Verification of a GCM entails simulating past climates, comparing the simulation with the actual conditions that occurred, and adjusting the GCM to minimize the differences.
Observed global average temperature changes versus simulations via the HadGEM1 GCM assuming only natural forcing factors, human factors, or the sum of the two. The gray band denotes the uncertainty of the GCM simulation. The baseline is the average for the whole period.
Bloom (2010) Global Climate Change: Convergence of Disciplines, Sinauer Assoc.