Still, the worst thing you can do for the environment, especially if you are a first worlder (as most reading this are), is have a child.

Stopping burning fossil fuels is much harder than it sounds. We need fertilizer from Haber-Bosch, we need tractors, we need trucks shipping food to population centers. Without fossil fuels, I know I am one of the billions that starve. I will probably eat some neighbors first though.