“This marks the fact that humans burning fossil fuels have caused global carbon dioxide concentrations to rise more than 120ppm since pre-industrial times,” added Tans. “Half of that rise has occurred since 1980.”

Reddit comment:

Question: If we stopped burning any fossil fuels right now would this level immediately start to go down, stay the same for a good bit, or continue rising for a bit?

This is a fantastic question. If the fossil fuels were suddenly "turned off" tomorrow, the surface/mid ocean would "rapidly" take up carbon over the next 50 years or so, but it wouldn't be completely removed from the system (by deep ocean burial) for many thousands of years. Thus CO2 initially falls rapidly after turning it off, but then levels out to something like 40% (roughly, its highly nonlinear) of the value where you stopped emitting and just stays at those levels for thousands of years (which is practically infinite compared to human timescales).

See the paper here for more information: http://www.pnas.org/content/early/2009/01/28/0812721106.abstract

1600+ Reddit comments:

http://reddit.com/r/science/comments/355z11/global_carbon_dioxide_levels_reach_400ppm/