Timelock

In Timelock, the thrilling finale to the Caretaker Trilogy, the end of life as we know it has arrived, and history will be made -- or lost -- at the hands of one young hero.
Jack Danielson has spent the last year saving the oceans and the Amazon, attempting to fulfill a prophecy that was written before his birth. Now he's more than ready to get back to life as a typical teenager and spend some quality time with his girlfriend, P.J. Too bad the world has other plans.
Wrenched away once more from those he knows and loves, Jack is thrust through time to the fiery deserts of the future and the frozen tundra of the Arctic, battling cyborgs, zombie warlocks, and scorpions the size of tanks. At least he has Gisco -- everyone's favorite surly telepathic canine -- to keep him company, not to mention the Ninja Babe, Eko. And he will finally be reunited with the parents who abandoned him so long ago, in order that he might save their dying planet.
But it isn't only a race to save Earth. As the clock ticks down before the final confrontation with the dreaded Dark Lord, Jack must decide once and for all who he really is -- prince of the future or humble human of the present -- and choose between the two women who love him.