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Today I read about climateprediction.net in the book "A Vast Machine", which is a project that uses idle computers to compute versions of GCMs for the timespan of the next 100 years, much like the SETI project did (with pattern matching for background radiation to find aliens, or something similar).
I always suspected that something like this existed, and it seems to be active still. But of course we should calculate how much CO2 is generated by computing a GCM instead letting a PC run idle :-)