Quick quiz: How long does carbon dioxide last in the atmosphere? If you said "a few hundred years," you're partially right -- but not completely. It turns out that the chemistry of atmospheric CO2 is a bit more complex than is generally thought. The University of Chicago's David Archer has a guest piece over at RealClimate, spelling out why pumping extra CO2 into the atmosphere makes for a real long-term mess.
When you release a slug of new CO2 into the atmosphere, dissolution in the ocean gets rid of about three quarters of it, more or less, depending on how much is released. The rest has to await neutralization by reaction with CaCO3 or igneous rocks on land and in the ocean... If one is forced to simplify reality into a single number for popular discussion, several hundred years is a sensible number to choose, because it tells three-quarters of the story, and the part of the story which applies to our own lifetimes.
However, the long tail is a lot of baby to throw out in the name of bath-time simplicity. Major ice sheets, in particular in Greenland, ocean methane clathrate deposits, and future evolution of glacial/interglacial cycles might be affected by that long tail. A better shorthand for public discussion might be that CO2 sticks around for hundreds of years, plus 25% that sticks around forever.