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1989; Phillipps and Held, 1994). Either the models are incomplete or orbitally induced solar insolation variations are at best only a catalyst for glacial/interglacial changes. Both of these conclusions have important implications for global change projections: the former implies that contemporary GCMs might not be sufficiently sensitive to solar radiative forcing (whether of orbital or solar activity origin), while the latter emphasizes that it is the climate system feedbacks that are most important in producing climate change, invalidating the use of simple transfer functions between radiative forcing perturbations and climatic responses.