The gleaming white Sapphire Princess docked in this deep-water port this month, unloading its passengers and taking on another 2,600 guests headed first to Glacier Bay and, eventually, Vancouver, B.C. Every day of that trip the cruise ship — whose Web site invites passengers to see Alaska’s “pristine landscapes” — will emit the same amount of sulfur dioxide as 13.1 million cars, according to the Environmental Protection Agency, and as much soot as 1.06 million cars.

But starting Aug. 1, the Sapphire Princess and every other large ship traveling within 200 miles of the coasts of the United States and Canada will have to burn cleaner fuel.