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The extent of grounded ice and buttressing by the Ronne Ice Shelf, which provides resistance to the outflow of ice streams, moderate West Antarctic Ice Sheet stability. During the Last Glacial Maximum, the ice sheet advanced and was grounded near the Weddell Sea continental shelf break. The timing of subsequent ice sheet retreat and the relative roles of ice shelf buttressing and grounding line changes remain unresolved. Here we use an ice core record from grounded ice at Skytrain Ice Rise to constrain the timing and speed of early Holocene ice sheet retreat. Measured δ18O and total air content suggest that the surface elevation of Skytrain Ice Rise decreased by about 450 m between 8.2 and 8.0 kyr before 1950 ce(±0.13 kyr). We attribute this elevation change to dynamic thinning due to flow changes induced by the ungrounding of ice in the area. Ice core sodium concentrations suggest that the ice front of this ungrounded ice shelf then retreated about 270 km (±30 km) from 7.7 to 7.3 kyr before 1950 ce. These centennial-scale changes demonstrate how quickly ice mass can be lost from the West Antarctic Ice Sheet due to changes in grounded ice without extensive ice shelf calving. Our findings both support and temporally constrain ice sheet models that exhibit rapid ice loss in the Weddell Sea sector in the early Holocene.

Antarctic ice cores can help determine ice mass loss from the Antarctic Ice Sheet (AIS) during past warm periods. We compile Last Interglacial (LIG) δ18O${\delta }^{18}O$measurements from eight Antarctic cores and compare these to new isotope‐enabled LIG simulations, which explore three plausible LIG AIS elevation and extent scenarios. We find that these simulations capture less than 10% of East Antarctic core‐mean δ18O${\delta }^{18}O$changes. Although our simulations do not fully explain the changes, they capture some inter‐core geographical δ18O${\delta }^{18}O$variations. Some LIG AIS configurations show higher skill than PI AIS configurations in simulating the inter‐core differences. The remaining discrepancies between the simulated and observed core‐mean water isotope changes suggest that LIG simulations also need to include the influences of reduced Antarctic sea ice, a warmer Southern Ocean, and resultant shifts in vapor source regions to produce a more satisfactory match to δ18O${\delta }^{18}O$observed at ice core sites. Reconstructions of Antarctic Ice Sheet (AIS) retreat during past warm periods provide important constraints for projections of future ice mass loss and sea level rise. The Last Interglacial (LIG) is a particularly useful example, when global temperatures were 1±1oC$\pm {1}^{o}C$warmer than preindustrial (PI) conditions, to explore future sea level rise projections associated with the successful implementation of the Paris Agreement. Here, we use data from six East Antarctic ice cores and two new, unpublished records from West Antarctica that capture LIG conditions. Water isotopes in ice cores are sensitive to changes in temperature, AIS elevation, atmospheric circulation pattern, sea ice area, and ocean conditions. We compare model simulations of the LIG with ice core data and find that elevation changes, an important indicator of ice mass loss, explain only around 9% of the isotope change captured in East Antarctic ice cores. Due to the limitation of our simulations in the coastal regions, our West Antarctic coastal sites are more challenging. In summary, we find that while the range of our simulations does not fully explain average East Antarctic PI‐to‐LIG isotope changes, they do capture some of the geographical variations in isotope change patterns. A compilation of Last Interglacial δ18O${\delta }^{18}O$ice core records shows anomalies of +${+}$1.5‰ and +${+}$3.3‰ for 127 kyr BP and LIG‐peak core‐meanSimulations run with plausible LIG Antarctic Ice Sheet (AIS)configurations, alongside greenhouse gas and orbital changes, capture <${< } $10% of core‐mean differencesTwo LIG AIS configurations yield lower geographical errors at 127 kyr, compared to their PI configurations A compilation of Last Interglacial δ18O${\delta }^{18}O$ice core records shows anomalies of +${+}$1.5‰ and +${+}$3.3‰ for 127 kyr BP and LIG‐peak core‐mean Simulations run with plausible LIG Antarctic Ice Sheet (AIS)configurations, alongside greenhouse gas and orbital changes, capture <${< } $10% of core‐mean differences Two LIG AIS configurations yield lower geographical errors at 127 kyr, compared to their PI configurations

The sea ice surface is thought to be a major source of sea salt aerosol, suggesting that sodium records of polar ice cores may trace past sea ice extent. Here we test this possibility for the Arctic, using a chemical transport model to simulate aerosol emission, transport, and deposition in the satellite era. Our simulations suggest that sodium records from inland Greenland ice cores are strongly influenced by the impact of meteorology on aerosol transport and deposition. In contrast, sodium in coastal Arctic cores is predominantly sourced from the sea ice surface and the strength of these aerosol emissions controls the ice core sodium variability. Such ice cores may therefore record decadal to centennial scale Holocene sea ice changes. However, any relationship between ice core sodium and sea ice change may depend on how sea ice seasonality impacts sea salt emissions. Field‐based observations are urgently required to constrain this. To better understand the variability of Arctic sea ice and its interaction with climate, we need to reconstruct its history back beyond the time range of observations. As the sea ice surface is a major source of sea salt aerosol in the polar regions, one possibility is to use sea salt sodium records in Arctic ice cores as tracers of sea ice change. However, there are other factors that influence ice core sodium, in addition to the strength of the sea ice source of aerosol. The most important is meteorology, or storminess, that impacts how aerosol is transported and deposited. We adopt a new approach to tackling this problem, using a numerical model that replicates the emission, transport, and deposition of sea salt to simulate Arctic ice core sodium records. This is used to investigate the competing controls of sea ice versus storminess. Our results suggest that sodium records of coastal Arctic ice cores, away from central Greenland, offer the greatest potential for Holocene sea ice reconstruction. However, this study also highlights the need for more field observations to improve understanding of aerosol emission over the sea ice. Simulated satellite-era interannual variability in Greenland ice core sodium concentrations is driven by meteorology not sea iceSodium budget of coastal Arctic ice cores is dominated by sea salt from the sea ice surface not the open oceanInterannual variability of sodium records from coastal Arctic cores is driven by aerosol emission strength, not transport or deposition

