The past rapid growth of Northern Hemisphere continental ice sheets, which terminated warm and stable climate periods, is generally attributed to reduced summer insolation in boreal latitudes1, 2, 3. Yet such summer insolation is near to its minimum at present4, and there are no signs of a new ice age5. This challenges our understanding of the mechanisms driving glacial cycles and our ability to predict the next glacial inception6. Here we propose a critical functional relationship between boreal summer insolation and global carbon dioxide (CO2) concentration, which explains the beginning of the past eight glacial cycles and might anticipate future periods of glacial inception. Using an ensemble of simulations generated by an Earth system model of intermediate complexity constrained by palaeoclimatic data, we suggest that glacial inception was narrowly missed before the beginning of the Industrial Revolution. The missed inception can be accounted for by the combined effect of relatively high late-Holocene CO2 concentrations and the low orbital eccentricity of the Earth7. Additionally, our analysis suggests that even in the absence of human perturbations no substantial build-up of ice sheets would occur within the next several thousand years and that the current interglacial would probably last for another 50,000 years. However, moderate anthropogenic cumulative CO2 emissions of 1,000 to 1,500 gigatonnes of carbon will postpone the next glacial inception by at least 100,000 years8, 9. Our simulations demonstrate that under natural conditions alone the Earth system would be expected to remain in the present delicately balanced interglacial climate state, steering clear of both large-scale glaciation of the Northern Hemisphere and its complete deglaciation, for an unusually long time.
At a glance
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Extended data figures and tables
Extended Data Figures
- Extended Data Figure 1: Ice sheet at 0 kyr bp. (154 KB)
The extension and elevation of simulated Northern Hemisphere ice sheets at the time corresponding to present-day (0 kyr bp, ‘OK’) insolation are shown for constant CO2 concentrations of 280 p.p.m. (a) and 240 p.p.m. (b). Experiments were performed with the coldest of the accepted model versions.
- Extended Data Figure 2: Detection of the critical CO2 value. (271 KB)
For the orbital configuration corresponding to 777 kyr bp we show the prescribed CO2 concentration (a), the simulated Northern Hemisphere (NH) ice volume in metres of sea-level equivalent (msl; b), and the simulated global mean surface air temperature (SAT) (c). Blue lines correspond to the coldest of accepted model versions and red lines to the warmest. The figure shows only the part of 200,000-year-long simulations for which glacial inceptions are simulated by both model versions. We note that the simulation time is ten times larger for the ice-sheet component than for the climate component (see bottom axis).
- Extended Data Figure 3: Past glacial inceptions. (259 KB)
Past glacial inceptions, detected using the critical insolation–CO2 relation, are shown in comparison to three different reconstructions of ice-volume variations over the past 800 kyr (refs 25, 29 and 30) and results of model simulations16.