Ice Sheet Volume Research Articles

Miocene ice sheet dynamics and sediment deposition in the central Ross Sea, Antarctica

Drill cores from the Antarctic continental shelf are essential for directly constraining changes in past Antarctic Ice Sheet extent. Here, we provide a sedimentary facies analysis of drill cores from International Ocean Discovery Program (IODP) Site U1521 in the Ross Sea, which reveals a unique, detailed snapshot of Antarctic Ice Sheet evolution between ca. 18 Ma and 13 Ma. We identify distinct depositional packages, each of which contains facies successions that are reflective of past baseline shifts in the presence or absence of marine-terminating ice sheets on the outermost Ross Sea continental shelf. The oldest depositional package (>18 Ma) contains massive diamictites stacked through aggradation and deposited in a deep, actively subsiding basin that restricted marine ice sheet expansion on the outer continental shelf. A slowdown in tectonic subsidence after 17.8 Ma led to the deposition of progradational massive diamictites with thin mudstone beds/laminae, as several large marine-based ice sheet advances expanded onto the mid- to outer continental shelf between 17.8 Ma and 17.4 Ma. Between 17.2 Ma and 15.95 Ma, packages of interbedded diamictite and diatom-rich mudstone were deposited during a phase of highly variable Antarctic Ice Sheet extent and volume. This included periods of Antarctic Ice Sheet advance near the outer shelf during the early Miocene Climate Optimum (MCO)—despite this being a well-known period of peak global warmth between ca. 17.0 Ma and 14.6 Ma. Conversely, there were periods of peak warmth within the MCO during which diatom-rich mudstones with little to no ice-rafted debris were deposited, which indicates that the Antarctic Ice Sheet was greatly reduced in extent and had retreated to a smaller terrestrial-terminating ice sheet, most notably between 16.3 Ma and 15.95 Ma. Post-14.2 Ma, diamictites and diatomites contain unambiguous evidence of subglacial shearing in the core and provide the first direct, well-dated evidence of highly erosive marine ice sheets on the outermost continental shelf during the onset of the Middle Miocene Climate Transition (MMCT; 14.2−13.6 Ma). Although global climate forcings and feedbacks influenced Antarctic Ice Sheet advances and retreats during the MCO and MMCT, we propose that this response was nonlinear and heavily influenced by regional feedbacks related to the shoaling of the continental shelf due to reduced subsidence, sediment infilling, and local sea-level changes that directly influenced oceanic influences on melting at the Antarctic Ice Sheet margin. Although intervals of diatom-rich muds and diatomite indicating open-marine interglacial conditions still occurred during (and following) the MMCT, repeated advances of marine-based ice sheets since that time have resulted in widespread erosion and overdeepening in the inner Ross Sea, which has greatly enhanced sensitivity to marine ice sheet instability since 14.2 Ma.

Modelling Mediterranean ocean biogeochemistry of the Last Glacial Maximum

Abstract. We present results of simulations performed with a physical–biogeochemical ocean model of the Mediterranean Sea for the Last Glacial Maximum (LGM) and analyse the differences in physical and biochemical states between the historical period and the LGM. Long-term simulations with an Earth system model based on ice sheet reconstructions provide the necessary atmospheric forcing data, oceanic boundary conditions at the entrance to the Mediterranean Sea, and river discharge to the entire basin. Our regional model accounts for changes in bathymetry due to ice sheet volume changes, reduction in atmospheric CO2 concentration, and an adjusted aeolian dust and iron deposition. The physical ocean state of the Mediterranean during the LGM shows a reduced baroclinic water exchange at the Strait of Gibraltar, a more sluggish zonal overturning circulation, and the relocation of intermediate and deep-water-formation areas – all in line with estimates from palaeo-sediment records or previous modelling efforts. Most striking features of the biogeochemical realm are a reduction in the net primary production, an accumulation of nutrients below the euphotic zone, and an increase in the organic matter deposition at the seafloor. This seeming contradiction of increased organic matter deposition and decreased net primary production challenges our view of possible changes in surface biological processes during the LGM. We attribute the origin of a reduced net primary production to the interplay of increased stability of the upper water column, changed zonal water transport at intermediate depths, and lower water temperatures, which slow down all biological processes during the LGM. Cold water temperatures also affect the remineralisation rates of organic material, which explains the simulated increase in the organic matter deposition, which is in good agreement with sediment proxy records. In addition, we discuss changes in an artificial tracer which captures the surface ocean temperature signal during organic matter production. A shifted seasonality of the biological production in the LGM leads to a difference in the recording of the climate signal by this artificial tracer of up to 1 K. This could be of relevance for the interpretation of proxy records like, e.g., alkenones. Our study not only provides the first consistent insights into the biogeochemistry of the glacial Mediterranean Sea but will also serve as the starting point for transient simulations of the last deglaciation.

