National Geographic’s new documentary film, “Time and Water,” grapples with a challenging, profound question: How do you say goodbye to what you never thought you could lose? Through archival footage, photos, art, and science, Academy Award-nominated director Sara Dosa follows acclaimed Icelandic writer and poet Andri Snær Magnason as he confronts the death of his country’s glaciers, the loss of his grandparents, and the kind of world he hopes future generations can experience. The story’s next chapters are being written at this very second. A ‘Polytemporal’ Film Dosa and her team combine contemporary footage with official archival materials and a massive library of personal films that Andri Magnason captured over the past three decades.…