Blue Mars by Kim Stanley Robinson

The red planet is red no longer, as Mars has become a perfectly inhabitable world. But while Mars flourishes, Earth is threatened by overpopulation and ecological disaster. Soon people look to Mars as a refuge, initiating a possible interplanetary conflict, as well as political strife between the Reds, who wish to preserve the planet in its desert state, and the Green “terraformers”. The ultimate fate of Earth, as well as the possibility of new explorations into the solar system, stand in the balance.