Although no glacial inception is projected to occur at current anthropogenically-driven CO₂ concentrations of 390 ppmv, considerable uncertainty remains over the natural length of the present interglacial in the absence of human interference. This arises from the unusually subdued present summer solar radiation minimum.  Low CO₂ concentrations can also promote glaciation, but modelling experiments indicate that pre-industrial CO₂ levels of 280 ppmv were not sufficiently low to lead to new ice growth.  Examination of an interglacial period ~780 thousand years ago, characterized by similar radiative signature to present, suggests that the current interglacial would be nearing its end, provided that CO₂ concentrations were 250-240ppmv.