Large-ensemble simulations of the North American and Greenland ice sheets at the Last Glacial Maximum with a coupled atmospheric general circulation–ice sheet model

Abstract. The Last Glacial Maximum (LGM) was characterised by huge ice sheets covering the Northern Hemisphere, especially over North America, and by its cold climate. Previous authors have performed numerical simulations of the LGM to better understand coupled climate–ice sheet systems. However, the results of such simulations are sensitive to many model parameters. Here, we perform a 200-member ensemble of simulations of the North American and Greenland ice sheets and climate of the LGM with a coupled ice sheet–atmosphere–slab ocean model (FAMOUS-BISICLES) to explore sensitivities of the coupled climate–ice system to 16 uncertain parameters. In the ensemble of simulations, the global mean surface temperature is primarily controlled by the combination of parameters in the large-scale condensation scheme and the cumulus convection scheme. In simulations with plausible LGM global mean surface temperatures, we find that the albedo parameters have only a small impact on the Greenland ice volume due to the limited area of surface ablation associated with the cold climate. Instead, the basal sliding law controls the ice volume by affecting ice transport from the interior to the margin. On the other hand, like the Greenland ice sheet in future climate change, the LGM North American ice sheet volume is controlled by parameters in the snow and ice albedo scheme. Few of our simulations produce an extensive North American ice sheet when the global temperature is above 12 °C. Based on constraints on the LGM global mean surface temperature, the ice volume and the southern extent of the North American ice sheet, we select 16 acceptable simulations. These simulations lack the southern extent of ice compared to reconstructions, but they show reasonable performance on the ice sheet configuration and ice streams facing Baffin Bay and the Arctic Ocean. The strong sensitivities of the North American ice sheet to albedo at the LGM may imply a potential constraint on the future Greenland ice sheet by constraining the albedo schemes.

The influence of glacial landscape evolution on Scandinavian ice-sheet dynamics and dimensions

Abstract. The Scandinavian topography and bathymetry have been shaped by ice through numerous glacial cycles in the Quaternary. In this study, we investigate how the changing morphology has influenced the Scandinavian ice sheet (SIS) in return. We use a higher-order ice-sheet model to simulate the SIS through a glacial period on three different topographies, representing different stages of glacial landscape evolution in the Quaternary. By forcing the three experiments with the same climate conditions, we isolate the effects of a changing landscape morphology on the evolution and dynamics of the ice sheet. We find that early Quaternary glaciations in Scandinavia were limited in extent and volume by the pre-glacial bathymetry until glacial deposits filled depressions in the North Sea and built out the Norwegian shelf. From middle–late Quaternary (∼0.5 Ma) the bathymetry was sufficiently filled to allow for a faster southward expansion of the ice sheet causing a relative increase in ice-sheet volume and extent. Furthermore, we show that the formation of The Norwegian Channel during recent glacial periods restricted southward ice-sheet expansion, only allowing for the ice sheet to advance into the southern North Sea close to glacial maxima. Finally, our experiments indicate that different stretches of The Norwegian Channel may have formed in distinct stages during glacial periods since ∼0.5 Ma. These results highlight the importance of accounting for changes in landscape morphology through time when inferring ice-sheet history from ice-volume proxies and when interpreting climate variability from past ice-sheet extents.

