Currently about half of the CO2 released by humans is absorbed by the land and sea. The other half contributes to rising GHG. It is believed that GHG levels translate into higher temperatures about 50 years later. Thus the original Kyoto Protocol aimed at reducing emissions 50% by way of stabilizing CO2 at 450 ppm and committing the earth to an inevitable extra 2°C.
With the rate of emissions at 2 ppm per year, currently at 430 ppm CO2 equivalent (380 ppm CO2, plus equivalent methane and other GHGs) and accelerating, stabilizing at 450 ppm seems unrealistic. The Stern Review finds 500-550 ppm more likely and recommends 80% reduction in emissions by 2050.
Considering that the GHG absorption rate is inversely proportional to the temperature. As temperatures rises it becomes ever more difficult to reduce the GHG. Therefore, overshooting 550 ppm is not wise as we may be crossing a threshold of no return.
None the less, a 550 ppm GHG (primarily CO2) level might commit us to unprecedented 5-6°C in the next century.
