A reasonable goal at this point in time is to stabilize CO2 concentrations around 600-650 ppm. Given the the current CO2 concentrations (~390 ppm) and the political challenges reaching a deal, an extremely ambitious goal of 350 or 450 is simply not realistic. The most important thing is that the global community agrees to take some type of action now. Setting a goal of around 650 ppm entails national policies that countries might actually be willing to agree to and, most importantly, is still aggressive enough to avoid/significantly reduce the worst risks of climate change. Most of the effort of our plan is supported by developed countries, but developing countries are still required to reduce their emissions relative to BAU.NOTE In comments L Carey clarifies: your post is not clear that the text is from a grad student submission and not an "official" MIT document - in light of the heated nature of the topic, I'd suggest that you might want to clarify that and forestall the likely "green doom-monger" comments.
Is 650 ppmv as a goal tantamount to doom? I mean, given how quickly we have slipped from 450 to 650, this is pretty scary. I hope we get some sort of a grip before even that goal slips out of reach. I think we can survive it, but not without tragic loss. We are committing our future selves and our descendants to a much diminihsed natural world, at very best.
But in any case, it makes it more urgent than ever to cut back on ancillary greenhouse gases, a goal that perhaps can achieve more support. (After all, the saturation argument, however wrongheaded, doesn't even apply to the trace GHGs.)
To that end, I will take up Alex Viets of INECE's suggestion to draw your attention to a recent New York Times article on expanding the Montreal protocol to limit HFC emissions.
In any case, the possibility of avoiding unprecedented climate change is likely out of reach. So it's biosphere management time for us. We just smashed the autopilot. Time to start thinking about steering.