Independent (UK): Pen Hadow is used, figuratively, to living on thin ice. The explorer — the first person ever to trek alone and unaided to the North Pole — habitually pushes himself far beyond the limits of normal human endurance. But for the past two and a half months he has been on it literally as well. He and two colleagues — Ann Daniels and Martin Hartley — have been out on the Arctic ice cap in temperatures down to minus 90C, measuring its thickness and finding far less of it beneath their …