In an interesting article The Naib on Sietch blog the uptake of CO2 by the environment is explained.  It highlights some interesting research from this weeks Science journal which goes into the details. 

What I find interesting about all of the carbon balancing is just how good the earth actually is at absorbing the abuse we hurl at it.  That is not to say that it will always keep up.  There is always a chance that we will manage to do just a bit too much damage and tip the balance just a bit too far and then we are really for it!