“All the way through the year, on average it’s been warmer than we’ve really ever seen [for the UK’s seas],” said Prof John Pinnegar, the lead adviser on climate change at Cefas.
“[The seas] have been warming for over a century and we’re also seeing heatwaves coming through now,” he added.
“What used to be quite a rare phenomenon is now becoming very, very common.”
Like heatwaves on land, sea temperatures are affected by natural variability and short-term weather. Clear, sunny skies with low winds – like much of the UK had in early July – can heat up the sea surface more quickly.
But the world’s oceans have taken up about 90% of the Earth’s excess heat from humanity’s emissions of planet-warming gases like carbon dioxide.
That is making marine heatwaves more likely and more intense.
“The main contributor to the marine heatwaves around the UK is the buildup of heat in the ocean,” said Dr Caroline Rowland, head of oceans, cryosphere and climate change at the Met Office.
“We predict that these events are going to become more frequent and more intense in the future” due to climate change, she added.