By Stephen Beech
An "unprecedented" marine heatwave in seas around the UK could become an annual occurrence, according to new research.
Record-breaking surface temperatures were recorded in the North Sea and Celtic Sea in June 2023, reaching 2.9° Celsius above the monthly average for 16 days.
Scientists say the phenomenon was primarily driven by high-pressure weather systems that brought high solar radiation, weak winds, and warm air.
But while unprecedented since observations began, the new study warns that rapid climate change means there is now around a 10% chance of a marine heatwave of the same scale occurring every year.
The June 2023 marine heatwave significantly disrupted phytoplankton blooms, according to the research conducted by the University of Exeter, the Met Office and the Centre for Environment, Fisheries and Aquaculture Science (Cefas).
Although their full impact on marine ecosystems remains to be assessed, scientists say such heatwaves can stress marine species and increase concentrations of bacteria that can harm humans.
Study leader Dr. Jamie Atkins, formerly of Exeter University, said: "Our findings show that marine heatwaves are a problem now - not just a risk from future climate change."
Dr. Atkins, now at Utrecht University in the Netherlands, added: "The unprecedented nature of the June 2023 event put European marine heatwaves firmly in the public consciousness.
"However, our study shows that - in today's climate - such events should not be unexpected."
Co-author Professor Adam Scaife, head of long-range forecasting at the Met Office, said: "This is another example of how steady climate warming is leading to an exponential increase in the occurrence of extreme events."
The study, published in the journal Communications Earth and Environment, used a large number of climate model simulations to assess the likelihood of heatwaves at the June 2023 level or above.
It focused on two areas.
In the Celtic Sea - off the south coast of Ireland - the annual chance of such a heatwave rose from 3.8% in 1993 to 13.8% now.
In the central North Sea, the chance rose from 0.7% in 1993 to 9.8% now.
Previous research showed that the June 2023 marine heatwave also contributed to record-breaking temperatures and increased rainfall over the British Isles.
Dr. Atkins said: "Warmer seas provide a source of heat off the coast, contributing to higher temperatures on land.
"Additionally, warmer air carries more moisture - and when that cools it leads to increased rainfall."
The team says more research is now needed to investigate the impacts of marine heatwaves in other European seas.