The sea around the UK is now significantly warmer than normal. Temperatures are running about 2°C above what they should be, with some areas as much as 4–5°C hotter. The UK's Met Office, the national weather authority, calls this a "severe" marine heatwave — meaning the water is dangerously warm The Guardian. This is the third such event in 2026, and it's the most intense so far. Dr Ségolène Berthou, who studies how air and sea interact at the Met Office, said there is "no sign of an end to" the heatwave as of early July.

The Met Office warned on July 3 that conditions were "likely to reach extreme levels" within days Met Office.

What caused this?

The heatwave at sea came from a June heatwave on land. Much of Europe experienced its worst heat on record that month, and the atmospheric conditions that caused it drove the ocean to warm rapidly The Guardian.

The waters around the English Channel have been unusually warm for much of 2026. The sea off the East Anglian coast is currently experiencing what the Met Office calls "category two intensity" heatwave conditions Yahoo News. A third heat wave on land arrived in early July, with temperatures above 30°C lasting up to ten consecutive days. These conditions reinforce the warm ocean but are not the root cause.

Last year was already a warning sign

2026 didn't start from a normal baseline. The Met Office confirmed that 2025 was the UK's warmest and sunniest year on record, and the waters surrounding the country had unusual warmth throughout that entire year Met Office. The current event stacks on top of that already-warm baseline.

A global pattern, not just a UK problem

The UK is experiencing something wider. On July 1, the Copernicus Climate Change Service and Copernicus Marine Service — which monitor Earth's climate — confirmed that global sea surface temperatures had broken records for the time of year Copernicus.

On June 21, sea surface temperatures outside the polar regions exceeded what had been recorded on that same calendar date in both 2023 and 2024 The Guardian — the two years that had themselves set successive global records. The Mediterranean broke its own June sea surface temperature record this year, according to imagery released by the European Space Agency ESA.

Scientists predicted that current El Niño conditions — a natural warming pattern in the Pacific Ocean that occurs roughly every five to seven years — would be the strongest in decades. UN Secretary-General António Guterres called the phenomenon's arrival an "urgent climate warning" and called for an end to fossil fuel dependence, faster deployment of renewable energy, protection for vulnerable populations, and early warning systems for extreme weather The Guardian.

Here's where the significance lies. El Niño is a natural cycle that warms the Pacific temporarily, but it sits on top of a longer warming trend caused by human greenhouse gas emissions. When both happen at once, the ocean absorbs heat in a way that looks compounding rather than simply additive. The fact that successive global records have been set in 2023, 2024, and now 2026 suggests a ratchet effect — each year's bottom line gets warmer, not just temporarily spiking and settling back.

What happens to fish and other sea life

Extreme marine heatwaves can kill many creatures in a single species and shift where commercially important fish and shellfish live. Prof John Pinnegar, a principal scientist at the Centre for Environment, Fisheries and Aquaculture Science (CEFAS), warned that sustained warm water can also allow new species to move into UK waters, establish themselves, and disrupt existing ecosystems The Guardian.

Octopus offers the clearest example. Record numbers were caught off the south-west coast of England in 2025, and in June 2026 Brixham market sold a record 100 tonnes of octopus in a single day. Fisheries scientists often point to octopus as a visible marker of warming UK seas, because the species prefers warm water and can outcompete native crabs and lobsters when it spreads into cooler areas.

The real concern for people who manage fisheries is not the octopus catch itself. It's the pattern: three heatwaves in one UK marine season, set against a record global June, layered on top of the warmest year the UK has ever recorded. These kinds of shifts — species moving to new areas — complicate the computer models that fisheries managers use to decide how many fish can be safely caught. They also put species with narrow temperature preferences at greatest risk. Shellfish beds and the nurseries where young fish grow are typically most exposed.

The question of whether this warming is "natural" from El Niño or "human-caused" matters for policy, though in practice the distinction has become blurry. El Niño cycles come and go on timescales of years, but greenhouse warming does not. The data from June 2026 — which broke records even compared to the El Niño-boosted years of 2023 and 2024 — suggests that the two are reinforcing each other, not substituting. That inference is what Guterres's call for faster renewable energy deployment and early warning systems rests on.