In 2018, the water in Hudson Bay didn't freeze until December 12.

That was very late. “Sea ice is finally forming," one scientist reported. “The polar bears are moving quickly offshore." The Arctic- the polar bears' habitat- is changing.

Temperatures have gone up about 3 degrees Celsius since 1900.

The ice cover is becoming smaller, too.

In 2019, it was 30 percent smaller than it was twenty-five years earlier. And each year, the remaining ice cover is melting faster and faster.

老师：这一句“ it was 30 percent smaller than it was twenty-five years earlier.” 中的earlier = before 吗？为什么不使用过去完成时...than it had been twenty-five years earlier(=before) ？？？