We know enough about the carbon cycle to say that at least of the
order of a quarter of the CO2 emitted in burning fossil fuels under a
BAU scenario will stay in the air for an eternity, the latter defined
practically as more than 500 years.

This implies, that whatever we do, CO2 levels will still exceed 305 ppm 500 years from now. 305 ppm was about the level CO2 peaked at during the Eemian. In addition to cutting emissions to slow warming, we must also prepare for Eemian climate and sea level.