Granted, the Northwest Passage has been navigable in the past. In fact, Norwegian explorer Roald Amundsen first transited the passage during his expedition of 1903-06. However, weather plays a huge part in climate, and weather is unpredictable on a large scale, as a modern day group of explorers have learned.
It seems that a climate scientist, explorer and adventurer decided that the ice would draw back far enough that the passage should surely be open. Mother Nature, of course, showed them the folly of their science. They're stuck in the ice. Granted, they're in the Northeast Passage, but....
There has been one small hiccup thus-far though: they are currently stuck in Murmansk, Russia because there is too much ice blocking the North East passage the team said didn’t exist in summer months,I'm no scientist, but I have noticed some general trends in weather, and climate. First, if you're going to attempt that trip, go in the summer. It's generally warmer during those months. Second, if you think that you can predict weather, you're doomed to spending half of your career being demonstrably wrong, at least past the three or four day mark.
Since Henry Hudson, sailors have dreamed of a reliable ocean passage in the Arctic regions of our globe. Sometimes it's navigable, sometimes it's not, and mariners seek it at their peril. For the past 500 years, it's been a craphoot thinking that you can point a boat generally Northwest and sail across the top of the world.