Ice Sheet Elevation Research Articles

Millennial and orbital-scale variability in a 54 000-year record of total air content from the South Pole ice core

Abstract. The total air content (TAC) of polar ice cores has long been considered a potential proxy for past ice sheet elevation. Recent work, however, has shown that a variety of other factors also influence this parameter. In this paper we present a high-resolution TAC record from the South Pole ice core (SPC14) covering the last 54 000 years and discuss the implications of the data for interpreting TAC from ice cores. The SPC14 TAC record shows multiple features of interest, including (1) long-term orbital-scale variability, (2) millennial-scale variability in the Holocene and last glacial period, and (3) a period of stability from 35 to 25 ka. The longer, orbital-scale variations in TAC are highly correlated with integrated summer insolation (ISI), corroborating the potential of TAC to provide an independent dating tool via orbital tuning. Large millennial-scale variability in TAC during the last glacial period is positively correlated with past accumulation rate reconstructions as well as δ15N-N2, a firn thickness proxy. These TAC variations are too large to be controlled by direct effects of temperature and too rapid to be tied to elevation changes. We propose that grain size metamorphism near the firn surface explains these changes. We note, however, that at sites with different climate histories than the South Pole, TAC variations may be dominated by other processes. Our observations of millennial-scale variations in TAC show a different relationship with accumulation rate than observed at sites in Greenland.

Unraveling the complexities of the Last Glacial Maximum climate: the role of individual boundary conditions and forcings

Abstract. In order to quantify the relative importance of individual boundary conditions and forcings, including greenhouse gases, ice sheets, and Earth's orbital parameters, on determining Last Glacial Maximum (LGM) climate, we have performed a series of LGM experiments using a state-of-the-art climate model AWI-ESM, in which different combinations of boundary conditions and forcings have been applied following the protocol of Paleoclimate Modelling Intercomparison Project phase 4 (PMIP4). In good agreement with observational proxy records, a general colder and drier climate is simulated in our full-forced LGM experiment as compared to the present-day simulation. Our simulated results from non-full-forced sensitivity simulations reveal that both the greenhouse gases and ice sheets play a major role in defining the anomalous LGM surface temperature compared to today. Decreased greenhouse gases in LGM as compared to present day leads to a non-uniform global cooling with polar amplification effect. The presence of LGM ice sheets favors a warming over the Arctic and northern Atlantic oceans in boreal winter, as well as a cooling over regions with the presence of ice sheets. The former is induced by a strengthening in the Atlantic meridional overturning circulation (AMOC) transporting more heat to high latitudes, whilst the latter is due to the increased surface albedo and elevation of ice sheets. We find that the Northern Hemisphere monsoon precipitation is influenced by the opposing effects of LGM greenhouse gases and ice sheets. Specifically, the presence of ice sheets leads to significant drying in the Northern Hemisphere monsoon regions, while a reduction in greenhouse gases results in increased monsoon rainfall. Based on our model results, continental ice sheets exert a major control on atmospheric dynamics and the variability of El Niño–Southern Oscillation (ENSO). Moreover, our analysis also implies a nonlinearity in climate response to LGM boundary conditions and forcings.

The Rise and Fall of Alaska and Yukon Glaciers Detected by TOPEX/Poseidon and Jason‐2 Altimeters Using a Novel Glacier‐Threshold Method

AbstractRadar altimetry has been used to monitor sea level changes and ice sheet elevation changes for decades. Over mountain glaciers, radar altimetry has limited applications due to contaminated waveforms caused by complex glacier surfaces and steep terrains. In this study, we develop a glacier‐threshold method (GTM) to determine glacier elevation changes in Alaska and Yukon. The GTM retracks waveforms, reduces terrain effect and detects invalid elevation observations from the TOPEX/Poseidon (T/P) and Jason‐2 (J2) altimeters, resulting in an average usable rate of 35% from original altimeter‐measured heights. The selected measurements are used to construct time series of glacier elevation changes over 1993–2002 (T/P) and 2008–2016 (J2) at 47 sites. A crossover analysis, validation by airborne laser altimetry observations, and comparisons with recent studies confirm the estimated glacier elevation changes. Our findings suggest that the thinning of glaciers in Alaska and Yukon has been ubiquitous in recent years. The site near Walsh Glacier has the highest thinning rate of −5.71 ± 0.09 m/yr, followed by Chitina Glacier at −4.51 ± 0.21 m/yr. Only 17% of sites show glacier thickening due to surges and mass accumulations. Using a sophisticated data processing algorithm like the GTM, we show that altimeter data from the repeat TOPEX‐Jason missions can be used to monitor long‐term glacier elevation changes at inaccessible spots, creating an additional value for altimeter missions originally purposed for monitoring long‐term sea level change.

