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

Findings in leading scientific journal that globe has breached key warming milestone challenged by climate science experts

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.