Record sea heatwave grips Wales but not all visitors will be welcome

North Wales Live

22-05-2025

Exotic sea creatures could start being washed up on Welsh beaches as water temperatures soar, scientists believe. The UK and Ireland in the grip of a marine heat wave with some waters now 4C warmer than usual for the time of year.
Already a surge in sea and land temperatures has seen more people flocking to beaches, with warmer waters reducing the cooling effect of sea breezes. However, Britain's waters remain distinctly chilly for most swimmers and toe-dippers.
The heatwave is most intense 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. Around the UK coastline, the sea is 1.5C-2.5C warmer, with hotspots in Cardigan Bay and along the North Wales coast.
These temperatures are 'what we would usually expect around mid-June,' said the national forecaster. Classing the event as 'extreme', the Met Office said surface temperatures (SSTs) reached levels for April and May that were the highest since satellite monitoring began in 1982.
Not only is the heatwave significant due to its intensity, observers have been surprised by its longevity. So far it's lasted more than two months: usually, marine heatwaves in UK waters dissipate after two to four weeks. Sea temperatures are finally expected to start dropping in the coming days as cooler weather settles over Britain.
Dr Ségolène Berthou, scientific manager for regional coupled modelling at the Met Office, 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.
'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.' Join the North Wales Live Whatsapp community now
Climate change is causing oceans to warm around the globe and is making marine heatwaves more likely. On May 19 the average UK sea surface temperature reached 12.69C – well above the country's marine heatwave threshold for May (11.3C).
Scientists suspect warmer waters could increase the annual influx of jellyfish to UK shores. Already a number of sightings have been made of the huge barrel jellyfish washed up on beaches in Gwynedd and Ceredigion.
A marine expert has warned it could see other exotic and unusual sea creatures arriving on Welsh shores in greater numbers. Frankie Hobro, director of Anglesey Sea Zoo, Brynsiencyn, believes some may pose a greater risk to beachgoers, underlining the need for greater awareness of marine medicine.
Some might be first-time visitors, other more mature versions of juveniles currently spotted in UK waters. While many will be welcome, such as bluefin tuna, sunfish and basking sharks, others will not, she said. Candidates include pufferfish and Box jellyfish, one of the most poisonous animals on Earth.
Others already on British beaches could become more of a pest, such as the weever fish. This little critter buries itself in the and can deliver an excruciating sting. Another occasional visitor which also packs a powerful sting is the Portuguese man o' war, which could arrive in larger numbers and sizes, said Frankie.
Algal blooms are also expected to become more commonplace due to calmer and warmer seas., with patches of green algae developing that can threaten other marine life. On the plus side, bioluminescence displays next to beaches may become more numerous and spectacular.
However, heat stress could drive other marine life away from Britain's shores. 'There is no escape from the heat for sea creatures within it,' Frankie told North Wales Live. 'They can't turn on air conditioners under water, or escape it.'
While the warmer waters might be discernible to some swimmers, they are still distinctly nippy for the casual bather. Dr Berthou said temperatures in the North Sea have already peaked, whereas west Ireland and the Celtic Sea are currently experiencing their highest levels.
During marine heatwave events, a layer of warm water forms at the surface. She added: 'This layer acts like a lid, trapping heat near the surface and preventing it from mixing into deeper, cooler waters.
'It can be compared 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 increase fast: the last nine days have seen a sudden additional 1C warming.'
Dr Jonathan Tinker, a Met Office marine climate scientist, said the prolonged warming can amplify land heatwaves by reducing the cooling effect of sea breezes and by altering rainfall patterns. Sign up for the North Wales Live newsletter sent twice daily to your inbox
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.'