I (quite deliberately) didn't say methane. And this isn't wikipedia, take it or leave it. Or, you know, contribute to the discussion.

EDIT:

sorry, didn't mean to be cranky, just sick of "citation" comments on HN. If I remember correctly it was an anecdote during a BBC documentary "Beautiful Minds" [1] which is well worth watching in full. Lovelock is one of my very favourite scientists - one of the few that write like poets (along with E.O. Wilson)

[1: http://www.youtube.com/watch?v=j6H9cYScMkE]