Miocene Antarctic Ice Sheet area adapts significantly faster than volume to CO2-induced climate change

Abstract. The strongly varying benthic δ18O levels of the early and mid-Miocene (23 to 14 Myr ago) are primarily caused by a combination of changes in Antarctic Ice Sheet (AIS) volume and deep-ocean temperatures. These factors are coupled since AIS changes affect deep-ocean temperatures. It has recently been argued that this is due to changes in ice sheet area rather than volume because area changes affect the surface albedo. This finding would be important when the transient AIS grows relatively faster in extent than in thickness, which we test here. We analyse simulations of Miocene AIS variability carried out using the three-dimensional ice sheet model IMAU-ICE forced by warm (high CO2, no ice) and cold (low CO2, large East AIS) climate snapshots. These simulations comprise equilibrium and idealized quasi-orbital transient runs with strongly varying CO2 levels (280 to 840 ppm). Our simulations show a limited direct effect of East AIS changes on Miocene orbital-timescale benthic δ18O variability because of the slow build-up of volume. However, we find that relative to the equilibrium ice sheet size, the AIS area adapts significantly faster and more strongly than volume to the applied forcing variability. Consequently, during certain intervals the ice sheet is receding at the margins, while ice is still building up in the interior. That means the AIS does not adapt to a changing equilibrium size at the same rate or with the same sign everywhere. Our results indicate that the Miocene Antarctic Ice Sheet affects deep-ocean temperatures more than its volume suggests.

Glacial seismic geomorphology and Plio-Pleistocene ice sheet history offshore NW Europe

Abstract Plio-Pleistocene records of ice-rafted detritus suggest NW European ice sheets regularly reached coastlines. However, these records provide limited insight on the frequency, extent and dynamics of ice sheets delivering the detritus. Three-dimensional reflection seismic data of the NW European glaciated margin have previously documented buried landforms that inform us on these uncertainties. This paper reviews and combines these existing records with new seismic geomorphological observations to catalogue landform occurrence along the European glaciated margin and considers how they relate to ice sheet history. The compilation shows Early Pleistocene ice sheets regularly advanced across the continental shelves. Early Pleistocene sea-level reconstructions demonstrate lower magnitude fluctuations compared to the Middle–Late Pleistocene, and more extensive/frequent Early Pleistocene glaciation provides a possible mismatch with sea-level reconstructions. This evidence is discussed with global records of glaciation to consider possible impacts on our wider understanding of Plio-Pleistocene climate changes, in particular how well Early Pleistocene sea-level records capture ice sheet volume changes. Resolving such issues relies on how well landforms are dated, whether they can be correlated with other proxy datasets, and how accurately these proxies reconstruct the magnitudes of past climatic changes. Many questions about Pleistocene glaciation in Europe and elsewhere remain.

Quantifying the uncertainty in the Eurasian ice-sheet geometry at the Penultimate Glacial Maximum (Marine Isotope Stage 6)

Abstract. The North Sea Last Interglacial sea level is sensitive to the fingerprint of mass loss from polar ice sheets. However, the signal is complicated by the influence of glacial isostatic adjustment driven by Penultimate Glacial Period ice-sheet changes, and yet these ice-sheet geometries remain significantly uncertain. Here, we produce new reconstructions of the Eurasian ice sheet during the Penultimate Glacial Maximum (PGM) by employing large ensemble experiments from a simple ice-sheet model that depends solely on basal shear stress, ice extent, and topography. To explore the range of uncertainty in possible ice geometries, we use a parameterised shear-stress map as input that has been developed to incorporate bedrock characteristics and the influence of ice-sheet basal processes. We perform Bayesian uncertainty quantification, utilising Gaussian process emulation, to calibrate against global ice-sheet reconstructions of the Last Deglaciation and rule out combinations of input parameters that produce unrealistic ice sheets. The refined parameter space is then applied to the PGM to create an ensemble of constrained 3D Eurasian ice-sheet geometries. Our reconstructed PGM Eurasian ice-sheet volume is 48±8 m sea-level equivalent (SLE). We find that the Barents–Kara Sea region displays both the largest mean volume and volume uncertainty of 24±8 m SLE while the British–Irish sector volume of 1.7±0.2 m SLE is the smallest. Our new workflow may be applied to other locations and periods where ice-sheet histories have limited empirical data.

Climate shifts orchestrated hominin interbreeding events across Eurasia.

When, where, and how often hominin interbreeding happened is largely unknown. We study the potential for Neanderthal-Denisovan admixture using species distribution models that integrate extensive fossil, archaeological, and genetic data with transient coupled general circulation model simulations of global climate and biomes. Our Pleistocene hindcast of past hominins' habitat suitability reveals pronounced climate-driven zonal shifts in the main overlap region of Denisovans and Neanderthals in central Eurasia. These shifts, which influenced the timing and intensity of potential interbreeding events, can be attributed to the response of climate and vegetation to past variations in atmospheric carbon dioxide and Northern Hemisphere ice-sheet volume. Therefore, glacial-interglacial climate swings likely played an important role in favoring gene flow between archaic humans.

