The satellite data showing Arctic ice at record low levels is pretty striking but, as I mentioned, unlikely to change many minds at this stage. One point that can reasonably be made is that satellites have only been around for thirty years – maybe there was less ice further back in history. There is one sort of evidence that goes back many centuries – the repeated failure to find the fabled Northwest Passage, by voyagers going back at least as far as John Cabot in 1497, and arguably as far back as the Vikings in the 11th century. Roald Amundsen managed it in three years in 1903-06 and the voyage was managed with the aid of icebreakers in the middle of the 20th century. in 1984, a specially designed cruise ship managed the voyage, but now it’s become almost routine. “ In 2011 16 private yachts made their way successfully through the once-dreaded passage. ” Here’s an example of the kind of boat that can now manage, within a season, a voyage that defeated the well-equipped expedition of Sir John Franklin, with the loss of the entire crew.
Of course, in a parallel universe, the feat was accomplished by a Chinese fleet in 1421.