It seems obvious to me that this is unrealistic - that we're not going to do the cuts that we "need"; People will not get worried enough to take the steps necessary to avert it, it's simply not going to happen, and it's wishful thinking to expect that people will change their behaviour so much before they have immediate first-hand horrible effects (and those effects won't happen before it's too late). If that is our plan, then why not just wish for rainbow-farting unicorns to fix our ecosystem, it's just as likely to happen but more effective? Everything we know about people's psychology, social structures and politics seems to indicate that if the global population will be given a choice between those reductions and a slow-motion catastrophe, they'll pick policies that result in this catastrophe. Such reductions won't be made unilaterally or willingly, they might happen only if enforced with extreme violence (i.e. short-term threats instead of long-term horrible expectations) but no one sufficiently powerful seems willing to fight internal unrest or threaten international wars to enforce drastic measures for the sake of preventing climate change.

So we have to start with an assumption that emission cuts of the required scale are not going to happen (or, at least, are not going to happen soon enough), and look for more realistic alternatives than convincing 7 billion people to voluntarily downshift their lifestyle by 2050. Large scale carbon sequestration is one such alternative option.