"The U.S. is the only industrialized nation in the world experiencing significant population growth," says Vicky Markham of the Center for Environment and Population. "That, combined with America's high rates of resource consumption, results in the largest ... environmental impact [of any nation] in the world."
No kidding! I'm just inclined to think that when weighing the utility and value of people versus SUVs, it's obvious what's got to go.
The US is not having too many children. If you want to see what cutting down on our birthrate will do, look at the problems Europe and Japan are having and the incredible difficulties they'll be having in the near future as their population ages and social services have to be cut.
And if you're really intent at keeping the US population static, cut immigration.