Projecting future emissions is by no means simply. The underlying conceit in much of the discussion (e.g. see Pielke et al.) of late is that the projected emissions in the scenarios used by all the climate models are too low. In other words, climate change will be even worse than the models say, and the runaway emissions train will be even harder to stop than we think.

Now here's the same graph with the high growth, medium (reference scenario) and low growth scenarios from the IEO (coarse grey/black lines). And the surprise: the path of the

There is a caveat. The red line on the graph comes from a recent paper by Sheehan in Climatic Change (see Pielke's commentary). This scenario attempts to take into account the "abrupt shift to rapid growth based on fossil fuels" that has occurred in Asian countries. The paper argues that the SRES scenarios assumed too much decarbonization of the energy system and the economy. The SRES approach is reasonable -- as I've argued before, the emissions intensity of the world economy, the amount of carbon pumped out per unit of GDP, has been declining for decades as we became more efficient at making producing stuff. The problem is that we're at a moment where Asian are using coal and oil to expand, so the global decarbonization is slowing down or even stopping (because the carbon emissions per unit energy production is rising). How and whether recent Asian fossil fuel growth should be extrapolated 25 years into the future, I can't say. I'd be interested to see a comparison of Sheehan-style scenarios with those of the IEO.
(By the way, the IPCC is planning a new set of scenarios for the fifth assessment)