From National Geographic:
In recent years, warmer temperatures have been changing the structure of the snow—with devastating effects for the lemmings.
Rather than remaining below freezing for most of the winter, temperatures have bounced above freezing a number of times, melting and then refreezing the snowpack.
"This enables water to enter the system, flooding the snow tunnels and then forming ice layers on the ground," Kausrud said.
Many lemmings drown when their burrows are flooded, and those that survive often starve when their food is trapped under an icy layer.
Unable to gorge themselves on lemmings every few years, predators like the Arctic fox and snowy owl have had to rely on other food sources, such as ptarmigan (a kind of grouse) and willow grouse.
As a result, the numbers of these ground-nesting birds have been diminishing. "Lemming population explosions help to take the pressure off ptarmigan and willow grouse," Coulson explained.
Although the study looks only at one region in Norway, Kausrud and his colleagues believe the effect could be widespread.