Use of the conjunction “before”

I think before in your example would be classified nowadays as an intransitive preposition; such prepositions often have an adverbial sense.

Before in your phrase goes with "never witnessed before"; it does not belong with which; before is not "taking" a relative clause.

... a phenomenon never witnessed before, ^{a phenomenon} which could be driven by rising temperatures

P.S. Here is an example where before takes which as its object:

You may enter the temple through this door, before which, please remove your shoes.

When before has as its complement something that could stand on its own as a full declarative sentence, traditionally before would be classed as a subordinating conjunction. There's no relative clause as a relative word could not be added in while keeping the meaning the same.

I came home [before [it started to rain]].

=/=

?I came home, [[before which] it started to rain].

When there's relative word which involved, before has as complement that word only, not the whole relative clause. In this case, before would traditionally be classed as a preposition, which being a relative pronoun.

Local news outlets reported that it took several hours and 50 people to return the Antarctic animal home,[{before[which]} it had wandered a whopping 10 blocks through the city center]. (NY Post)

Here, which is understood as returning home, as in

It had wandered a whopping 10 blocks through the city center before returning home.

The example given in the OP however falls into a different category as there is no written out complement of before, moving before here into the traditional classification of adverb, and the whole string being a noun phrase that acts as an ascriptive supplement to a subglacial lake erupted through the Greenland ice sheet.

Satellite images reveal how a subglacial lake erupted through the Greenland ice sheet – a phenomenon never witnessed before which could be driven by rising temperatures. (New Scientist)

Or reworded slightly,

A subglacial lake erupting through the Greenland ice sheet is a phenomenon never witnessed before.

This phenomenon could be driven by rising temperatures.

However, clearly before has an implied complement now, as in never witnessed before now - or possibly before some unspecified, but obvious time, like the time at which this was observed.

But, what is a conjunction?

That largely depends on your definition of this frequently used, but loosely defined item of grammar terminology. If your only criteria is that a conjunction appears between two strings that could stand on their own as complete sentences, then it appears that both before and before which would be instances of conjunctions in the first two examples above, but not before in the example from the OP - that is, unless you believe there is an elided clause along the lines of before it was observed on this date.

The Cambridge Grammar of the English Language proposes that before is simply a preposition in all these cases, and that prepositions, like verbs, should not change their part of speech classification based on their complement. In fact, they recommend doing away with the troublesome category of 'conjunction' altogether in favor of simply having prepositions (which the vast majority of 'subordinating conjunctions' would fall into), coordinators, and a very small class of subordinators. This would simplify the present conundrum significantly.

“never witnessed before” is a compound adjective

The “phenomenon” (noun) has the property “never witnessed before” (adjective).

So, “before” is not being used as a conjunction. You would see this more clearly if it was reworded “a phenomenon never before witnessed which could be driven by rising temperatures,”.