Ancient sea sponges at centre of controversial claim world has already warmed by 1.7C

A deep-sea diver takes a specimen of Ceratoporella nicholsoni – a sponge that can take hundreds of years to grow. Prof Malcolm McCulloch says ‘findings show that global warming is more advanced than we thought’. Photograph: Clark Sherman

Between 30 metres and 90 metres below the surface of the Caribbean Sea, an ancient sponge species that grows a hard skeleton has been quietly recording changes in the ocean temperature for hundreds of years.

Now those sponges are at the centre of a bold and controversial claim made in a leading scientific journal that, since the start of the Industrial Revolution, the planet may have already warmed by 1.7C – half a degree more than estimates used by the United Nation’s climate panel.