Appositive vs Relative Clause

Both are grammatically correct, and in their current form they convey similar meanings. However, they don't need to be worded identically in other respects. For instance, your second sentence could be shortened to: "Global warming, which most scientists agree is caused by humans, will soon make humans pay." Let's look at some contrasting examples:

1) Japan, a country with a long tradition of multigenerational households, is seen by some Americans as a model to be emulated.

2) Japan, which has a long tradition of multigenerational households, is seen by some Americans as a model to be emulated.

Either of these options would work. I think #1 would be better to introduce a new idea, while #2 sounds like something from the middle of a paragraph. Again, note that when we use "which," we drop the noun that was used in the appositive. If we kept it in ("which is a country"), it would imply that we are telling our reader something they are unlikely to know. Consider this opener:

1) The GMAT, a test used for admission to MBA programs . . .
2) The GMAT, which is a test used for admission to MBA programs . . .
3) The GMAT, which is used for admission to MBA programs . . .

Note that the third option implies that we have heard of the GMAT, or at least that we know it is a test, but that we may not know that it is used for MBA admissions. In other cases, the appositive may serve this function:
1) Even the elephant, one of the largest animals in the world, begins its life as a single cell.
2) Even the elephant, which is one of the largest animals in the world, begins its life as a single cell.

Here there is no equivalent to #3 (we can't drop the word "animals"), so #1 is probably best. #1 stresses that elephants are very large, while #2 makes it seem like we don't already know that.

Feel free to try out other sentences, and I will be happy to weigh in on them, but the basic idea here is that there is no strict rule determining which form to use. They are quite similar in effect, but fine shades of meaning can make one form preferable to the other. In terms of the GMAT, I don't see this making a big difference one way or the other, except that you might go with the form that allows you to shave off a few excess words.