2023 ocean heatwave ‘unprecedented but not unexpected’

The June 2023 heatwave in northern European seas was “unprecedented but not unexpected”, new research shows.

During the heatwave, temperatures in the shallow seas around the UK (including the North Sea and Celtic Sea) reached 2.9°C above the June average for 16 days.

While unprecedented since observations began, the study warns that rapid climate change means there is now about a 10% chance of a marine heatwave of this scale occurring each year.

The June 2023 marine heatwave significantly disrupted phytoplankton blooms. Although its full impact on marine ecosystems remain to be assessed, such heatwaves can stress marine species and increase concentrations of bacteria that can harm humans.

The study was carried out by the University of Exeter, the Met Office and Cefas.

“Our findings show that marine heatwaves are a problem now – not just a risk from future climate change,” said Dr Jamie Atkins, who led the study during his PhD at Exeter, and is now at Utrecht University.

“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, of the University of Exeter and 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 used a large number of climate model simulations to assess the likelihood of heatwaves at the June 2023 level or above.

It focussed on two locations:

• 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.

Explaining this, 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 say more research is now needed to investigate the impacts of marine heatwaves in European North-West shelf seas.

Dr Atkins’ work was funded by the Natural Environment Research Council (NERC) via the GW4+ Doctoral Training Partnership.

The paper, published in the journal Communications Earth & Environment, is entitled: “Recent European marine heatwaves are unprecedented but not unexpected.”