Sea temperatures around Britain are set to reach unusually high levels this week amid a sustained marine heat wave, the country’s weather service reported, raising concerns about potential harm to marine ecosystems. The Met Office indicated on Wednesday that sea temperatures are significantly above average, currently ranked as “severe,” just one step below its highest alert level. Some areas of the English Channel are expected to escalate to the top category of “extreme” later this week.

This marine heat wave is part of a broader pattern of rising global sea surface temperatures. Two European organizations specializing in climate and ocean monitoring confirmed that worldwide ocean temperatures have reached record highs for this time of year. The Copernicus Climate Change Service and Copernicus Marine Service stated that the average global sea surface temperature recently hit 20.86 degrees Celsius (69.55 degrees Fahrenheit), surpassing previous records set in 2023 and 2024.

Extended periods of marine heat pose significant risks to aquatic life. Experts warn that elevated water temperatures can displace fish populations, damage key plant species such as sea grasses and kelp, and encourage the formation of harmful algal blooms. Marie Drévillon, head of operations at Mercator Ocean International, described marine heat waves as comparable to forest fires but occurring in the ocean, exerting severe stress on local ecosystems and causing species mortality and migration.

In Britain, the Met Office said surface water temperatures may rise as much as 5 degrees Celsius (9 degrees Fahrenheit) above average in some locations, particularly in the south. Current sea surface temperatures in the area have already reached approximately 19 degrees Celsius (66 degrees Fahrenheit), levels typically seen in August after prolonged summer heating. According to Mercator Ocean International, the first half of 2026 has seen record-breaking warmth in large portions of the English Channel, Irish Sea, and Bay of Biscay compared to the last 30 years.

The ongoing marine heat wave follows a period of extreme heat over Britain and much of Europe in June, driven by a “heat dome” event that set new temperature records. With forecasts predicting continued sunny and calm conditions this week, the Met Office’s air-sea interaction specialist, Ségoène Berthou, explained that the ocean’s surface may retain more heat, potentially intensifying the marine heat wave to reach “extreme” classification, particularly in southern waters.

Global ocean temperatures are also being influenced by the emergence of a strong El Niño event, which meteorologists expect to be one of the most intense in decades. Carlo Buontempo, director of Copernicus, indicated that further ocean temperature records could be anticipated in the coming months due to this phenomenon, underscoring ongoing challenges posed by climate variability.