Sea temperatures of 30C or above are not unprecedented in the Med in late summer.

But they are highly unusual for June, according to data from the European Copernicus climate service, Mercator Ocean International, and measurements at Spanish ports.

"What is different this year is that 30C sea temperatures have arrived much earlier, and that means that we can expect the summer to be more intense and longer," said Marta Marcos, associate professor at the University of the Balearic Islands in Spain.

"I grew up here, so we are used to heatwaves, but this has become more and more common and intense."

In fact, the number of days of extreme sea surface heat globally has tripled over the past 80 years, according to research published earlier this year.

"Global warming is the main driver of marine heat waves… it's essentially transferring heat from the atmosphere to the ocean. It's very simple," said Dr Marcos.

The Mediterranean is particularly vulnerable because it's a bit like a bathtub, largely surrounded by continents rather than open ocean.

That means water cannot escape easily, so its surface heats up quickly in the presence of warm air, sunny skies and light winds - as happened in June.

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