The influence of inter-annual temperature variability on the Greenland Ice Sheet volume

Abstract The Greenland Ice Sheet has become an increasingly larger contributor to sea level rise in the past two decades and is projected to continue to lose mass. Climate variability is expected to increase under future warming, but the effect of climate variability on the Greenland Ice Sheet volume is poorly understood and is adding to the uncertainty of the projected mass loss. Here we quantify the influence of inter-annual temperature variability on mass loss from the Greenland Ice Sheet using the PISM model. We construct an ensemble of temperature-forcing fields that accounts for inter-annual variability in temperature using reanalysis data from NOAA-CIRES over the period 1851–2014. We investigate the steady-state and transient response of the Greenland Ice Sheet. We find that the simulated steady-state ice-sheet volume decreases by 1.9 ± 0.4 cm of sea level equivalent when forced with a varying temperature forcing compared to a constant temperature forcing, and by 11.5 ± 1.4 cm when the variability is doubled. The northern basins are particularly sensitive with a change in volume of 0.9–1.1%. Our results emphasize the importance of including temperature variability in projections of future mass loss.

Mass balance of the Greenland and Antarctic ice sheets from 1992 to 2020

Abstract. Ice losses from the Greenland and Antarctic ice sheets have accelerated since the 1990s, accounting for a significant increase in the global mean sea level. Here, we present a new 29-year record of ice sheet mass balance from 1992 to 2020 from the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE). We compare and combine 50 independent estimates of ice sheet mass balance derived from satellite observations of temporal changes in ice sheet flow, in ice sheet volume, and in Earth's gravity field. Between 1992 and 2020, the ice sheets contributed 21.0±1.9 mm to global mean sea level, with the rate of mass loss rising from 105 Gt yr−1 between 1992 and 1996 to 372 Gt yr−1 between 2016 and 2020. In Greenland, the rate of mass loss is 169±9 Gt yr−1 between 1992 and 2020, but there are large inter-annual variations in mass balance, with mass loss ranging from 86 Gt yr−1 in 2017 to 444 Gt yr−1 in 2019 due to large variability in surface mass balance. In Antarctica, ice losses continue to be dominated by mass loss from West Antarctica (82±9 Gt yr−1) and, to a lesser extent, from the Antarctic Peninsula (13±5 Gt yr−1). East Antarctica remains close to a state of balance, with a small gain of 3±15 Gt yr−1, but is the most uncertain component of Antarctica's mass balance. The dataset is publicly available at https://doi.org/10.5285/77B64C55-7166-4A06-9DEF-2E400398E452 (IMBIE Team, 2021).

Reconstruction of the palaeo‐sea level of Britain and Ireland arising from empirical constraints of ice extent: implications for regional sea level forecasts and North American ice sheet volume

ABSTRACTReconstructions of palaeo‐sea level are vital for predicting future sea level change and constraining palaeo‐ice sheet reconstructions, as well as being useful for a wide array of applications across Quaternary Science. Previous reconstructions of the palaeo‐sea level of Britain and Ireland relied on a circular tuning of glacio‐isostatic models: input ice sheet thicknesses and extents were iteratively altered to fit relative sea level data. Here we break that circularity by utilizing new data from the BRITICE‐CHRONO project, which constrains the position of the British–Irish ice sheet margin through time, and we compare derived glacio‐isostatic modelling to the rich relative sea level record. We test a combination of plausible ice thickness scenarios which account for the uncertainty of ice margin position over the North Sea, demonstrating the region where regional sea level data could distinguish between different glaciation scenarios. Our optimal reconstruction is then combined with several global‐scale reconstructions. As the signal of the British–Irish Ice Sheet is constrained, we demonstrate how the relative sea level record of Britain and Ireland can be used to test reconstructions of far‐field ice sheets (e.g. Antarctica, Eurasia and the Laurentide). The derived palaeo‐topography data are likely to be useful for multiple disciplines. Finally, our improved method of sea level reconstruction impacts predictions of contemporary vertical land motion.

