In all seriousness? Having 19 kids.
Researchers & scientists at the University of Oregon have actually done the math.
http://oregonstate.edu/ua/ncs/archiv...ental-emphasis
If any of those 19 kids has any kids, and any of those kids has any kids, then it creates the carbon footprint that keeps on giving.
They actually came up with a term for the ongoing impact: Carbon Legacy. Basically, for each child you have (who then creates a carbon footprint of their own), that has an impact that is 20 times greater than the impact of driving a car, recycling, using energy efficient appliances, etc etc.
Wearing a condom will be 20 times more effective at curbing greenhouse gases than trading in your Excursion for a Prius.