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.