The biggest greenhouse gas is H20, true. It's not an issue of the biggest it's the issue of the gas that tips the temperature over time. Methane is a more concentrated green house gas but dissipates rather quickly. CO2 does not. That's why it's long term effect is more pronounced at tipping the temperature.

I'm sure you all knew that though.

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The crows seemed to be calling his name, thought Caw.
- Jack Handy