oompa, are you really interested in the physics and chemistry equations that go into how scientists model this? Because I will really get them for you if you want... I know many posters here just post rhetorical (sometimes beligerent) comments but I will actually get you that info if you want me to.
Short answer, you and I know that scientists have not been taking measurements for thousands of years (on anything) yet we still know thousands of things about the natural world in the past. We know the physical mechanisms for uptake of carbon in the ocean, by plants by direct measurements and understanding the physics and chemistry behind the measurements. We know the long-term uptake by carbonates in the sea-floor and in rock as well.
So-- there is long term uptake for carbon dioxide: this is on the order of 10s of thousands of years -or more.
--we know the immediate-term uptake for carbon dioxide which is pretty fast- on the order of years to decades- this is done by the surface ocean and plants (photosynthesis etc).
--we know the ~mid-term uptake for carbon dioxide into the deep ocean and soils which is tens to hundreds of years
All in all- it IS pretty hard to give ONE number for the length of time that a specific CO2 stays in the atmosphere. BUT we do know that 15 - 30% of CO2 remains in the atmosphere for 200 - 300 years. For ALL of anthropogenic CO2 to be absorbed...? You are looking at 1000s of years. This isn't just me accepting what people tell me. And the 15 - 30% remainng ~300 years really isn't diputed... ...it's all physics and chemistry.
oompa-- you are quite vehement that you aren't going to believe this 1000 years thing, that I am just brainwashed into believing what I want to hear from scientists... Could you please (and I am serious, not being facetious) explain to me how you formulate your ideas (and you seem to have strong ones) about climate change? What evidence to you choose to accept? Where do you get it? I feel quite confident about what I am learning. I wouldn't spout off ideas about something I have not studied like, say, economics or African politics for example- I would listen to the scientists that actually study these topics, not sensational media coverage of economics or African politics... well, I am sure you understand the point I am making.
We are here to listen and learn from one another, not to call names! A bunch of us might need a time out!
-K
p.s. I will post a graph (I can't find the exact one I want -which is waaaaay cooler) that is sort of what I said I will post. The real cooler one is on a network of mine that I can't map to right now. rrrgh.