Climate changes in the North Atlantic region during the last glacial cycle were dominated by the slow waxing and waning of the North American ice sheet as well as by intermittent, millennial-scale Dansgaard–Oeschger climate oscillations. However, prior to the last deglaciation, the responses of North American vegetation and biomass burning to these climate variations are uncertain. Ammonium in Greenland ice cores, a product from North American soil emissions and biomass burning events, can help to fill this gap. Here we use continuous, high-resolution measurements of ammonium concentrations between 110,000 to 10,000 years ago from the Greenland NGRIP and GRIP ice cores to reconstruct North American wildfire activity and soil ammonium emissions. We find that on orbital timescales soil emissions increased under warmer climate conditions when vegetation expanded northwards into previously ice-covered areas. For millennial-scale interstadial warm periods during Marine Isotope Stage 3, the fire recurrence rate increased in parallel to the rapid warmings, whereas soil emissions rose more slowly, reflecting slow ice shrinkage and delayed ecosystem changes. We conclude that sudden warming events had little impact on soil ammonium emissions and ammonium transport to Greenland, but did result in a substantial increase in the frequency of North American wildfires.

Ice could play a role in identifying and defining the Anthropocene. The recurrence of northern hemisphere glaciation and the stability of the Greenland Ice Sheet are both potentially vulnerable to human impact on the environment. However, only a very long hiatus in either would be unusual in the context of the Quaternary Period, requiring the definition of a geological boundary. Human influence can clearly be discerned in several ice-core measurements. These include a sharp boundary in radioactivity due to atmospheric nuclear testing; increases, unprecedented at least in the Holocene, in Greenland concentrations of sulphate, nitrate and metals such as lead; the appearance in ice-core air bubbles of previously undetectable compounds such as SF6; and the rise, unprecedented in the last 800 ka, in concentrations of carbon dioxide and methane. Some combination of these changes could be used by future generations to clearly identify the onset of a new epoch defined at a particular calendar date. However, it is not yet clear what the character of the fully developed Anthropocene will be, and it might be wise to let future generations decide, with hindsight, when the Anthropocene started, acknowledging only that we are in the transition towards it.

Over the past 50 years, warming of the Antarctic Peninsula has been accompanied by accelerating glacier mass loss and the retreat and collapse of ice shelves. A key driver of ice loss is summer melting; however, it is not usually possible to specifically reconstruct the summer conditions that are critical for determining ice melt in Antarctic. Here we reconstruct changes in ice-melt intensity and mean temperature on the northern Antarctic Peninsula since AD 1000 based on the identification of visible melt layers in the James Ross Island ice core and local mean annual temperature estimates from the deuterium content of the ice. During the past millennium, the coolest conditions and lowest melt occurred from about AD 1410 to 1460, when mean temperature was 1.6?°C lower than that of 1981–2000. Since the late 1400s, there has been a nearly tenfold increase in melt intensity from 0.5 to 4.9%. The warming has occurred in progressive phases since about AD 1460, but intensification of melt is nonlinear, and has largely occurred since the mid-twentieth century. Summer melting is now at a level that is unprecedented over the past 1,000 years. We conclude that ice on the Antarctic Peninsula is now particularly susceptible to rapid increases in melting and loss in response to relatively small increases in mean temperature.

Environmental context. Investigating the past is often the only way we have of determining whether we have included all processes correctly into models, and then of verifying their behaviour. Ice cores provide an excellent way of finding out about the past. Air bubbles trapped in the ice allow us to directly access the concentration of stable trace gases, including important greenhouse gases. However, there are also tantalising possibilities to learn about aerosols and shorter-lived gases. This article describes some of the information we have already learnt from ice cores, but also describes the challenges that require understanding of atmospheric chemistry in the polar regions today in order to extract the full value of the records of the past trapped in the ice sheet.  Abstract. Ice cores provide the most direct evidence available about the past atmosphere. For long-lived trace gases, ice cores have provided clear evidence that in the last two centuries, concentrations of several greenhouse gases have risen well outside the natural range observed in the previous 650000 years. Major natural changes are also observed between cold and warm periods. Aerosol components have to be interpreted in terms of changing sources, transport and deposition. When this is done, they can also supply evidence about crucial aspects of the past environment, including sea ice extent, trace element deposition to the ocean, and about the aerosols available for cloud nucleation, for example. It is much more difficult to extract information about shorter-lived chemical species. Information may be available in components such as nitrate and formaldehyde, but to extract that information, detailed modern atmospheric studies about air to snow transfer, preservation in the ice, and the link between the polar region boundary layer and other parts of the atmosphere are urgently required.

Many attempts have been made to trace increases in global pollution from the record of impurities preserved in polar snow and ice layers. Although inadequate sampling and analysis techniques have hampered some of the studies, others have improved uniquely our understanding of the history of changes in airborne pollution.

An analysis of air up to 2 million years old, trapped in Antarctic ice, shows that a major shift in the periodicity of glacial cycles was probably not caused by a long-term decline in atmospheric levels of carbon dioxide.