The evolution of past global ice sheets is highly uncertain. One example is the missing ice problem during the Last Glacial Maximum (LGM, 26 000-19 000 years before present) – an apparent 8-28 m discrepancy between far-field sea level indicators and modelled sea level from ice sheet reconstructions. In the absence of ice sheet reconstructions, researchers often use marine δ18O proxy records to infer ice volume prior to the LGM. We present a global ice sheet reconstruction for the past 80 000 years, called PaleoMIST 1.0, constructed independently of far-field sea level and δ18O proxy records. Our reconstruction is compatible with LGM far-field sea-level records without requiring extra ice volume, thus solving the missing ice problem. However, for Marine Isotope Stage 3 (57 000-29 000 years before present) - a pre-LGM period - our reconstruction does not match proxy-based sea level reconstructions, indicating the relationship between marine δ18O and sea level may be more complex than assumed.

The last deglaciation is one of the best constrained global-scale climate changes documented by climate archives. Nevertheless, understanding of the underlying dynamics is still limited, especially with respect to abrupt climate shifts and associated changes in the Atlantic meridional overturning circulation (AMOC) during glacial and deglacial periods. A fundamental issue is how to obtain an appropriate climate state at the Last Glacial Maximum (LGM, 21 000 yr before present, 21 ka BP) that can be used as an initial condition for deglaciation. With the aid of a comprehensive climate model, we found that initial ocean states play an important role on the equilibrium timescale of the simulated glacial ocean. Independent of the initialization, the climatological surface characteristics are similar and quasi-stationary, even when trends in the deep ocean are still significant, which provides an explanation for the large spread of simulated LGM ocean states among the Paleoclimate Modeling Intercomparison Project phase 2 (PMIP2) models. Accordingly, we emphasize that caution must be taken when alleged quasi-stationary states, inferred on the basis of surface properties, are used as a reference for both model inter-comparison and data model comparison.

The simulated ocean state with the most realistic AMOC is characterized by a pronounced vertical stratification, in line with reconstructions. Hosing experiments further suggest that the response of the glacial ocean is dependent on the ocean background state, i.e. only the state with robust stratification shows an overshoot behavior in the North Atlantic. We propose that the salinity stratification represents a key control on the AMOC pattern and its transient response to perturbations. Furthermore, additional experiments suggest that the stratified deep ocean formed prior to the LGM during a time of minimum obliquity (~ 27 ka BP). This indicates that changes in the glacial deep ocean already occur before the last deglaciation. In combination, these findings represent a new paradigm for the LGM and the last deglaciation, which challenges the conventional evaluation of glacial and deglacial AMOC changes based on an ocean state derived from 21 ka BP boundary conditions

Glacial climate is marked by abrupt, millennial-scale climate changes known as Dansgaard-Oeschger cycles. The most pronounced stadial coolings, Heinrich events, are associated with massive iceberg discharges to the North Atlantic. These events have been linked to variations in the strength of the Atlantic meridional overturning circulation. However, the factors that lead to abrupt transitions between strong and weak circulation regimes remain unclear. Here we show that, in a fully coupled atmosphere-ocean model, gradual changes in atmospheric CO 2 concentrations can trigger abrupt climate changes, associated with a regime of bi-stability of the Atlantic meridional overturning circulation under intermediate glacial conditions. We find that changes in atmospheric CO 2 concentrations alter the transport of atmospheric moisture across Central America, which modulates the freshwater budget of the North Atlantic and hence deep-water formation. In our simulations, a change in atmospheric CO 2 levels of about 15 ppmv-comparable to variations during Dansgaard-Oeschger cycles containing Heinrich events-is su cient to cause transitions between a weak stadial and a strong interstadial circulation mode. Because changes in the Atlantic meridional overturning circulation are thought to alter atmospheric CO 2 levels, we infer that atmospheric CO 2 levels may serve as a negative feedback to transitions between strong and weak circulation modes.A brupt climate changes associated with Dansgaard-Oeschger (DO) events as recorded in Greenland ice cores are characterized by rapid warming from stadial to interstadial conditions. This is followed by a phase of gradual cooling before an abrupt return to cold stadial conditions 1,2 . A common explanation for these transitions involves changes in the Atlantic meridional overturning circulation (AMOC) 3 , perhaps controlled by freshwater perturbation (for example, refs 4,5) and/or Northern Hemisphere ice sheet changes (for example, refs 6-8). To reproduce the abrupt transitions into and out of cold conditions across the North Atlantic (that is, AMOC weak or 'off' mode 3 ), a common trigger mechanism is related to the timing of North Atlantic freshwater perturbations 9,10 that is mainly motivated by unequivocal ice-rafting events during Heinrich Stadials (HS) 11 . However, recent studies suggest that the Heinrich ice-surging events are in fact triggered by sea subsurface warming associated with an AMOC slow-down 12,13 . Furthermore, the duration of ice-rafting events does not systematically coincide with the beginning and end of the pronounced cold conditions during Heinrich Stadials 14,15 . This evidence thus challenges the current understanding of glacial AMOC stability 5,8 , suggesting the existence of additional control factors that should be invoked to explain abrupt millennial-scale variability in climate records. In contrast to the north, the rapid climate transitions are characterized by interhemispheric anti-phased variability, with more gradual changes in southern high latitudes 16...