"50 billion billion tons is 5E+19 tons. Earth is ~5E+14 square metres in area, so that much CO2 is 100,000 tons per square metre. The equivalent of 10,000 atmospheres pressure! There's only an estimated 45 atmospheres locked up as carbonates in the crust! Someone has slipped too many zeros into the figure."

Earth:
Total mass of atmosphere: 5.1 x 10^18 kg
http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html
Venus has:
Total mass of atmosphere: ~4.8 x 10^20 kg
http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html

Hmm, at first blush something does seem wrong.

You do have the 2 billion year time period to consider, for instance, does anyone know how much CO2 Venus has lost to space over last 2 billion years?