No, all the scenarios do not assume carbon capture and storage. The RCP 8.5 scenario is actually a high emissions scenario that experts think overestimates carbon emissions given existing trends.
As to the other point, the IPCC has studied the possibility of positive feedback loops and has concluded they’re unlikely.
Does that scenario include destabilization of societies and war? If the current level of migration is causing political strife in the US and EU, do you think that orders of magnitude more might cause much more severe problems?
As to the other point, the IPCC has studied the possibility of positive feedback loops and has concluded they’re unlikely.