Without big feedback loops you actually have to go out of your way to get above a certain level of warming. RCP8.5, for 8.5C of warming, involves a frankly silly quantity of emissions, I assume this is because CO2 absorbs at certain wavelengths and there are diminishing returns as concentrations get really high.

Of course, we don’t know what the feedback loops are, they could kick in and take over. The IPCC is quite conservative about including feedback loops, and mainly deals with direct effects. But the same logic applies either way, within any reasonable range any reduction in CO2 emissions is worth pursuing.

I also personally think the economics for certain low carbon technologies will dominate over a middle/long time period, just that as it stands they won’t take over quick enough to dramatically bring down emissions.