There is a solid scientific basis for concluding that humanity should be working as hard as possible to keep total warming under 2 °C. (thinkprogress.org)
In any case, the IPCC makes clear that 750 ppm would result in total warming from preindustrial levels of 4 °C by 2100. (cato-unbound.org)
If we take strong actions to limit further emissions starting today — what the authors call the AS [alternative scenario]-- we can limit total warming from preindustrial levels to about 2 °C, but even that risks sea level rise of one meter per century. (thinkprogress.org)