Modelling feedbacks between the Northern Hemisphere ice sheets and climate during the last glacial cycle

Abstract. During the last glacial cycle (LGC), ice sheets covered large parts of Eurasia and North America, which resulted in ∼120 m of sea level change. Ice sheet–climate interactions have considerable influence on temperature and precipitation patterns and therefore need to be included when simulating this time period. Ideally, ice sheet–climate interactions are simulated by a high-resolution Earth system model. While these models are capable of simulating climates at a certain point in time, such as the pre-industrial (PI) or the Last Glacial Maximum (LGM; 21 000 years ago), a full transient glacial cycle is currently computationally unfeasible as it requires a too-large amount of computation time. Nevertheless, ice sheet models require forcing that captures the gradual change in climate over time to calculate the accumulation and melt of ice and its effect on ice sheet extent and volume changes. Here we simulate the LGC using an ice sheet model forced by LGM and PI climates. The gradual change in climate is modelled by transiently interpolating between pre-calculated results from a climate model for the LGM and the PI. To assess the influence of ice sheet–climate interactions, we use two different interpolation methods: the climate matrix method, which includes a temperature–albedo and precipitation–topography feedback, and the glacial index method, which does not. To investigate the sensitivity of the results to the prescribed climate forcing, we use the output of several models that are part of the Paleoclimate Modelling Intercomparison Project Phase III (PMIP3). In these simulations, ice volume is prescribed, and the climate is reconstructed with a general circulation model (GCM). Here we test those models by using their climate to drive an ice sheet model over the LGC. We find that the ice volume differences caused by the climate forcing exceed the differences caused by the interpolation method. Some GCMs produced unrealistic LGM volumes, and only four resulted in reasonable ice sheets, with LGM Northern Hemisphere sea level contribution ranging between 74–113 m with respect to the present day. The glacial index and climate matrix methods result in similar ice volumes at the LGM but yield a different ice evolution with different ice domes during the inception phase of the glacial cycle and different sea level rates during the deglaciation phase. The temperature–albedo feedback is the main cause of differences between the glacial index and climate matrix methods.

Ice Volume and Insolation Forcing of Abrupt Strengthening of East Asian Winter Monsoon During Glacial Inceptions

AbstractIt is generally accepted that the glacial‐interglacial variations of the East Asian winter monsoon (EAWM) are controlled by the volume of the Northern Hemisphere ice sheets (NHIS), but the fact that they exhibit different evolution patterns during glacial inceptions is often overlooked. By generating an independent chronology framework and integrating multi‐proxy records from the loess sections on the central Chinese Loess Plateau, here we show that the rapid intensifying of the EAWM during glacial inceptions reflects millennial perturbations in the large‐scale atmospheric circulation in East Asia in response to insolation‐triggered abrupt North Atlantic cooling. This climate teleconnection between North Atlantic and East Asia is found to be particularly effective only when the NHIS reaches a critical large size. Our integrated multi‐proxy records thus highlight the key role of ice volume in modulating the response of the EAWM to insolation‐triggered North Atlantic cooling during the interglacial‐glacial transitions.

Rapid southeastern Laurentide Ice Sheet thinning during the last deglaciation revealed by elevation profiles of in situ cosmogenic 10Be

Accurate reconstruction of Laurentide Ice Sheet volume changes following the Last Glacial Maximum is critical for understanding ice sheet contribution to sea-level rise, the resulting influence of meltwater on oceanic circulation, and the spatial and temporal patterns of deglaciation. Here, we provide empirical constraints on Laurentide Ice Sheet thinning during the last deglaciation by measuring in situ cosmogenic 10Be in 81 samples collected along vertical transects of nine mountains in the northeastern United States. In conjunction with 107 exposure age samples over five vertical transects from previous studies, we reconstruct ice sheet thinning history. At peripheral sites (within 200 km of the terminal moraine), we find evidence for ∼600 m of thinning between 19.5 ka and 17.5 ka, which is coincident with the slow initial margin retreat indicated by varve records. At locations >400 km north of the terminal moraine, exposure ages above and below 1200 m a.s.l. exhibit different patterns. Ages above this elevation are variable and older, while lower elevation ages are indistinguishable over 800−1000 m elevation ranges, a pattern that suggests a subglacial thermal boundary at ∼1200 m a.s.l. separating erosive, warm-based ice below and polythermal, minimally erosive ice above. Low-elevation ages from up-ice mountains are between 15 ka and 13 ka, which suggests rapid thinning of ∼1000 m coincident with Bølling-Allerød warming. These rates of rapid paleo-ice thinning are comparable to those of other vertical exposure age transects around the world and may have been faster than modern basin-wide thinning rates in Antarctica and Greenland, which suggests that the southeastern Laurentide Ice Sheet was highly sensitive to a warming climate.

