Population Zero by Wrath James White

When Todd was just a child, he learned that sometimes it was necessary to sterilize or euthanize animals in order to keep their population from growing too rapidly. It was the humane thing to do, the best thing for the environment. Yet, every day at his job at the Welfare Department, Todd sees the dregs of humanity multiplying unchecked, overburdening the earth with a tidal wave of humanity. But if he can convince them not to reproduce, if he can convince everyone to voluntarily sterilize themselves, then he might just prevent the coming population explosion. And those who can't be convinced…

Population Zero is the story of an environmental activist named Todd Hammerstein who is on a mission to save the planet. In just 50 years the population of the planet is expected to double. But not if Todd can help it.