PARIS (AFP) - The North Pole once had a balmy, sub-tropical sea because of extreme global warming, according to European scientists who have carried out the world's deepest drilling into ancient sediment on the far northern seabed. Cores retrieved from up to 430 metres (1,397 feet) below the seafloor in waters 1,300 metres (4,550 feet) deep show that, for a brief period which occurred around 55 million years ago, the Arctic Ocean was around 20 C (68 F), compared with today's typical average temperature of minus 1.5 C (29.3 F), they said on Tuesday. "It occurred during a period called...