Reconstructing Continuous Ice Sheet Elevation Changes in the Amundsen Sea Sector During 2003–2021 by Merging Envisat, ICESat, CryoSat‐2, and ICESat‐2 Multi‐Altimeter Observations

AbstractThe Amundsen Sea (AS) sector in West Antarctica accounts for a significant proportion of Earth's ice losses and is the largest contributor of Antarctica's mass loss. To evaluate its contribution to global sea‐level rise, we reconstruct the long‐term continuous surface elevation changes (CSEC) record of the AS sector by an improved least‐squares plane fitting method (ILSPFM), which merged the relative surface elevation change (SEC) series instead of height from Envisat, ICESat, CryoSat‐2, and ICESat‐2 missions during 2003–2021. The accuracy of CSEC is improved by 25.9% using ILSPFM. The average rate of CSEC in the AS sector was −24.25 ± 0.48 cm yr−1 during 2003–2021. The largest signals of SEC are found over Pine Island, Thwaites, and Pope Glaciers, with the largest decline of SEC over Pope Glacier with a total SEC of −82.44 ± 7.21 m and an annual change rate of −4.34 ± 0.38 m yr−1. The ridge between Pine Island and Thwaites Glaciers is found in the AS sector, indicating that the change of ice sheet is dynamic thinning and closely related to the topography and the distance from the grounding line. Compared with meteorological data sets, we find that the codirectional fluctuation in CSEC is delayed by 3 months with surface temperature, and the precipitation leading SEC series as the phase arrow points straight down from the cross wavelet transform. Our new record shows that the AS sector thinned rapidly from 2003 to 2021 but decelerated from 2019 to 2021, and it was clearly correlated to the surface temperature, precipitation, and local terrain.

Wilkes subglacial basin ice sheet response to Southern Ocean warming during late Pleistocene interglacials

The response of the East Antarctic Ice Sheet to past intervals of oceanic and atmospheric warming is still not well constrained but is critical for understanding both past and future sea-level change. Furthermore, the ice sheet in the Wilkes Subglacial Basin appears to have undergone thinning and ice discharge events during recent decades. Here we combine glaciological evidence on ice sheet elevation from the TALDICE ice core with offshore sedimentological records and ice sheet modelling experiments to reconstruct the ice dynamics in the Wilkes Subglacial Basin over the past 350,000 years. Our results indicate that the Wilkes Subglacial Basin experienced an extensive retreat 330,000 years ago and a more limited retreat 125,000 years ago. These changes coincide with warmer Southern Ocean temperatures and elevated global mean sea level during those interglacial periods, confirming the sensitivity of the Wilkes Subglacial Basin ice sheet to ocean warming and its potential role in sea-level change.

Extraction and analysis of elevation changes in Antarctic ice sheet from CryoSat-2 and Sentinel-3 radar altimeters

New radar altimeters have supported elevation modeling with greater accuracy and extended periods in recent years. Sentinel-3 has been rarely used in previous studies for monitoring the Antarctic ice sheet. The CryoSat-2 and Sentinel-3 satellite radar altimeter data were combined to extract the elevation changes in the Antarctic ice sheet. First, a subregional data filtering method based on a clustering algorithm was proposed to improve the accuracy of the elevation change results for the outliers in the original measurements. Next, an improved model-fitting model that incorporated multiple parameters was used to extract elevation changes, and the results were validated using airborne laser altimetry data and ICESat-2 data. The average changes in the Antarctic ice sheet elevation from 2016 to 2019 were −4.3 ± 0.9 cm / y. The elevation changes of the ice shelf edges with large slopes and complex topography were much larger than the interior. Some specific areas showed substantial elevation changes. The most significant surface decreases occurred at the Pine Island Glacier and the Totten Glacier, while a pronounced surface heightening existed at the Kamb Ice Stream. We reveal an effective method for combining Antarctic ice sheet data from new radar altimeters. This method supports the long-term monitoring of the Antarctic ice sheet and global climate change in the future.

