Determining the natural length of the current interglacial

No glacial inception is projected to occur at the current atmospheric CO2 concentrations of 390 ppmv (ref. 1). Indeed, model experiments suggest that in the current orbital configuration-which is characterized by a weak minimum in summer insolation-glacial inception would require CO2 concentrations below preindustrial levels of 280 ppmv (refs 2-4). However, the precise CO2 threshold(4-6) as well as the timing of the hypothetical next glaciation(7) remain unclear. Past interglacials can be used to draw analogies with the present, provided their duration is known. Here we propose that the minimum age of a glacial inception is constrained by the onset of bipolar-seesaw climate variability, which requires ice-sheets large enough to produce iceberg discharges that disrupt the ocean circulation. We identify the bipolar seesaw in ice-core and North Atlantic marine records by the appearance of a distinct phasing of interhemispheric climate and hydrographic changes and ice-rafted debris. The glacial inception during Marine Isotope sub-Stage 19c, a close analogue for the present interglacial, occurred near the summer insolation minimum, suggesting that the interglacial was not prolonged by subdued radiative forcing(7). Assuming that ice growth mainly responds to insolation and CO2 forcing, this analogy suggests that the end of the current interglacial would occur within the next 1500 years, if atmospheric CO2 concentrations did not exceed 240 +/- 5 ppmv.