Why bother with 400ppm when there are so many, well, "burning sites" untapped?
The intuitive counter-argument is that long term there shouldn't be any "burning sites" left and that capturing all burning sites would only ever get us to zero but not to negative emissions, but that's not true when you include burning sites that run on biological sources. Let plants deal with the 400ppm problem, use those plants as fuel, sequester where those plants are burnt. Negative emissions.
The intuitive counter-argument is that long term there shouldn't be any "burning sites" left and that capturing all burning sites would only ever get us to zero but not to negative emissions, but that's not true when you include burning sites that run on biological sources. Let plants deal with the 400ppm problem, use those plants as fuel, sequester where those plants are burnt. Negative emissions.