Greenhouse hot air

About once or twice a month I get an email from some global warming denier who mocks my stance that humans are the cause of most of this effect. In general, they insult me for a bit (always charming), and then pull out their trump card: "Why don't you ever talk about methane?! It's a far more potent greenhouse gas than carbon dioxide!" The triumphantly close out their missive with another insult or two, and I am chastised, sobbing, beaten. Or am I? Duh, no. I don't talk about methane because I understand math. There's a lot more CO2 in the air than methane! CO2 makes up roughly about 0.04% of the Earth's atmosphere by volume. Methane? 0.0002%. Methane is in fact a more efficient greenhouse gas than CO2, but there's so much less of it that the overall effect is much lower. Methane's contribution to the greenhouse effect is ...