High interstadial sea levels over the past 420ka from the Huon Peninsula, Papua New Guinea

The history of sea level across the Quaternary is essential for assessing past and future climate. Global sea-level reconstructions are typically derived from oxygen isotope curves, but require calibration with geological constraints that are scarce prior to the last glacial cycle (>130 thousand years ago). Here we show that the coral reef terrace sequence at the Huon Peninsula (Papua New Guinea) provides such constraints up to ∼420 thousand years ago, through a geometric analysis of high-resolution topographic data. We derive a northward tectonic tilt as regional deformation pattern, and estimate relative sea level for 31 Quaternary periods, including several periods for which no relative sea level data exists elsewhere. Supported by numerical reef models, these estimates suggest that oxygen isotope-based global mean sea-level curves systematically underestimate interstadial sea-level elevations, by up to ∼20 m. Compared to those curves, our results imply a stronger degree of non-linearity between ice-sheet volumes and global temperatures within Quaternary glacial cycles.

Simulations of firn processes over the Greenland and Antarctic ice sheets: 1980–2021

Abstract. Conversion of altimetry-derived ice-sheet volume change to mass requires an understanding of the evolution of the combined ice and air content within the firn column. In the absence of suitable techniques to observe the changes to the firn column across the entirety of an ice sheet, the firn column processes are typically modeled. Here, we present new simulations of firn processes over the Greenland and Antarctic ice sheets (GrIS and AIS) using the Community Firn Model and atmospheric reanalysis variables for more than four decades. A data set of more than 250 measured depth–density profiles from both ice sheets provides the basis of the calibration of the dry-snow densification scheme. The resulting scheme results in a reduction in the rate of densification, relative to a commonly used semi-empirical model, through a decreased dependence on the accumulation rate, a proxy for overburden stress. The 1980–2020 modeled firn column runoff, when combined with atmospheric variables from MERRA-2, generates realistic mean integrated surface mass balance values for the Greenland (+390 Gt yr−1) and Antarctic (+2612 Gt yr−1) ice sheets when compared to published model-ensemble means. We find that seasonal volume changes associated with firn air content are on average approximately 2.5 times larger than those associated with mass fluxes from surface processes for the AIS and 1.5 times larger for the GrIS; however, when averaged over multiple years, ice and air-volume fluctuations within the firn column are of comparable magnitudes. Between 1996 and 2019, the Greenland Ice Sheet lost nearly 5 % of its firn air content, indicating a reduction in the total meltwater retention capability. Nearly all (94 %) of the meltwater produced over the Antarctic Ice Sheet is retained within the firn column through infiltration and refreezing.

Dynamic regimes of the Greenland Ice Sheet emerging from interacting melt–elevation and glacial isostatic adjustment feedbacks

Abstract. The stability of the Greenland Ice Sheet under global warming is governed by a number of dynamic processes and interacting feedback mechanisms in the ice sheet, atmosphere and solid Earth. Here we study the long-term effects due to the interplay of the competing melt–elevation and glacial isostatic adjustment (GIA) feedbacks for different temperature step forcing experiments with a coupled ice-sheet and solid-Earth model. Our model results show that for warming levels above 2 ∘C, Greenland could become essentially ice-free within several millennia, mainly as a result of surface melting and acceleration of ice flow. These ice losses are mitigated, however, in some cases with strong GIA feedback even promoting an incomplete recovery of the Greenland ice volume. We further explore the full-factorial parameter space determining the relative strengths of the two feedbacks: our findings suggest distinct dynamic regimes of the Greenland Ice Sheets on the route to destabilization under global warming – from incomplete recovery, via quasi-periodic oscillations in ice volume to ice-sheet collapse. In the incomplete recovery regime, the initial ice loss due to warming is essentially reversed within 50 000 years, and the ice volume stabilizes at 61 %–93 % of the present-day volume. For certain combinations of temperature increase, atmospheric lapse rate and mantle viscosity, the interaction of the GIA feedback and the melt–elevation feedback leads to self-sustained, long-term oscillations in ice-sheet volume with oscillation periods between 74 000 and over 300 000 years and oscillation amplitudes between 15 %–70 % of present-day ice volume. This oscillatory regime reveals a possible mode of internal climatic variability in the Earth system on timescales on the order of 100 000 years that may be excited by or synchronized with orbital forcing or interact with glacial cycles and other slow modes of variability. Our findings are not meant as scenario-based near-term projections of ice losses but rather providing insight into of the feedback loops governing the “deep future” and, thus, long-term resilience of the Greenland Ice Sheet.

