Seekers: Return to the Wild #2: The Melting Sea by Erin Hunter

Erin Hunter, bestselling author of the Warriors and Survivors series, continues to explore the world of bears and their environment in Seekers, the thrilling ecological adventure saga.
 
In Return to the Wild #2: The Melting Sea, Kallik has almost reached home. The bears who have become her family--Lusa, Yakone and Toklo—are devastated that they’ll have to split up, but polar bears Kallik and Yakone belong on the ice, while the black and brown bears belong in the forest.
 
At least Kallik will be reunited with her brother, Taqqiq. But when she and the others arrive at the Melting Sea, she is horrified to find the white bears almost starving due to an unnaturally early thaw. Worse, Taqqiq has rejoined a group of bullies who are terrorizing the other bears for food.