Uptake by the Ocean
The main way anthropogenic CO2 is removed from the atmosphere is through oceanic uptake.  This occurs because CO2 dissolves in seawater.  The excess carbon is eventually incorporated into the skeletons of marine organisms and buried in deep sea sediments as the organisms die and fall to the bottom of the ocean.
This process is thought to remove about 2 gigatons of carbon per year from the atmosphere.  So we started with 7.5 gigatons, 3 are in the atmosphere, 2 are taken up by the ocean, leaving us with 2.5 gigatons still to be accounted for.