As far as hiking buddies go, Barack Obama and Bear Grylls make for an unlikely pairing. Grylls is a TV outdoorsman with a flair for drama and a propensity for drinking his own urine. Obama is cerebral and stolidly even-keeled, and, you know, the president of the United States. Nevertheless, the strange duo is slated for some bonding Tuesday while hiking the Exit Glacier, a south Alaskan river of ice that has shrunk by 1.25 miles in recent decades. They have no plans to drink urine, Grylls has confirmed, and the adventurer’s usually madcap exploits will be scaled down to...