Climatic and Climactic

Climatic means "regarding the climate."  Pineapples won't grow in Canada because of climatic conditions.  Climatic changes are causing the polar ice caps to shrink.

Climactic refers to the climax of something.  The highest point of tension in a story, when the main conflict is about to be resolved, is the climax of the story.  When Luke Skywalker faces Darth Vader, it is a climactic moment.