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The Greenland Ice Sheet (GIS) is losing mass at a high rate1. Given the short-term nature of the observational record, it is difficult to assess the historical importance of this mass-loss trend. Unlike records of greenhouse gas concentrations and global temperature, in which observations have been merged with palaeoclimate datasets, there are no comparably long records for rates of GIS mass change. Here we reveal unprecedented mass loss from the GIS this century, by placing contemporary and future rates of GIS mass loss within the context of the natural variability over the past 12,000 years. We force a high-resolution ice-sheet model with an ensemble of climate histories constrained by ice-core data2. Our simulation domain covers southwestern Greenland, the mass change of which is dominated by surface mass balance. The results agree favourably with an independent chronology of the history of the GIS margin3,4. The largest pre-industrial rates of mass loss (up to 6,000 billion tonnes per century) occurred in the early Holocene, and were similar to the contemporary (ad 2000–2018) rate of around 6,100 billion tonnes per century5. Simulations of future mass loss from southwestern GIS, based on Representative Concentration Pathway (RCP) scenarios corresponding to low (RCP2.6) and high (RCP8.5) greenhouse gas concentration trajectories6, predict mass loss of between 8,800 and 35,900 billion tonnes over the twenty-first century. These rates of GIS mass loss exceed the maximum rates over the past 12,000 years. Because rates of mass loss from the southwestern GIS scale linearly5 with the GIS as a whole, our results indicate, with high confidence, that the rate of mass loss from the GIS will exceed Holocene rates this century. Rates of ice-mass loss from southwestern Greenland this century will exceed the maximum rate over the past 12,000 years, and would not be the result of natural variation.

The Greenland Ice Sheet (GIS) is losing mass at a high rate

Measurements of cosmic-ray-produced 10Be and 26Al in a bedrock core from beneath the summit of the Greenland Ice Sheet show that Greenland was nearly ice-free for extended periods under Pleistocene climate forcing. The Greenland Ice Sheet is a main contributor to modern sea-level rise, but its stability during past warm periods is uncertain, thus compromising our ability to robustly predict future rates and magnitudes of ice loss. Joerg Schaefer and colleagues present cosmogenic isotope evidence, from bedrock samples from beneath an ice core near the summit of the ice sheet, to show that the Greenland Ice Sheet was largely ice-free at some point during the Pleistocene. The data cannot constrain the time or duration of ice-free conditions, and seem incompatible with our current understanding of ice-sheet behaviour. Nevertheless, alternative explanations for the cosmogenic data are lacking, suggesting that our understanding of the response of the Greenland Ice Sheet to past warmth remains incomplete. The Greenland Ice Sheet (GIS) contains the equivalent of 7.4 metres of global sea-level rise1. Its stability in our warming climate is therefore a pressing concern. However, the sparse proxy evidence of the palaeo-stability of the GIS means that its history is controversial (compare refs 2 and 3 to ref. 4). Here we show that Greenland was deglaciated for extended periods during the Pleistocene epoch (from 2.6 million years ago to 11,700 years ago), based on new measurements of cosmic-ray-produced beryllium and aluminium isotopes (10Be and 26Al) in a bedrock core from beneath an ice core near the GIS summit. Models indicate that when this bedrock site is ice-free, any remaining ice is concentrated in the eastern Greenland highlands and the GIS is reduced to less than ten per cent of its current volume. Our results narrow the spectrum of possible GIS histories: the longest period of stability of the present ice sheet that is consistent with the measurements is 1.1 million years, assuming that this was preceded by more than 280,000 years of ice-free conditions. Other scenarios, in which Greenland was ice-free during any or all Pleistocene interglacials, may be more realistic. Our observations are incompatible with most existing model simulations that present a continuously existing Pleistocene GIS. Future simulations of the GIS should take into account that Greenland was nearly ice-free for extended periods under Pleistocene climate forcing.

The Younger Dryas — a period of sudden cooling in the Northern Hemisphere about 12,900 years ago — is perhaps the best-known example of abrupt climate change. But the global extent of the Younger Dryas is a topic of intense debate, particularly in the record of glacial behaviour in New Zealand. A new reconstruction of the growth and retreat patterns of glaciers in the Southern Alps in New Zealand at the time of the Younger Dryas supports the suggestion that temperature reductions in the north caused warming and glacial retreat in the Southern Hemisphere through a series of climate feedbacks. The Younger Dryas — during which Northern Hemisphere temperatures cooled drastically in just a few years — is perhaps the best-known example of abrupt climate change, but its global extent is under debate, particularly in the record of glacial behaviour in New Zealand. These authors present evidence for glacial retreat in New Zealand during the Younger Dryas, supporting the hypothesis that Northern Hemisphere climate changes caused Southern Hemisphere warming through a series of climate feedbacks. Millennial-scale cold reversals in the high latitudes of both hemispheres interrupted the last transition from full glacial to interglacial climate conditions. The presence of the Younger Dryas stadial (∼12.9 to ∼11.7 kyr ago) is established throughout much of the Northern Hemisphere, but the global timing, nature and extent of the event are not well established. Evidence in mid to low latitudes of the Southern Hemisphere, in particular, has remained perplexing1,2,3,4,5,6. The debate has in part focused on the behaviour of mountain glaciers in New Zealand, where previous research has found equivocal evidence for the precise timing of increased or reduced ice extent1,2,3. The interhemispheric behaviour of the climate system during the Younger Dryas thus remains an open question, fundamentally limiting our ability to formulate realistic models of global climate dynamics for this time period. Here we show that New Zealand’s glaciers retreated after ∼13 kyr bp, at the onset of the Younger Dryas, and in general over the subsequent ∼1.5-kyr period. Our evidence is based on detailed landform mapping, a high-precision 10Be chronology7 and reconstruction of former ice extents and snow lines from well-preserved cirque moraines. Our late-glacial glacier chronology matches climatic trends in Antarctica, Southern Ocean behaviour and variations in atmospheric CO2. The evidence points to a distinct warming of the southern mid-latitude atmosphere during the Younger Dryas and a close coupling between New Zealand’s cryosphere and southern high-latitude climate. These findings support the hypothesis that extensive winter sea ice and curtailed meridional ocean overturning in the North Atlantic led to a strong interhemispheric thermal gradient8 during late-glacial times, in turn leading to increased upwelling and CO2 release from the Southern Ocean9, thereby triggering Southern Hemisphere warming during the northern Younger Dryas.