WASHINGTON — A centuries-old dream of merchants and sailors, the fabled Northwest Passage across the ice-covered top of the world, may become open for commercial shipping as soon as five summers from now. Climate records show that the Arctic zone is warming much faster than midlatitude and tropical regions of our planet, causing ice to melt at an accelerating rate. Some experts say the polar ice cap could disappear during peak months of summer well before the end of this century. The ice reportedly shrank more dramatically this year than at any time since detailed records have been kept. "The...