Marine heatwave in Wales could see beachgoers facing new threats

Wales Online

22-05-2025

Marine heatwave in Wales could see beachgoers facing new threats
The UK and Ireland are currently in the middle of a marine heatwave with some waters now 4C warmer than usual for the time of year
Beachgoers will need to look out for these potentially deadly creatures
(Image: Dwalad/Wiki )
Exotic sea creatures may start appearing on Welsh shores as water temperatures rise, according to scientists. The UK and Ireland are currently experiencing a marine heatwave, with some waters now 4C warmer than the normal for the spring months.
The increase in both sea and land temperatures has already led to more people heading to the beaches, with the warmer waters reducing the cooling effect of sea breezes. However, for most swimmers and those dipping their toes, Britain's waters remain rather nippy.
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The heatwave is most severe in the Celtic Sea and off the west coast of Ireland, where current sea surface temperatures are up to 4C warmer, according to the Met Office. Along the UK coastline, the sea is 1.5C-2.5C warmer, with hotspots in Cardigan Bay and along the North Wales coast.
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These temperatures are "what we would usually expect around mid-June," stated the national forecaster. Labelling the event as "extreme", the Met Office reported that surface temperatures (SSTs) reached levels for April and May that were the highest since 1982 when satellite monitoring first began.
Temperature anomaly map showing seas off the UK and Ireland much warmer than usual
(Image: Met Office/Ostia )
The heatwave is not only significant due to its intensity but also its duration, which has taken observers by surprise. So far, it's lasted more than two months: typically, marine heatwaves in UK waters dissipate after two to four weeks, reports North Wales Live.
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Sea temperatures are expected to start dropping in the next few days as cooler weather settles over the country.
Dr Ségolène Berthou, the Met Office's scientific manager for regional coupled modelling, has attributed the heatwave to a combination of factors. For our free daily briefing on the biggest issues facing the nation, sign up to the Wales Matters newsletter here
She said: "The heatwave is the result of several overlapping factors. Prolonged high-pressure systems brought a dry, sunny spring and weak winds and waves, creating ideal conditions for warming.
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"This means the sea started to warm mid-February, one month earlier than usual. Additionally, the waters around the UK were already warmer than usual coming out of winter, a trend that has been building over the past 40 years, with an average increase of 0.3C per decade."
Global climate change is leading to higher ocean temperatures worldwide, increasing the frequency of marine heatwaves. On May 19, the average UK sea surface temperature was recorded at 12.69C – significantly higher than the May marine heatwave threshold of 11.3C in the UK.
The rise in sea temperatures may lead to more jellyfish visiting UK coasts annually. There have already been sightings of large barrel jellyfish stranded on beaches in Gwynedd and Ceredigion.
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Frankie Hobro, director of Anglesey Sea Zoo in Brynsiencyn, has issued a warning that this could result in more exotic and potentially dangerous sea creatures reaching Welsh beaches. She emphasises the importance of increased knowledge in marine medicine to safeguard beachgoers.
Some may be newcomers, while others are grown versions of the young sea creatures already seen in UK waters. While the arrival of species like bluefin tuna, sunfish, and basking sharks is welcomed, not all are as desirable, she mentioned.
Potential unwelcome visitors include pufferfish and Box jellyfish, one of Earth's most venomous creatures.
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Existing residents on British coasts, such as the weever fish, might become more troublesome. This small creature conceals itself in the sand and can inflict a painful sting.
Box jellyfish have cube-shaped bell and numerous tentacles. Their stings can paralyse and they can be fatal
(Image: Peter Southwood/Wiki )
The Portuguese man o' war, known for its potent sting, could also start appearing more frequently and in greater sizes, Frankie noted.
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Algal blooms are likely to occur more often due to calmer and warmer seas, leading to expanses of green algae that pose risks to other marine life. On a brighter note, bioluminescent displays near beaches may grow more frequent and spectacular.
Yet, rising temperatures could push some marine species away from Britain's coastlines.
"There is no escape from the heat for sea creatures within it," Frankie explained.
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"They can't turn on air conditioners under water, or escape it."
Although the warmer waters might be noticeable to some swimmers, they remain quite chilly for the average beachgoer. Dr Berthou has observed that the North Sea has reached its temperature peak, while west Ireland and the Celtic Sea are currently at their warmest.
During marine heatwave events, a warm water layer forms at the surface.
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She further explained: "This layer acts like a lid, trapping heat near the surface and preventing it from mixing into deeper, cooler waters.
"It can be likened to a layer of olive oil floating on water; this thin surface layer allows heat from solar radiation to accumulate near the surface. When this layer forms, surface temperatures can rise rapidly: the last nine days have seen a sudden additional 1C warming."
Dr Jonathan Tinker, a marine climate scientist at the Met Office, stated that prolonged warming can intensify land heatwaves by diminishing the cooling effect of sea breezes and altering rainfall patterns.
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He added: "With projections suggesting UK summer sea temperatures could rise by up to 2.5C by 2050, such events are likely to become more frequent, intense and longer-lasting. Marine heatwaves offer a glimpse into how changing ocean conditions could shape future weather conditions."