In November 2013, 61,000 reindeer starved to death on Russia's Yamal Peninsula, marking the largest regional "mortality episode" of reindeer ever recorded. An additional 20,000 had succumbed to famine in November 2006. The immediate cause, according to a team of researchers from Europe, the United States and Asia, was an unusual ice barrier that smothered the reindeer pastures. Reindeer can stamp through ice about three-quarters of an inch thick, using their feet to access the nutritious lichen and plants below. But in early November 2006 and 2013, the ice was an order of magnitude deeper, too tough even for the reindeer's sharp hoofs. Unable to eat, the animals died. The study linked several events into a chain, some of which had previously been observed only as discrete events, the authors noted. As the scientists summarized the sequence, "Warming - Sea ice decline - Increased precipitation and winter temperatures - Rain-on-snow events - Reindeer mortality."