Daily global sea surface temperatures outside the polar regions reached 20.86°C on 21 June, according to the EU’s Copernicus Climate Change Service, exceeding the previous June highs of 20.83°C recorded in both 2023 and 2024. A separate dataset from the Copernicus Marine Service, using independent methods, registered 21.0°C on the same day, 0.1°C above the earlier records. The monthly average for June stood at 20.98°C, surpassing the 20.89°C set in June 2024. The first half of 2026 was the second-warmest January–June period for the global ocean, marginally behind 2024.

The record reflects the early stages of an El Niño event, formally declared by the World Meteorological Organization on 2 June, layered on top of the long-term ocean heating driven by greenhouse gas emissions. El Niño, characterised by anomalously warm waters in the central and eastern equatorial Pacific, releases additional heat into the atmosphere and alters global wind and rainfall patterns. The tropical Pacific recorded its hottest June on record at 27.26°C, matching the January–June record from 2016. Seasonal forecast models assembled by Copernicus suggest the current El Niño could reach an intensity not seen in decades, amplifying the warming trend through the remainder of 2026 and into 2027.

Marine heatwaves affected 82% of the world’s oceans during the first half of the year, the second-largest extent after 2024. The Mediterranean was particularly exposed: 98% of its surface experienced heatwave conditions, and the basin set a June record of 24.3°C. A heatwave in the northwestern Mediterranean reached a record intensity on 30 June, with sea surface temperatures averaging 5.2°C above normal, according to the Institute of Marine Sciences in Barcelona. Warmer oceans sustain atmospheric heat longer, supply extra energy to storms, increase evaporation and the risk of extreme rainfall, and contribute to sea-level rise through thermal expansion. They also place stress on marine ecosystems, with prolonged heat capable of triggering mass coral bleaching and mortality in less mobile species.

Carlo Buontempo, director of the Copernicus Climate Change Service, said the conditions “could indicate the beginning of a new phase, leading, once more, to uncharted territory,” and that further temperature records are likely in the coming months. Oceanographer Simon van Gennip of Mercator Ocean International noted that 2026 is expected to rank among the warmest years on record, driven by both El Niño and continued greenhouse gas emissions. The next milestone will be the evolution of the El Niño event through the boreal autumn, when its atmospheric coupling typically strengthens and its influence on global surface temperatures becomes more pronounced.