In2018, the water in Hudson Bay didn't freeze until December 12.
Thatwas very late. “Sea ice is finally forming," one scientist reported. “Thepolar bears are moving quickly offshore." The Arctic| the polar bears'habitat| is changing.
Temperatureshave gone up about 3 degrees Celsius since 1900.
Theice cover is becoming smaller, too.
In2019, it was 30 percent smaller than it was twenty|five years earlier. And eachyear, 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？为何不用过去完成时...than ithad been twenty|five years earlier ？？？
earlier可以用before替换。在非正式语体中，时间关系明确的状况下，可以用一般过去年代替过去完成时。比如，More people came than was/had been invited.
