At Stoat, an excellent post explaining how Global Circulation Models work, why you cannot find the exact same calculations twice using the same model, and why it’s meaningless to try to do so anyway :
For scientific purposes, bit-reproducibility is not necessary. The weather isn’t supposed to be any individual events anyway, so you don’t care if you get different weather.
But bit-reproducibility is only possible on the same processor type, with the same compiler (probably the same version) and the same compiler options, and exactly the same code, and exactly the same input constants, and exactly the same start files, which are themselves enormous.
If you started a model (assumed to be 100% correct) back at 1860 with 99.9999% accurately known initial conditions you wouldn’t expect it to track the individual years accurately (which is why those stupid sci-fi novels about time-travel, where they wander around being careful not to toouch anything, and only get into trouble when they accidentally crush a butterfly, are nonsense. Just being there, standing in the way of the air currents, is enough to totally change the weather and hence all of history).
I’m not sure people who comment on GCMs (asking for example “give me the code and the initialization files”) realize the sheer size and heaviness of these apps… They are not point-and-click applications with a nice GUI – usually you need dedicated multi-processor servers, where you have to fight with several users all waiting for a free slot. If you don’t know what you’re doing they are basically useless.