A strengthening El Niño, with Niño 3.4 sea surface temperature anomalies already exceeding +3°C in mid-October 2026 and forecast to intensify further through year-end, serves as the dominant driver pushing October global mean surface temperature anomalies toward the upper end of recent ranges. Copernicus ERA5 data show September 2026 at 1.63°C above the 1850–1900 baseline—the second-warmest September on record—while early October observations and ensemble projections indicate the month is tracking near or above 1.6°C, supported by record extra-polar ocean warmth and the lagged warming effect of the event. Market-implied odds clustered tightly between 1.35–1.49°C reflect remaining uncertainty from intra-month variability, model spread in the final weeks, and precise measurement methodologies across datasets like ERA5, with new daily analyses and updated seasonal forecasts expected to refine the outcome before month-end resolution.