No general stability conditions for marine ice-sheet grounding lines in the presence of feedbacks

The “marine ice-sheet instability” hypothesis continues to be used to interpret the observed mass loss from the Antarctic and Greenland ice sheets. This hypothesis has been developed for conditions that do not account for feedbacks between ice sheets and environmental conditions. However, snow accumulation and the ice-sheet surface melting depend on the surface temperature, which is a strong function of elevation. Consequently, there is a feedback between precipitation, atmospheric surface temperature and ice-sheet surface elevation. Here, we investigate stability conditions of a marine-based ice sheet in the presence of such a feedback. Our results show that no general stability condition similar to one associated with the “marine ice-sheet instability” hypothesis can be determined. Stability of individual configurations can be established only on a case-by-case basis. These results apply to a wide range of feedbacks between marine ice sheets and atmosphere, ocean and lithosphere.

CryoSat-2 interferometric mode calibration and validation: A case study from the Austfonna ice cap, Svalbard

Satellite radar altimetry is widely used to measure glacier and ice sheet elevation changes, but can suffer from uncertainties related to geolocation and signal penetration. The unique capabilities of ESA's CryoSat-2 allow for accurate geolocation but impacts from signal penetration persist. This study uses surface elevations from Global Navigation Satellite System and airborne laser transects over the Austfonna ice cap, Svalbard, to measure the elevation bias of CryoSat-2 Point-of-closest-approach (POCA) and swath points, and to provide validation for dhdt estimates derived through the application of a least-squares plane-fit algorithm to these data. The mean elevation bias of swath points varies between 1 and 1.5m of penetration, which is close to observed winter snow depths. Histograms of POCA elevation bias for the applied leading-edge retracker peak near the surface, with a distribution skewed towards the sub-surface. At the onset of surface melt, surface scattering dominates backscatter, and penetration reduces. This results in spurious peaks in derived elevation and mass change time series. In spite of this seasonal variability in elevation bias, the validation dhdt dataset demonstrates that the CryoSat-2 dhdt estimates are robust on multi-year timescales. The transition from volume to surface scattering suggests the potential to estimate yearly snowpack thickness.

ALPS: A Unified Framework for Modeling Time Series of Land Ice Changes

Modeling time series (TS) is a research focus in cryospheric sciences because of the complexity and multiscale nature of events of interest. Highly nonuniform sampling of measurements from different sensors with different levels of accuracy, as is typical for measurements of ice sheet elevations, makes the problem even more challenging. In this article, we propose a spline-based approximation framework (ALPS—Approximation by Localized Penalized Splines) for modeling TS of land ice changes. The localized support of the B-spline basis functions enable robustness to nonuniform sampling, a considerable improvement over other global and piecewise local models. With features like discrete-coordinate-difference-based penalization and two-level outlier detection, ALPS further guarantees the stability and quality of approximations. ALPS incorporates rigorous model uncertainty estimates with all approximations. As demonstrated by examples, ALPS performs well for a variety of data sets, including TS of ice sheet thickness, elevation, velocity, and terminus locations. The robust estimation of TS and their derivatives facilitates new applications, such as the reconstruction of high-resolution elevation change records by fusing sparsely sampled TS of ice sheet thickness changes with modeled firn thickness changes, and the analysis of the relationship between different outlet glacier observations to gain new insights into processes and forcing.

The effect of climate factors on Antarctic Ice Sheet Changes from Radar Altimetry

The Antarctic ice sheet change is one of the key factors affecting global climate change. The influence of climate factors obscures the understanding to elevation change of the ice sheet. Due to the lack of understanding of the quantitative relationship between climate events and elevation change of Antarctic ice sheet, we here combined climate factor data to quantify the impact of climate events on the elevation change of Antarctic ice sheet. A strong correlation between climate factors and elevation change of ice sheet was found. And the influence between them exists time delay in some extent. Meanwhile, the influence of climate factors in the Antarctic ice sheet change was separated from the ice sheet change based on the Independent Component Analysis (ICA) and multivariate linear regression model. The study also indicates that the influence of climate factors may be an important contribution for the accelerated decline of ice sheet elevation in the Amundsen Sea.

Brief communication: Ice sheet elevation measurements from the Sentinel-3A and Sentinel-3B tandem phase

Abstract. Over the coming decade, the quartet of Copernicus Sentinel-3 satellite altimeters will provide a continuous record of ice sheet elevation change. Ensuring consistency of measurement between the four satellites requires rigorous in-flight inter-comparison. To facilitate this, Sentinel-3B was initially flown in a unique tandem formation with Sentinel-3A, enabling near-instantaneous, co-located measurements of surface elevation to be acquired. Here, we analyse tandem measurements of ice sheet elevation, to show that both instruments operate with statistically equivalent accuracy and precision, even over complex ice margin terrain. This analysis demonstrates that both satellites can be used interchangeably to study ice sheet evolution.

