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 only about half or less that of carbon dioxide.
Incidentally, water vapor is far and away the biggest contributor to greenhouse warming. The amount in the air is hugely variable, but relatively unaffected by man's activities. So over time, those variations even out, and the contribution of warming from water vapor is steady. The increase we see in temperature -- and there is an increase in temperature -- cannot be from water vapor, and the methane contribution is small. We also know it's not from the Sun, either.
That's why atmospheric scientists primarily study carbon dioxide.
And they've been studying it a long, long time. It's a very difficult field of research, fraught with hidden variables, difficult measurements, and political landmines. But chances are they know more about this than you and I do. There's a reason they're called experts, folks.
So the next time you want to send me some snarky email to embarrass me about some piece of info you just found on the intertoobs, please do yourself a favor: stop, think for just a moment, and ask yourself: "Is this really likely to have been missed by thousands of really smart highly educated people who have been studying this field for a combined length of time equaling many man-millennia?" The embarrassment you save just might be your own.