That's essentially the though experiment discussed by Davis et al. in last week's Science [LINK]. They tried to estimate what climate change would occur if CO2-emitting devices that currently exist were used until their expected lifetime, at which point they are taken offline and non-emitting devices are installed. Basically, they say, okay, we've got a lot of coal-fired power plants (and gas or oil plants and also cars and trucks), so what if we just use them until they're not good (say ~35 years for power plants)? In the interim, no additional CO2-emitting devices are introduced. The conclusion is that, based on a vast set of rough approximations and an intermediate complexity climate model, the atmospheric CO2 could be kept below 430 ppm, keeping global warming at a "comfortable" 1.3 degrees or so.
From my reading of the paper, the important insight is that the most dangerous CO2 emissions -- those that will take us past 450ppm and then past 550ppm -- have not yet been built. This is a scary realization because if their estimates are close, then we know we aren't committed to "dangerous anthropogenic warming" yet, but we're about to be. We see what's coming, but won't do anything about it. We're like the proverbial lemmings heading over the cliff.
Now, realistically we might be in worse shape than the scenarios addressed in the paper, and the authors acknowledge that. In particular, since there are such strong economic/societal incentives to use the cheap (but dirty) fuel sources, it is impossible to achieve a scenario that is even remotely like the one simulated by Davis et al. At the end, they also touch on the China and India issues. These countries are in the midst of rapid industrialization, and they aren't going to turn back. And they are going to account for a large amount of the accumulated emissions over the next 50 years or more. The developing world, under the Davis et al. scenario, would be left behind, too, unless they employ an incredible leap-frog to using clean energy sources. In the end, this study the idealistic scenario, one small step past the most idealized scenario of 'what if we stop emitting CO2 completely today?' The lessons we learn directly are mostly about what we are committed to already. That makes these kinds of studies very policy relevant, but of course few policy makers seem to care. Indirectly, science also progresses, as these are the kinds of studies that help us understand the interactions between the societal decisions and the climate system, and can lead to more fundamental understanding of timescales in the perturbed climate system.
Steven J. Davis, Ken Caldeira, H. Damon Matthews
Emissions and Climate Change from Existing Energy Infrastructure
Science, Vol. 329 no. 5997 pp. 1330-1333
10 September 2010