Could this be The Caretaker Trilogy? There's a guy who has to save the oceans, rain forest, and polar ice caps. He finds a wizard named Kidah.
The story focuses on Jack Danielson, a teenager sent back from the future to save the world’s oceans. Whirlwind, the second book in the Caretaker Trilogy, tells the story of Jack’s efforts to save the Amazon rain forest; [...] In the rain forest Jack finds Eko and after some romantic scenes they find Kidah, a mysterious wizard lost in time, [...] in the final novel of the trilogy Jack has to save the polar ice caps. He visits his own time, a world of burning deserts, and has to choose between his destiny as a prince of the future and his life as an ordinary present-day teenager.
Also, according to the Cast of Characters on The Caretaker Trilogy website, Jack has a girlfriend in the present and there's a girl from the future named Eko who helps train him to fight.
P.J. PETERS: Jack’s true love, kidnapped by the Dark Lord to lure Jack into a trap. What will he endure to save her?
EKO: Sexy sensei sent back in time, Eko gives Jack a big dose of tough love while training him to use his enhanced powers to fight the Dark Army.