Different tense in explaining scientific fact

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!

Answer

Reference verb tables

"Studies show that the polar ice ______(reduce) by 9.8% every 10 years
since 1978."

I would use "has reduced". Per the verb table reference, the present perfect arises from a combination of the present tense "studies show" and the perfect aspect that expresses a past event "reduction every 10 years" that has present consequences.

• "has reduced": correct


• "has been reducing": This is 'Present perfect continuous' and shows that something started in the past and has continued up until now. In order to use this, the sentence would need to read "The polar ice has been reducing continuously". This does not connect a past event to the present tense, this describes a past event that is continuing right now. The sentence focus is entirely on the event, bring it from the past, continuously to the present.


• "has been reduced": check the verb tables, this isn't grammatical at all. This is some horrible hybrid between Present perfect continuous and Past perfect continuous. This would work if you consider the verb table for "to be" and use "reduced" as an adjective to describe the noun "ice", but even in this context it is very final. The ice has been reduced, end of story.