The world's oceans reached their highest temperatures ever recorded for June, with global average sea surface temperatures hitting 21.0 degrees Celsius, according to the European Union's Copernicus Marine Service. The reading, announced on Wednesday, surpassed the previous records set in the same month in both 2023 and 2024, capping six months of what the service described as "persistently elevated" and "exceptional" ocean warmth. Scientists warned the situation could deteriorate further in the months ahead.

Marine heatwaves expanded steadily throughout the first half of 2026, ultimately affecting around 82 percent of the global ocean — the second-largest extent on record. Regional hotspots included the Mediterranean, where temperatures in June reached 24.3°C, the central North Atlantic, and the equatorial Pacific. The Belgian coast of the North Sea, a relatively shallow body of water bordered by Belgium, the Netherlands, and the United Kingdom, experienced what researchers described as a continuous marine heatwave since February — 144 consecutive days — with temperatures in late June running 3.1 degrees above the long-term average, the warmest three-day period in 26 years. In the western Mediterranean, a marine heatwave classified as "extreme" recorded temperatures up to 8 degrees above seasonal norms.

Compounding the long-term trend driven by greenhouse gas emissions, scientists point to the onset of a new El Niño weather pattern as a significant additional factor. El Niño — a periodic natural climate phenomenon characterised by unusually warm surface waters in the central and eastern Pacific Ocean — releases heat into the atmosphere and disrupts weather patterns globally, raising the risk of floods in South America, droughts across parts of Africa, and wildfires in Australia. "With ocean temperatures at these levels and El Niño on the horizon, we are likely to see more temperature records fall in the coming months," said Carlo Buontempo, director of the Copernicus Climate Change Service. "Current conditions could indicate the beginning of a new phase, leading, once more, to uncharted territory."

The implications extend well beyond temperature readings. Oceans absorb approximately 90 percent of the excess heat generated by human-caused greenhouse gas emissions, making them a critical buffer for the global climate — but one under growing strain. Warmer seas fuel more intense tropical cyclones and heavier rainfall events, contribute directly to sea-level rise as water expands with heat, and devastate coral reef ecosystems through prolonged bleaching events. Increased absorption of atmospheric carbon dioxide is also accelerating ocean acidification, threatening shellfish, crustaceans, and the plankton that underpin the entire marine food chain. Researchers at Belgium's Flanders Marine Institute (VLIZ) noted that plankton die-offs in summer are already being observed, with knock-on effects visible in shifting fish populations — cod retreating from northern European waters while squid numbers rise.

The Copernicus report follows a major UN scientific assessment last month that declared the world's oceans were in a "deepening crisis," with seas warming and rising faster than previously projected. Scientists acknowledged that some of the phenomena now being observed have outpaced even the most advanced climate models, a development they described as deeply concerning. Land and sea temperatures together reached an all-time high in 2024 at the close of the previous El Niño cycle; with a new cycle now beginning, 2026 is on course to rank among the warmest years in recorded history.