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Abrupt climate changes constitute a relatively new field of research, which addresses variations occurring in a relatively short time interval of tens to a hundred years. Such time scales do not correspond to the tens or hundreds of thousands of years that the astronomical theory of climate addresses. The latter theory involves parameters that are external to the climate system and whose multi-periodic variations are reliably known and almost constant for a large extent of Earth history. Abrupt changes, conversely, appear to involve fast processes that are internal to the climate system; these processes varied considerably during the past 2.6 Myr, and yielded more irregular fluctuations. In this paper, we re-examine the main climate variations determined from the U1308 North Atlantic marine record, which yields a detailed calving history of the Northern Hemisphere ice sheets over the past 3.2 Myr. The magnitude and periodicity of the ice-rafted debris (IRD) events observed in the U1308 record allow one to determine the timing of several abrupt climate changes, the larger ones corresponding to the massive iceberg discharges labeled Heinrich events (HEs). In parallel, abrupt warmings, called Dansgaard-Oeschger (DO) events, have been identified in the Greenland records of the last glaciation cycle. Combining the HE and DO observations, we study a complex mechanism that may lead to the observed millennial-scale variability corresponding to the abrupt climate changes of last 0.9 Myr. This mechanism relies on amended Bond cycles, which group DO events and the associated Greenland stadials into a trend of increased cooling, with IRD events embedded into every stadial, the latest of these being an HE. These Bond cycles may have occurred during the last 0.9 Ma when Northern Hemisphere ice sheets reached their maximum extent and volume, thus becoming a major player in this time interval’s climate dynamics. Since the waxing and waning of ice sheets during the Quaternary period are orbitally paced, we conclude that the abrupt climate changes observed during the Mid and Upper Pleistocene are therewith indirectly linked to the astronomical theory of climate.

We present the first transient simulations of Marine Isotope Stage 3 (MIS 3) performed with an oxygen isotope-enabled climate model. Our simulations span several Dansgaard-Oeschger cycles and three Heinrich stadials and are directly compared with oxygen isotope records from 13 sediment and 2 ice cores. Our results are consistent with a 30–50% weakening of the Atlantic Meridional Overturning Circulation during Dansgaard-Oeschger stadials and a complete shutdown during Heinrich stadials. We find that the simulated δ 18 O anomalies differ significantly between Heinrich stadials and non-Heinrich stadials. This difference is mainly due to different responses in ocean circulation, and therefore climate, impacting oceanic δ 18 O, while the volume of 18 O-depleted meltwater plays a secondary role.

We perform idealized experiments of a Heinrich Stadial using an oxygen-isotope-enabled Earth System Climate Model. Our results compare relatively well with the planktic and benthic δ18O records of Heinrich Stadials 1 and 4. We find that changes in surface δ18Oc can be equally attributed to the “temperature effect” due to fractionation during calcification; the “circulation and climate effect” due to changes in circulation, precipitation, and evaporation; and the “meltwater effect” due to the addition and subsequent advection of depleted fresh water from continental ice sheets. In contrast, the meltwater effect and circulation and climate effect have only a small impact on benthic δ18Oc during the cessation of the Atlantic Meridional Overturning Circulation (AMOC). Significant temperature changes simulated in both the deep Atlantic and Pacific Oceans substantially influence benthic δ18Oc during that time. During the AMOC recovery, however, anomalously low δ18Ow surface waters are advected to the deep Atlantic, thus generating a large (∼0.5‰) δ18Oc decrease in the deep North Atlantic. This is in contrast to a hypothesis that links such depletion seen in Atlantic sediment cores during the last glacial stadials to deepwater formation caused by brine rejection and sea ice formation.