Standard Carbon

How can I know that carbon dioxide is an acid at standard conditions?

The pKa of carbon dioxide is 6.3 and 10.3. This does not convince me that CO2 is acid because 10.3 indicates me that it is very weak acid, perhaps a base.

The CO2 must be aqueous to be an acid
H2O + CO2 <==> H2CO3

This is a very weak acid.
One proton can be neutralized at pKa 6.3. This leaves
HCO3- The second proton is more difficult to remove - it comes off at pKa 10.3.