As permafrost thaws it releases carbon dioxide and in turn contributes

I think the ~option D~ IMO. Even though it has non-essential modifies, I believe that it helps to define what "speed" is. Moreover the use of "defrosts" signifies scientific facts hence i support the use of present tense. The parallelism marker "and" used in options A&B is misleading. Also the participle "defrosting" in option A, B and C is quiet of a problem whether it acts as a modifier!!!
Option E may be eliminated because of the use of present perfect tense "has been".

+1 for Option D

Open for criticism and corrections.

VERITAS PREP OFFICIAL SOLUTION:

This problem is filled with several “which” modifiers so start by analyzing those. There are two choices for the second “which” modifier at the end: in (A) and (C), the modifier goes right to the end of the sentence, while in the others, “defrosting”, “defrosts” and “has defrosted” are left outside of the modifier after a comma. A close look back shows that “Greenland” – the noun modified by the “which” clause – must have a verb: the sentence cannot end with “helps increase the speed THAT Greenland.” As a result, you can eliminate (A) and (C) because when you slash-and-burn the “which” modifier you discover a sentence construction error (no verb for Greenland).

To compare (B), (D), and (E) you should get rid of that second “which” modifier and look at each choice carefully. For (B), the sentence states: “As permafrost thaws it releases carbon dioxide and in turn contributes to global warming and helps increase the speed of Greenland...defrosting.” This sentence contains an improper series with three elements all linked with “and,” and the “speed of Greenland defrosting” is incorrect – it should be the “speed that Greenland defrosts”. (D) and (E) are very similar but feature the verb tense choice of “defrosts” and “has defrosted.” A careful analysis of the verbs used earlier in the sentence shows that the present perfect “has defrosted” is illogical: you cannot say, “As permafrost thaws it releases carbon dioxide, which…helps increase the speed that Greenland HAS defrosted.” The verb “helps increase” demands a verb in the present tense as the permafrost thawing is only changing the speed at that same time, not before. Correct answer is (D).