I can't be the only one who's noticed this, but no one else has come out with this theory, so here it goes. Throughout the last two books, there has been constant mentions of weird weather all around the globe (e.g. Mt. Vesuvius, the Pacific northwest, the eastern coast, Germany, and so on). But anyways, all these references to strange weather can't be coincidental. The Vesper's grand plan has to involve the earth's climate patterns in some way.
On a related note, with all the celestial-related things that Amy and Dan have had to fetch, I'm making an educated guess that these two things are connected, and that the plan involves objects in space as well, and that it is all woven together somehow. I guess all will be revealed in Book 5! (I have another theory which received much aclame on the 39 Clues site, click here to read it). Thanks!