None of the RCP scenarios consider potential positive feed back loops. This is understandable because from a climate modeling perspective these are complex and involve a lot of uncertainty/unknowns. However historical evidence suggests that in all mass extinction events involving rapid increase in atmospheric CO2 there is a point at which CO2 concentrations appear to start to dramatically increase due to some systemic trigger. We don’t know what exactly that may be, but many climate change concerns are happing “faster than expected”.
Events that cause positive feedback include things like increased CO2 emissions from wild fire, destruction of the amazon rainforest, increased albedo from melt arctic sea ice, methane release from melting permafrost, etc. We really don’t know how to account for all of these but past climate/co2 events suggest there is a “tipping point” for radical climate change.
It is also worth pointing out that climate change is only one of the ways in which our current system is completely unsustainable.
Our current economy demands perpetual growth. We are already living unsustainably. The consequences for this are obvious, and if we were remotely capable of surviving in a way remotely resembling our current standard of living we would have to immediately start scaling back production and consumption. The green new deal is a joke, and we clearly are incapable of changing our path.
As to the other point, the IPCC has studied the possibility of positive feedback loops and has concluded they’re unlikely.