Remote and moving targets are most accurately hit by guided systems and not by an unguided system such as a cap set 40 years in advance. The standard approach is to use periodic estimates of the location of the target and the trajectory of the system to adjust one or more control variables, such as price.
Either an emission target or a global price target will need adjusting many times, and it will be far easier to reset one price target than to renegotiate more than one-hundred emission targets. Moreover the ultimate target should not be an emission level, but some climate variable such as a global temperature. It will be easier to set and meet such a long-range goal under a system of price commitments than a system of caps.