Very unusual weather in 2006 and 2013 caused a build-up of ice on the ground in the Russian Arctic, causing famine and mass starvation among reindeer, according to a new paper in Biology Letters. In a seeming paradox, a warming climate led to thick ice covering the reindeer’s grazing territories both years, killing 20,000 reindeer in 2006 and 61,000 in 2013.

The loss of nearly a quarter of the region’s reindeer population has serious implications for the future of the indigenous Nenets nomads, who use the reindeer for food, clothing, tools, transportation, and more as they migrate seasonally more than 700 miles across the tundra.

Researchers from the University of Lapland in Rovaniemi, Finland, note that Arctic warming has significantly exceeded warming in lower latitudes in recent decades. During the month of November in both 2006 and 2013—a time of year when ice typically starts to build up—ice in the Barents and Kara seas in northern Russia began to retreat, they say. This ice retreat produced unusually high levels of evaporation and humidity.

In 2006 and 2013, unseasonably warm temperatures caused rain clouds, which in turned caused torrential rains where Nenet herders pastured their reindeer. With the ground completely covered in waterlogged snow, the temperature then plummeted to minus-40 degrees C., resulting in pastures so encrusted with ice that reindeer couldn’t chip through.

During the 2013 famine, the ice covered an area around 10,425 square miles, so it wasn’t easy to for the Nenets herders to find forage for their herds. With the ice melting earlier in the spring and not freezing until later, the herders are also being forced to change centuries-old migration patterns because reindeer have difficulty walking over the snowless tundra, which is boggy when it thaws. READ MORE