Marco Werman climbs aboard the largest passenger ship ever to sail through the Northwest Passage.

Also: we speak to residents of the Arctic with mixed feelings about cruise ships sailing past their towns; we meet climate change scientists risking their lives to gather data in the field; we visit a marshland that's worth millions of dollars; we spend the day with teens saving songbirds in Washington, DC; and we learn how American climate change policies have changed this past year.

(Image:The Crystal Serenity docked at the Boston cruise terminal near the end of its 32-day Northwest Passage journey. Credit: PRI’s The World)