Xone of Contention — Piers Anthony

Climate and culture shock ensue when Xanthians swap bodies with mere humans in this madcap adventure in the New York Timesâ€"bestselling series.
 
All Breanna of the Black Wave and her newfound love, Justin Tree, want is a little time to get to know each other better, but a climatic catastrophe is causing a massive meltdown that threatens to inundate the ancient forests of Xanth.
 
The only way to avert this dire disaster is to undertake a voyage into the distant mists of the past, to find the moment when this ghastly greenhouse effect began. And the only people who can safely make that journey are those unaffected by Xanth's magic -- normal, ordinary humans from our own world.
 
So Breanna and Justin turn to the Demon X(A/N)th (a.k.a. Nimby) and his lovely consort Chlorine for help. Together, they devise a daring plan. Making use of the O-Xone, a magical computer network that links the worlds of Xanth and Earth, Nimby and Chlorine make contact with a young couple from Earth who are working on a Xanth game, and arrange to exchange bodies with them. But an unexpected surprise awaits them on their arrival . . .