Suppose you want to shut down all global coal-fired power stations in five years. Can any sustainable supply be ramped up at the rate this would require?

OK, that's a bit of an arbitrary target, but let's see. Using figures from the REN21 Global Status Report 2012, and the IEA.

Coal was about 40% of global electricity, which was around 2500GW, so we'd be looking for about 1000GW of electricity.

PV has been increasing at about 70% a year. Wind at about 30% per year. And that's without global co-ordinated effort. What could be done with such an effort? Shall we double those rates? After all, this is as serious as a World War, and so merits the same sort of industrial response.

Global annual PV production capacity was around 60GW at the start of this year. Global annual wind production capacity was around 50GW or so (hard to be sure). Let's take a PV capacity factor of 15%, and a wind capacity factor of 30%, assuming most deployment in high-yield places. So that gives us a base figure of 9GW of extra electricity from PV, and 15GW from wind, for 2012. Let's see what could be produced for the five years 2013-2017, in terms of additional electricity (not capacity, but mean power) per year:

Total (2013-2017): +1627GW of additional electricity production

So that's easily met the 1000GW, before we've accounted for any extra hydro, and any conversion of coal plant to biomass.

So, it doesn't seem entirely unfeasible, at least at the level of generation.

And how much extra nuclear could we bring on board in that time, assuming planning and design started tomorrow? Well, in round figures, as close to zero as makes no odds. And would you really want anyone living on the same planet as a panic-built nuclear fleet anyway?

Now, maybe you've got additional questions about balancing. But that would need a systems model for the entire world, and that's a decent-sized research project (call it a million Euro, off the top of my head - please send a formal request for quotation to andrew at {my EuroTrib username} dot info ;-). The bottom line is probably going to be something about building a few million gigawatt-km of extra transmission infrastructure, and converting as much of the world's existing storage hydro to pumped-storage as possible, and maybe building a hundred GW of peaking gas plant). I don't yet know how to assess the feasibility of those things.