The influence of Antarctic ice loss on polar motion: an assessment based on GRACE and multi-mission satellite altimetry

Increasing ice loss of the Antarctic Ice Sheet (AIS) due to global climate change affects the orientation of the Earth’s spin axis with respect to an Earth-fixed reference system (polar motion). Here the contribution of the decreasing AIS to the excitation of polar motion is quantified from precise time variable gravity field observations of the Gravity Recovery and Climate Experiment (GRACE) and from measurements of the changing ice sheet elevation from altimeter satellites. While the GRACE gravity field models need to be reduced by noise and leakage effects from neighboring subsystems, the ice volume changes observed by satellite altimetry have to be converted into ice mass changes. In this study we investigate how much individual gravimetry and altimetry solutions differ from each other. We show that due to combination of individual solutions systematic and random errors of the data processing can be reduced and the robustness of the geodetic derived AIS polar motion excitations can be increased. We investigate the interannual variability of the Antarctic polar motion excitation functions by means of piecewise linear trends. We find that the long-term behavior of the three ice sheet subregions: EAIS (East Antarctic Ice Sheet), WAIS (West Antarctic Ice Sheet) and APIS (Antarctic Peninsula Ice Sheet) is quite different. While APIS polar motion excitations show no significant interannual variations during the study period 2003-2015, the trend of the WAIS and EAIS polar motion excitations increased in 2006 and again in 2009 while it started slightly to decline in 2013. AIS mass changes explain about 45% of the observed magnitude of the polar motion vector (excluding glacial isosatic adjustment). They cause the pole position vector to drift along 59^{circ } East longitude with an amplitude of 2.7 mas/yr. Thus the contribution of the AIS has to be considered to close the budget of the geophysical excitation functions of polar motion.

Antarctic Ice Sheet Elevation Impacts on Water Isotope Records During the Last Interglacial

AbstractChanges of the topography of the Antarctic Ice Sheet (AIS) can complicate the interpretation of ice core water stable isotope measurements in terms of temperature. Here, we use a set of idealized AIS elevation change scenarios to investigate this for the warm Last Interglacial (LIG). We show that LIG δ18O against elevation relationships is not uniform across Antarctica and that the LIG response to elevation is lower than the preindustrial response. The effect of LIG elevation‐induced sea ice changes on δ18O is small, allowing us to isolate the effect of elevation change alone. Our results help to define the effect of AIS changes on the LIG δ18O signals and should be invaluable to those seeking to use AIS ice core measurements for these purposes. Especially, our simulations strengthen the conclusion that ice core measurements from the Talos Dome core exclude the loss of the Wilkes Basin at around 128 kyr.

Laser pointing and characterization parameter determination methods based on laser profile arrays of ICESat/GALS.

Satellite laser altimeter data are used for polar ice sheet elevation mapping, vegetation mapping, etc. Data quality mainly depends on complex relationships among several factors in the path of laser transmission and on illuminated surfaces, including clouds, atmospheric aerosol, satellite pointing, laser energy, topography, footprint size, shape and orientation. The precise pointing of the transmitted laser pulse is critical for improving the horizontal accuracy of the footprint on the ground. Thus, we extracted the centroid of the laser profile array (LPA) image of ICESat/GLAS by 1/e2 maximum energy distribution method. The results show that the accuracy of extraction of the LPA's centroid improved by 0.3 pixel, and the relative positioning accuracy improved by 0.11 pixel. The fast Fourier transform and Fourier series fitting of the LPA centroid has been implemented to detect the periodic change and analyze the model regularity. The results show that the centroid of the LPA undergoes four periodic changes: 1.83 × 10-4, 3.36 × 10-4, 5.19 × 10-4, and 6.71 × 10-4Hz. The correlation of fit is a good indicator (R2=0.86) and accurate up to 0.4 arcsec (approximately 0.13 pixel). Finally, we extract and estimate the LPA characteristic parameters (eccentricity, orientation, total intensity, and major axis) in different campaigns. We observe that the results obtained by the 1/e2maximum energy distribution are only approximate.

Sky-high to the sea floor

Seafloor Bathymetry The ICESat-2 satellite, launched in 2018, was developed primarily to measure the surface elevation of polar ice sheets, but it can do much more than that. One of its other capabilities is determining water depths to as much as 40 meters. Thomas et al. used this ability, combined with remote optical imagery, to map the shallow seafloor in coastal areas of Florida, Crete, and Bermuda. This work demonstrates the viability of using ICESat-2 to help make global, space-borne estimates of nearshore bathymetry in shallow, clear water environments. Geophys. Res. Lett. 10.1029/2020GL092170 (2021).