Dynamic boreal summer atmospheric circulation response as negative feedback to Greenland melt during the MIS-11 interglacial

Abstract. The unique alignment of orbital precession and obliquity during the Marine Isotope Stage 11 (MIS-11) interglacial produced perhaps the longest period of planetary warmth above preindustrial conditions in the past 800 kyr. Reconstructions point to a significantly reduced Greenland ice sheet volume during this period as a result, although the remaining extent and volume of the ice sheet are poorly constrained. A series of time slice simulations across MIS-11 using a coupled climate model indicates that boreal summer was particularly warm around Greenland and the high latitudes of the Atlantic sector for a period of at least 20 kyr. This state of reduced atmospheric baroclinicity, coupled with an enhanced and poleward-shifted intertropical convergence zone and North African monsoon, favored weakened high-latitude winds and the emergence of a single, unified midlatitude jet stream across the North Atlantic sector during boreal summer. Consequent reductions in the lower-tropospheric meridional eddy heat flux over the North Atlantic therefore emerge as negative feedback to additional warming over Greenland. The relationship between Greenland precipitation and the state of the North Atlantic jet is less apparent, but slight changes in summer precipitation appear to be dominated by increases during the remainder of the year. Such a dynamic state is surprising, as it bears stronger resemblance to the unified-jet state postulated as typical for glacial states than to the modern-day interglacial state.

Layered seawater intrusion and melt under grounded ice

Abstract. Increasing melt of ice sheets at their floating or vertical interfaces with the ocean is a major driver of marine ice sheet retreat and sea level rise. However, the extent to which warm, salty seawater may drive melting under the grounded portions of ice sheets is still not well understood. Previous work has explored the possibility that dense seawater intrudes beneath relatively light subglacial freshwater discharge, similar to the “salt wedge” observed in many estuarine systems. In this study, we develop a generalized theory of layered seawater intrusion under grounded ice, including where subglacial hydrology occurs as a macroporous water sheet over impermeable beds or as microporous Darcy flow through permeable till. Using predictions from this theory, we show that seawater intrusion over flat or reverse-sloping impermeable beds may feasibly occur up to tens of kilometers upstream of a glacier terminus or grounding line. On the other hand, seawater is unlikely to intrude more than tens of meters through permeable till. Simulations using the Ice-sheet and Sea-level System Model (ISSM) show that even just a few hundred meters of basal melt caused by seawater intrusion upstream of marine ice sheet grounding lines can cause projections of marine ice sheet volume loss to be 10 %–50 % higher. Kilometers of intrusion-induced basal melt can cause projected ice sheet volume loss to more than double. These results suggest that further observational, experimental and numerical investigations are needed to determine the conditions under which seawater intrusion occurs and whether it will indeed drive rapid marine ice sheet retreat and sea level rise in the future.

A large West Antarctic Ice Sheet explains early Neogene sea-level amplitude.

Early to Middle Miocene sea-level oscillations of approximately 40-60 m estimated from far-field records1-3 are interpreted to reflect the loss of virtually all East Antarctic ice during peak warmth2. This contrasts with ice-sheet model experiments suggesting most terrestrial ice in East Antarctica was retained even during the warmest intervals of the Middle Miocene4,5. Data and model outputs can be reconciled if a large West Antarctic Ice Sheet (WAIS) existed and expanded across most of the outer continental shelf during the Early Miocene, accounting for maximum ice-sheet volumes. Here we provide the earliest geological evidence proving large WAIS expansions occurred during the Early Miocene (~17.72-17.40 Ma). Geochemical and petrographic data show glacimarine sediments recovered at International Ocean Discovery Program (IODP) Site U1521 in the central Ross Sea derive from West Antarctica, requiring the presence of a WAIS covering most of the Ross Sea continental shelf. Seismic, lithological and palynological data reveal the intermittent proximity of grounded ice to Site U1521. The erosion rate calculated from this sediment package greatly exceeds the long-term mean, implying rapid erosion of West Antarctica. This interval therefore captures a key step in the genesis of a marine-based WAIS and a tipping point in Antarctic ice-sheet evolution.