In the new National Geographic series “Arctic Ascent,” Alex Honnold sets his sights higher than ever before, on a 4,000-foot sea cliff in Greenland. This time he’s got ropes, but he also has the climate crisis on his mind as he’s joined by a team of scientists led by glaciologist Heïdi Sevestre. Honnold and Sevestre talked with Hari Sreenivasan about what they discovered.