The tipping point for irreversible global warming may have already been triggered, the scientist who led the biggest-ever expedition to the Arctic warned yesterday. "The disappearance of summer sea ice in the Arctic is one of the first landmines in this minefield, one of the tipping points that we set off first when we push warming too far," said Markus Rex. Rex, who led the world's biggest mission to the North Pole, an expedition involving 300 scientists from 20 countries, said we might have already stepped on the mine. He said the next year's data is crucial to confirm his fear.