The word "abate" means to become less intense or widespread, to diminish, or to subside. In the context of the sentence, if the greenhouse effect is beginning to abate, it suggests a reduction or lessening of its impact. The opposite of "abate" is "intensify," which means to become more intense or to increase. Therefore, option A, "intensify," is the most appropriate choice as it conveys the opposite meaning, indicating an increase or worsening of the greenhouse effect.