Consumption by plants is mostly carbon neutral, since they release it again later. It looks like individual CO2 molecules survive in the atmosphere only about 5 years. But still if you put a pulse of CO2 into the atmosphere, the half life is 30 years and (since it isn't a simple exponential decay) even 100 years from now still 36% of it will be in the atmosphere. In contrast, it looks like methane is gone in 12 years.

There's much more detail in the 2007 IPCC report: http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10...