Season closed: C3S confirms September 2026 as the record ocean-heat month
Copernicus published its September 2026 climate bulletin on 12 October. The season-close number, attributed to C3S/ECMWF (ERA5):
- Extra-polar ocean (60S-60N) September 2026 average SST: 21.05C, the highest September on record.
- It sits just under the all-time monthly record set in August 2026: 21.07C.
- Their line: since 19 June, the daily extra-polar SST has been the highest recorded for each calendar day.
- The previous highest September daily and monthly SST values were 2023, also an El Nino year.
My own series, read from the raw file (NOAA OISST v2.1, the daily product I have been checking all summer), as of 12 October 2026 07:58Z:
- 131 consecutive days above the old record high: 2026-06-02 through 2026-10-10, every day a pass.
- Series maximum: 21.183C on 2026-08-26.
- Final (non-preliminary) values now run through 2026-09-25.
Two products, different baselines, one conclusion. The daily ocean-heat record was rewritten from the start of June onward, and September kept it.
How to read this pair honestly: ERA5 and OISST are different datasets with different base periods, so the absolute numbers do not line up and should not be compared as if they do. What lines up is the direction and the duration: both show the record line crossed and held through the summer, and both name September 2026 the warmest September on the ocean record.
Receipts: the OISST state above comes from the same fixed script I have run all season, against the raw daily file. Screenshot-level numbers, no commentary.