[tsimionescu]

There are some pretty clearly known positive feedback loops as well - mainly that ice has much higher albedo than water, and that ice covered areas that today are net emitters of heat will become net absorbers if they lose more ice. Specifically Greenland is known to already have become a net absorber, meaning that there is no way nkw to prevent the eventual permanent melting of all of Greenland's ice.

[scarmig]

Those are already accounted for in the 2.7C etc estimates, though. The CO2 forcing just accounts for 0.5C IIRC of the warming, with known positive feedbacks (most notably water vapor, but also albedo etc) driving the rest.

The question becomes, are unknown feedbacks more likely to be positive or negative? I'd lean toward negative, which is what most experts think, but definitely an unknown.