You can try this exercise yourself. The text below explains the variables and steps involved. You can download the climate data here and the model code here. And you can compare your results with mine, which are here. You can also change the variables to see what other future scenarios are possible. One note: the model runs on MatLab software, which can be obtained here.
Matlab source code for the energy balance model, data used in the calculations and the simulation results discussed in the Scientific American article are available at: www.meteo.psu.edu/holocene/public_html/supplements/EBMProjections