While the ocean’s large-scale overturning circulation is thought to have been significantly different under the climatic conditions of the Last Glacial Maximum (LGM), the exact nature of the glacial circulation and its implications for global carbon cycling continue to be debated. Here we use a global array of ocean–atmosphere radiocarbon disequilibrium estimates to demonstrate a ∼689±53 14C-yr increase in the average residence time of carbon in the deep ocean at the LGM. A predominantly southern-sourced abyssal overturning limb that was more isolated from its shallower northern counterparts is interpreted to have extended from the Southern Ocean, producing a widespread radiocarbon age maximum at mid-depths and depriving the deep ocean of a fast escape route for accumulating respired carbon. While the exact magnitude of the resulting carbon cycle impacts remains to be confirmed, the radiocarbon data suggest an increase in the efficiency of the biological carbon pump that could have accounted for as much as half of the glacial–interglacial CO2 change.

Changes in deep ocean ventilation are commonly invoked as the primary cause of lower glacial atmospheric CO2. The water mass structure of the glacial deep Atlantic Ocean and the mechanism by which it may have sequestered carbon remain elusive. Here we present neodymium isotope measurements from cores throughout the Atlantic that reveal glacial–interglacial changes in water mass distributions. These results demonstrate the sustained production of North Atlantic Deep Water under glacial conditions, indicating that southern-sourced waters were not as spatially extensive during the Last Glacial Maximum as previously believed. We demonstrate that the depleted glacial δ13C values in the deep Atlantic Ocean cannot be explained solely by water mass source changes. A greater amount of respired carbon, therefore, must have been stored in the abyssal Atlantic during the Last Glacial Maximum. We infer that this was achieved by a sluggish deep overturning cell, comprised of well-mixed northern- and southern-sourced waters.

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The AIS is an important component of the global climate system. Human activities have caused the atmosphere and especially the oceans to warm. However, the full effect of human caused climate change on the AIS has not currently been realised because the ice sheet responds on a range of timescales and to many different Earth processes. Modern observations show that West Antarctica has been melting at an accelerating rate since the 2000s, while the data for East Antarctica are less clear. Environmental records preserve the history of the climate and AIS, which extend beyond the instrumental record, and reveal how the AIS responded to past climate warming. Estimates of how much the AIS will contribute to sea-level rise by the year 2100 have changed as a result of new information on how the AIS evolved in the past, and research into the interactions between the ice sheet, solid Earthatmosphere and ocean systems. This review brings together our knowledge of the major processes and feedbacks affecting the AIS, and the evidence for how the ice sheet changed since the Pliocene. We consider the future estimates and consequences of global sea-level rise from melting of the AIS, and highlight priority research areas.