Years ago I heard a woman who told me she is working with her son on a campaign to spread the word that CO2 emissions from burning gasoline are worse than the excrement we defecate. That's true for anybody that uses more than one gallon of gas a week. Being a non car owner I have some other problem that is "my worst". (Probably sucking electricity out of the wall or eating food.) It's a minor carbon footprint issue compared to what driving is though.