In February, the floating sea ice around Antarctica hit a record low for the second year running. But the moderate alarm from scientists at that record low is now being overlaid with astonishment – with some scientists worried they could be witnessing the start of a slow collapse of the delicate southernmost ecosystem. Guardian Australia environment reporter Graham Readfearn and oceanographer Dr Will Hobbs tell Laura Murphy-Oates about why this melting is happening, and the major knock-on effects for the rest of the planet