Three hikers were killed on Tanzania's 19,340-foot Kilimanjaro when rocks apparently fell from the crater wall and struck them as they climbed a gully at around 17,000 feet; other hikers and porters were injured and were evacuated from the mountain. Could global warming be at fault? Mountains are inherently instable, of course, but glaciers on Kilimanjaro have been melting back at a rapid rate in the past decade or so, and, as any mountaineer knows, warm conditions often produce rock fall as the ice and snow that cements loose rocks in place melts away. In news reports, a park official blamed "a rapid change of weather" or possibly high winds for the rock fall; indeed, high winds often trigger slides of rock loosened by melting ice. If the warming trend persists, more rock-fall incidents seem inevitable, and some scientists predict the snows of Kilimanjaro will disappear entirely within two decades. Read more about research on the vanishing glaciers of tropical and subtropical mountains in Mark Bowen's comprehensive new book, "Thin Ice." Scary stuff.