A comment posted in a thread occasioned by Jonathon Porritt on Green prudence :
Alex Lockwood makes a useful addition to this thread. He and some others will be aware that stabilisation at 450ppmv, never mind 550 may pose very large risks -- risks that may come to seem unacceptably high once they are better understood. James Hansen and some colleagues recommend 350 (see also the campaign group 350.org). It may be that the only 'safe' concentration is even lower - close to pre-industrial levels!
It's hard to see how this can be achieved without a vast array of policies and technologies including, even, scrubbers of the kind championed by Wally Broeker.
Broeker suggests the scrubbers could remove CO2 for around $10 to $20 per tonne. On top of that you have sequestration cost, assuming sequestration is feasible. And as Broeker states, this would only be a part of a solution -- that is, we'd to have a wholesale green energy revolution, massively increase efficiency and much else besides as well.
Maybe the costs are close to impossible to characterize beyond arm waving at this stage. They might be on the order of defense budgets as a percentage of GDP - tat is something like twice the 2% of GDP I understand Stern is now talking about. How many election cycles, economic downturns and other events of a bumpy nature before that becomes an acceptable idea for journalists, politicians and others, if it ever does?