The last two sentences of P2 don’t describe observations that have been predicted by the greenhouse theory. The claim about sulfates and how they can counteract the heating effect of greenhouse gases isn’t something predicted by the greenhouse theory.

The author doesn’t argue that a new theory will need to be formulated. In fact, we know the author accepts the greenhouse theory at the end of P3. So it wouldn’t make sense for the author to argue that we need a new theory besides the greenhouse theory.

Actually, the last two sentences of P2 establish that scientists were able to show how the greenhouse theory can still account for observed phenomena. Also, there’s no indication that the greenhouse theory was “previously accepted.” In fact, the author describes the link between greenhouse gases and global warming as “controversial” in P1.

This best captures the purpose. The first half of P2 describes how early models of the greenhouse effect were not consistent with observed temperature increases, and some people therefore doubted the greenhouse theory. But in the last two sentences, the author shows how scientists reconciled the greenhouse theory with observed data by taking into account a previously overlooked variable (sulfates) that affects global temperature. Once additional data concerning sulfates is considered, the greenhouse theory can be defended.

We don’t get any discussion of competing theories in P2. Although we do know that some opponents doubted the greenhouse theory because of data that appeared to be inconsistent with early models of the greenhouse effect, we’re not told that those opponents advocated a competing theory. In any case, the author’s purpose in the last two sentences of P2 isn’t to propose a way of resolving a dispute. It’s to show that scientists did in fact reconcile apparently inconsistent evidence with the greenhouse theory and that current models of the greenhouse effect are consistent.