To improve our understanding of climate system dynamics and their variability, the numerical atmospheric model ECHAM6 is used to simulate a complete glacial cycle over the past 120 000 years. However, performing such simulations takes a long time even on state-of-the-art supercomputers. To accelerate the model simulation, we propose calculating radiative transfer processes in parallel with adiabatic processes in the atmosphere, which reduces the simulation time by nearly half.