernicus summary has global October surface air temperature at 15.35°C, 0.90°C above the 1991-2020 average and 1.75°C above the 1850-1900 pre-industrial level. It was the third consecutive month above 1.5°C; the previous such run ended in April 2025. Average sea surface temperature between 60°S and 60°N was 20.95°C, the highest for any October on record, edging out October 2023.
Copernicus frames El Niño as an addition to human-driven warming, not the cause of it. Burgess described October as continuing a pattern of remarkable global warmth, with record global and surface ocean temperatures, and pointed to the human-caused warming already baked into the climate system. The El Niño is layering natural variability on a baseline that has been rising for decades, which is the plain meaning of her wording about adding further heat.
Year, four-year average and the forecast
Copernicus’s statistical model, run on ERA5 data through October, puts the odds that 2026 exceeds 1.5°C above pre-industrial levels at better than 99 percent. The predicted range is 1.56°C to 1.61°C, which would make 2026 the second calendar year above that line, after 2024. The service gives about an 80 percent chance that 2026 ends as the second-warmest year and about 20 percent that it ties or beats 2024. It also expects the 2023-2026 four-year average to exceed 1.5°C, a point Burgess called particularly significant for how sustained the warm period has become.
On the ocean itself, Copernicus expects El Niño to peak toward the end of 2026. The Climate Prediction Center’s Sept. 10 discussion had put the chance of a historic event, defined as a three-month Relative Oceanic Niño Index of +2.5°C or more, at 75 percent for October-December, and its Oct. 8 update raised that to 83 percent. It said a strong-to-very strong El Niño through January-March 2027 had a better than 83 percent chance.
What follows for weather and for the climate
Heat is already showing up outside the Pacific. Climate Central’s October attribution overview found Sioux City, Iowa, running 9.0°F above normal for the month, the largest anomaly among 192 U.S. cities, and 33 of those cities had one of their five hottest Octobers on record. That analysis does not tie the Midwest warmth to El Niño, and neither does Copernicus, so the two findings are best read as concurrent rather than linked.
What Copernicus does say is what remains to be settled. A reading of 3.21°C is a single month’s anomaly, and the Climate Prediction Center’s next ENSO discussion, scheduled for Nov. 12, will show how NOAA’s own index has moved since its Oct. 8 reading. Burgess’s phrasing is a pointer in the same direction: the El Niño is expected to keep adding heat into the new year, with 2026’s final ranking among the warmest years decided by the last two months of data.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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