the ice beneath the bears' feet starts to break up and then melt.北极熊脚下的冰层开始分裂融化

break up:破碎, 解散

The Government has broken up the large private estates.

政府把大片的私人地产分割开。

The ice began to break up on the river.

河上的冰开始碎开。

because they can't survive for as long without feeding as their mother.因为如果像母亲那样长时间不进食它们就无法生存

as long as:和 ... 一样长

This line is four times as long as that one.

这条线是那条线的四倍长。

It's now summer and these bears have a choice-take their chances on the shrinking ice floes,
现在处于夏季北极熊们有两种选择，前往正在消融的浮冰上冒险觅食

take chance:冒险

You should never take chances when driving a car.

开车时决不应冒险。

We mustn't take chances,we'd better play safe.

我们不能冒险，我们最好来稳当的。