The figures I mentioned are in Working Group III of the IPCC report. I wasn't part of that, so I'm not intimately familiar with all of the arguments.
In the end, the IPCC chose to set a range. That's because there's a whole lot of assumptions in the models about what individual countries could do. That's why the range for the strongest levels is somewhere between about 40% and 95%.
You're right that if we're going to do that--I think Dr. Weaver mentioned something similar--we will certainly need to de-carbonize our economy. We'll need to reduce the amount of carbon in the fuels we use, which means using less and less coal, oil, and gas, and using more and more renewable energy sources such as biofuels, solar energy, wind energy, tidal energy, and the like, in addition to conserving.