450 ppm is a stretch goal and would require average global emissions over the century to drop more than 1/3 below current emissions. Given an apparent commitment to rapid growth in emissions from China, it is almost certainly unachievable, but it is worth aspiring to.
To stabilize at 450ppm, for carbon dioxide alone, the policy maker summary from working group 1 estimates at page 16 that we need to limit carbon dioxide emissions to somewhere 1800 GtCO2 (1800 billion metric tons of carbon dioxide) on average over the course of the century. The 90% confidence band around this number is fairly large: 1370 to 2200. This recent estimate is reduced from prior estimates because of better understanding of how carbon absorption will decline as temperatures rise. Older estimates at page 196 of the Stern Report run to the high end of the range. For the sake of argument, let us be optimistic and assume that cumulative emissions of 2000 GtCO2 over the century will not take us much beyond 450ppm (note the likely overrun of another 50ppm from other GHG’s — see Stern report at page 196). The resulting average emission level of 20 GtCO2 per year (from 2000 to 2100) is just a little below 1990 levels — which were 21.5 GtCO2 for carbon dioxide alone excluding the impact of deforestation.
Current global emissions in 2004 were at 27 GtCO2/year, according to the Energy Information Administration — in other words, 35% (and probably more) above the average annual 20 GtCO2/year level required over the next century to stabilize at 450ppm for carbon dioxide alone. This total does not include the carbon dioxide impact of land use change — deforestation — which in 2000 generated another 7 GtCO2 per year according to the World Resources Institute.
So, the cut that we need to make is more than 1/3 from current levels. The true necessary cut is as perhaps half — given that we are assuming a relatively generous carbon budget and further that we haven’t included other greenhouse gases or the impact of deforestation — but let’s stay with 1/3, for the analysis here.
If we continue to overspend the annual carbon budget early in the century, deeper cuts will be necessary later in the century to stay on the carbon budget for the century. It will take us some time to reach a even one third cut and every year we spend above that level is a year we will have to spend below that level later in the century. If you imagine a straight line declining from current levels through the course of the century, and averaging 1/3 below current levels, it would make the full 1/3 cut by 2050 and reach a 2/3 cut by the end of the century.