The study provides a model of CO2 in the atmosphere based on the global carbon cycle and the Kaya identity. The influences of: 1) energy intensity of GDP; 2) carbon intensity of energy on CO2 trajectories are given under four scenarios. The results from the most optimistic and technologically challenging scenario show that the atmospheric CO2 concentration can stabilise at 610 ppmv. It is also shown that the annual growth rates of atmospheric CO2 peak for all the scenarios before 2100 due to the expected world population peak in 2075 and the large share of fossil fuel energy.