In the following sentence:
"Studies show that the polar ice ______(reduce) by 9.8% every 10 years since 1978."
In the blank we should write the corresponding tense of reduce.
At the first sight I thought it is "is reduced", because I think it is a basic scientific fact, which is quite widely accepted. Perhaps "is reducing" is also acceptable; is there a difference in meaning between the two?
But my friend asks me if "has reduced", "has been reducing", and "has been reduced" are also grammatically correct, or if they express some subtlety different meaning.
I can't tell the difference or "right or wrong" of these options. Can anyone help?
Thanks in advance!