If one looks at actual evidence (rather than modeled responses), as do Hansen and Michaels, it is the lower end of the range that is more likely. The impacts associated with warming at the low end of the IPCC TAR range are far less dramatic and infinitely more manageable than those that accompany high-end warming.
It seems about time to dispense with the notion that future warming will be catastrophic and begin to focus on the implications of a modest warming where benefits are likely to outweigh costs.
Then you said,
I.e., if it warms to the point that large-scale melting of glaciers and ice sheet commences, we're going to be in trouble.
Give it up. If humans made a maximum effort to reduce production of carbon dioxide, the consequent reduction in heating is so small as to be virtually unmeasurable. Worse, our economic ability to fund the means to accommodate climate variation will have been seriously curtailed. Both humans and nature would be better off if we expend our efforts learning to manage habitat wisely on microscopic bases.
Hansen's alternative scenario addresses future increases in atmospheric CO2 via technological advances, which I agree with. The main area of emissions where he thinks control is required is on black soot, not CO2. As for your latter point, I agree that effective habitat management would be a significant boon for several areas of environmental concern.