We all know the climate changes, it has in the past, it is changing now and it will in the future, but what is different about what we are doing now compared to what happened in the Cretaceous?

Would it have been more appropriate to use had in the sentence above? Does using the perfect here imply the repeatability of climate change events, that they occurred many times before?

  • I have a bigger concern about the number matching: changes versus it has, it is and it will. – virolino Feb 18 '19 at 5:58
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    I suppose that "changes" here is a verb and "it" refers to "the climate". – user74785 Feb 18 '19 at 6:02
  • Wow! I did not see that, but in this case a that was really welcome: "We all know that the climate changes...". Or maybe use semi-colon instead of comma after "changes"? Verbally, the speaker can add intonation, but in writing... Tnx for the heads-up. – virolino Feb 18 '19 at 6:05
  • I agree. Plus, combining various tenses in a list, such as in the example above, seems to me (as a learner) to be a dangerous area. Just exploring the limits how to use them properly. – user74785 Feb 18 '19 at 6:25

If you put “had” it implies the change happened then stopped. Ie it was a “one off” change that was complete and finished.

Using “has” implies that the change was not a unique event and it can still change.

  • So, if I used the present simple here, it would necessarily mean that an event occurred only once in the past? – user74785 Feb 18 '19 at 6:16
  • I mean, it's now a matter of style, right? The past simple just don't fit here? – user74785 Feb 18 '19 at 6:20

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