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It's true that methane (CH4) is a potent greenhouse gas, but unlike CO2, it doesn't have a very long lifetime in the atmosphere. Release a molecule of methane today, and in a few years it will combine with oxygen: CH4 + 2O2 -> CO2 + 2H2O. That's an exothermic reaction, meaning you get energy out. A molecule of CO2, stays around for thousands to millions of years, since the reactions to convert it to C + O2 (or various carbohydrates, etc) are all endothermic, meaning you have to put energy in via e.g. photosynthesis.
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