Usually these thought experiments are framed in terms of Gt C (gigatons of carbon), not CO2 by weight. This allows for easier conversion.<p>I was going to bang out some numbers from my undergrad climate science course notes... but Wikipedia's article about the carbon cycle has the figures I was looking for. Check out the box on the right under "main components":
<a href="http://en.wikipedia.org/wiki/Carbon_cycle" rel="nofollow">http://en.wikipedia.org/wiki/Carbon_cycle</a><p>The fossil fuel estimates look low to me, particularly "other (peat)". Permafrost soils and peatlands contain about as much carbon as the atmosphere does. Circumboreal (e.g. Canada, Russia, and Fennoscandia) permafrost soils and peatlands are estimated to contain 273 to 547 Gt C alone (and I even have some references: Gorham 1991; Turunen et al. 2002; Yu et al. 2010).<p>Merely doubling atmospheric carbon content by burning up all available fossil fuels would be nice... but the reality is much worse.