The temperatures in the British and Ireland oceans have soared over the past week, and now some areas are warmer than usual 4C, with potential impacts on marine life and people swimming.

According to scientists at the National Oceanographic Center and the Metropolitan Office, the heat wave is the strongest, the most intense on the west coast of Ireland and the pockets of the Cornwall and Devon coasts.

Since monitoring began, the SST in April and the first half of May was the highest record.

Climate change is causing oceans to warm globally and making ocean heat waves more likely.

Scientists expect 2025 to be one of the hottest periods to record air temperatures.

Dr. Ségolène Berthou of the Metropolitan Office said: "It's very intense at the moment.

Dr. Zoe Jacobs, located at the National Oceanographic Center, first noticed the ocean temperature a few weeks ago. She found that UK pockets have emerged from mild heat waves since the end of 2024. This heat intensified and spread in March and has now surged.

Ocean heat waves are defined as seasonal thresholds exceeding seasonal thresholds for more than five consecutive days. In the UK, the threshold for ocean heat waves in May is 11.3c.

On May 19, the sea surface temperature reached 12.69c.

Now, the entire West Coast of Britain is about 2.5 degrees Celsius above average. A large portion of Scottish waters is 2-3C warmer than usual during the year.

"It started in the North Sea and the Celtic Sea. Now, the North Sea is a little cooler, but the west of Ireland is very hot."

One of the warmest springs recorded is driving the tides as high temperatures and wind energy warms the top layer of the ocean.

Ocean heat waves in the UK are considered a relatively new phenomenon, but their frequency and intensity are expected to increase.

How they will exactly affect marine life remains a mystery, but so far the signs are not good.

"It's interesting that it started in winter and spring when most people thought ocean heat waves were only in summer," Dr. Jacobs said.

People who swim on the west coast of England and Ireland may notice higher temperatures, although the waters are still cooler than they were at peak late summer.

Dr. Jacob explained that it is possible to avoid the greatest impact on species at the moment, as temperatures have not yet exceeded the upper limit that marine organisms can tolerate.

But this may be disrupting the breeding patterns of species and may bring in large quantities of jellyfish, just like warmer waters, including huge barrel peels, to the ocean and beaches.

This can also cause harmful algae to get out of control, resulting in broad green algae that can poison other lives.

Previous heat waves caused harmful reproduction of algae and in 2018 caused mass deaths of mussels.

In 2023, jellyfish witnesses increased by 32% after temperatures above average ocean heat waves.

The calories can also encourage different fish to enter British waters, including bluefin tuna, which may increase the number of fish caught.

Usually, ocean heat waves last about two weeks, so scientists were surprised at how long this lasts.

"This is the exception. We are about two and a half months old, and it's a long time," Dr. Batu said.

Higher sea temperatures can also increase land temperatures, as sea breezes take heat away from the ocean.

This occurred in May 2024, when a brief ocean heat wave contributed above-average land temperatures, according to the Metropolitan Office.

In certain parts of the world, including Australia, the United States and the Pacific, ocean heat waves can damage coral reefs or local fisheries, as well as important seagrass meadows.

Often, the UK is more protected because the waters are colder overall. But scientists still know a lot about them, and there is still a lot to discover.

A new study by Dr. Jacobs found that British hotspots are the Southern North Sea and the UK Channel, where heat waves can last longer than the rest of the country.

The ocean absorbs 90% of the excess heat generated by human burning fossil fuels, so overall it heats 1C.

"The waters are already in a hotter state until we start this good weather this spring," Dr. Batu said.

In the North Atlantic, sea surface temperatures have increased by about 0.3c per decade in the past 40 years, according to the Metropolitan Office.

Warm, dry weather is expected to break slightly this weekend.

"The ocean is behind the atmosphere for at least a few days, so maybe next week we may start to see the ocean cool down."

But it may be just a "temporary dip" as long-term forecasts suggest that the weather will warm again.

Erwan Rivault's Graphics