Caribou migrate with the seasons to search for food, avoid predators, find relief from harassing insects, and give birth. Their ability to get from place to place at precise times of the year is a condition of their survival and the survival of others in the food chain, including many Alaska Native peoples.

Weather, food supply, and predators have always been determining factors in the pathways caribou follow and the time it takes to complete their journeys. Now, with the climate changing, caribou must adapt rapidly if they are to survive. While there are some signs of shifts in migratory routes over the years to compensate for changing conditions, it is clear that adaptation will come with a cost to caribou populations.

Because warmer air holds more water vapor than colder air, warmer winter temperatures in the Arctic result in heavier snowfalls. The temperature rise also produces more "freeze-thaw" cycles, where the snow melts because of warmer temperatures, then freezes as the temperature drops at night. This is significant because the deep snow and icy cover limit access to the moss and lichen that caribou rely on for food during winter. Caribou also expend more energy walking in deeper snow and take longer to cover distances.

Steadily warmer and wetter conditions increase mosquito and black fly populations. These and other relentless insect pests drive caribou to higher, drier, and windier ground, onto ice patches, and into lakes or shallow saltwater to avoid them. These evasive maneuvers may tire the caribou or distract them from feeding. With the warmer temperatures caused by global warming, the insects will be able to follow the caribou farther north and higher up the mountains.

The caribou's survival has always been dependent on the timing of their seasonal migrations. With the timing of the seasonal changes shifting, many caribou are encountering lakes where in years past ice provided safe migration. Caribou can take longer routes around the lakes, facing exhaustion, or traverse the ice, with the risk of breaking through and dying. Earlier snow and ice melt in spring also adds danger to river crossings. Some northbound caribou drown in an onslaught of meltwater and ice.

Pregnant female caribou, called cows, that are unable to make a river crossing or take more time traveling in deep snow do not reach traditional calving grounds in time for giving birth. This is significant, as calving grounds offer plentiful and nutritious food for nursing mothers and calves if pregnant cows arrive on time. The grounds are also largely free of predators such as wolves and grizzlies, and inhabited by relatively few biting insects. In years when deep snow along the migration routes prevents caribou from reaching this preferred terrain, fewer calves survive.

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