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!

  • 5
    Why complicate things? Studies show that the polar ice reduced by 9.8% every 10 years since 1978. Present tense would imply "universal / timeless scientific truth", which isn't really appropriate for such a short range of observations. – FumbleFingers May 29 '17 at 15:22
  • 2
    Pretty much all of your options are correct. The difference (to the extent that there is one) is in the immediacy of the information -- "is reduced" and "is reducing" is predictive of the future (or at least refers to the assumed present conditions), while "has..." refers to historical information and does not predict (beyond the point that history tends to repeat). – Hot Licks May 29 '17 at 20:01
  • 1
    I'd use "decline" instead of "reduce," and delete the. "Studies show that polar ice has declined 9.8 percent every 10 years since 1978. (That's a little short of three 10-year periods.) "has been declining" would also be good. – Xanne May 29 '17 at 20:14

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.

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

The sentence may be better if it reads...

"Studies have shown that, since 1978, the polar ice reduce by 9.8% every 10 years."


"Studies have shown that the polar ice have reduced by 9.8% every 10 years, since 1978."

Referring to this writing guide LINK, I would suggest writing in the present perfect tense.

The present perfect is mostly used for referring to previous research in the field or to your own previous findings. Since the present perfect is a present tense, it implies that the result is still true and relevant today.

The present simple could also be used here, but the present perfect focuses more on what has been done than on what is known to be true now (present simple).

If you are in the United States I would refer you to the Modern Language Association (MLA) guidelines as a widely accepted set of academic writing procedures. Though, there are other formats, like the American Psychological Association (APA) guidelines, so check with your specific institution to confirm which is used.

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