Influence of the height of Antarctic ice sheet on its climate

The influence of the height of the Antarctic Ice Sheet (AIS) on its climate is investigated through numerical modeling, using NCAR-CESM-CAM5, which is found to be one of the best performing models over the region. The investigation reveals that on the reduction of its height, the troposphere over Antarctica will become warmer and wetter, annual precipitation and snow accumulation would enhance, and there will be a net enhancement in southward energy transport towards the Antarctic from the lower latitudes. The response will be opposite if the height of the AIS increases. The effect is found to be driven by the radiation imbalance developed from the changes in the outgoing longwave radiation at the top of the atmosphere due to perturbations in the ice sheet's elevation. An increase (decrease) in precipitation will occur on reduction (enhancement) of the AIS, which will try to stabilize its height, but this will be accompanied by an increase (decrease) in the surface temperature, which would have an opposite effect. A significant influence of the AIS elevation was also felt on the surface winds, stationary waves, and the eddy-driven jet around the continent, highlighting the impact that changes in the height of AIS have on the surrounding atmospheric circulation.

Past Warmth and Its Impacts During the Holocene Thermal Maximum in Greenland

Higher boreal summer insolation in the early to middle Holocene drove thousands of years of summer warming across the Arctic. Modern-day warming has distinctly different causes, but geologic data from this past warm period hold lessons for the future. We compile Holocene temperature reconstructions from ice, lake, and marine cores around Greenland, where summer temperatures are globally important due to their influence on ice sheet mass balance, ocean circulation, and sea ice. Highlighting and accounting for some key issues with proxy interpretation, we find that much of Greenland experienced summers 3 to 5°C warmer than the mid-twentieth century in the early Holocene—earlier and stronger warming than often presumed. Warmth had dramatic consequences: Many glaciers disappeared, perennial sea ice retreated, plants and animals migrated northward, the Greenland Ice Sheet shrank rapidly, and increased meltwater discharge led to strong marine water stratification and enhanced winter sea ice in some areas. ▪ Summer air temperatures and open ocean temperatures around much of Greenland peaked in the early Holocene in response to elevated summer insolation. ▪ Peak summer air temperatures ranged from 3 to 5°C warmer than the mid-twentieth century in northwest and central Greenland to perhaps 1 to 2°C warmer in south Greenland. ▪ Many differences between records can be explained by proxy seasonality, ice sheet elevation changes, vegetation analogs and lags, and the nearshore effects of ice sheet meltwater. ▪ Early Holocene warmth dramatically affected glaciers and the Greenland Ice Sheet; meltwater discharge, nearshore ocean salinity, and sea ice; and diverse flora and fauna.

Elevation Changes of the Antarctic Ice Sheet from Joint Envisat and CryoSat-2 Radar Altimetry

The elevation changes of ice sheets have been recognized as an essential climate variable. Long-term time series of these changes are an important parameter to understand climate change, and the longest time-series of ice sheet elevation changes can be derived from combining multiple Ku-band satellite altimetry missions. However, unresolved intermission biases obscure the record. Here, we revise the mathematical model commonly used in the literature to simultaneously correct for intermission bias and ascending–descending bias to ensure the self-consistency and cohesion of the elevation time series across missions. This updated approach is applied to combine Envisat and CryoSat-2 radar altimetry in the period of 2002–2019. We tested this approach by validating it against airborne and satellite laser altimetry. Combining the detailed temporal and spatial evolution of elevation changes with firn densification-modeled volume changes due to surface processes, we found that the Amundsen Sea sector accounts for most of the total volume loss of the Antarctic Ice Sheet (AIS), mainly from ice dynamics. However, surface processes dominate the volume changes in the key regions, such as the Totten Glacier sector, Dronning Maud Land, Princess Elizabeth Land, and the Bellingshausen Sea sector. Overall, accelerated volume loss in the West Antarctic continues to outpace the gains observed in the East Antarctic. The total volume change during 2002–2019 for the AIS was −68.7 ± 8.1 km3/y, with an acceleration of −5.5 ± 0.9 km3/y2.

Greenland Ice Sheet Elevation Change: Direct Observation of Process and Attribution at Summit

AbstractGreenland Ice Sheet surface elevation is changing as mass loss accelerates. In understanding elevation change, the magnitudes of physical processes involved are important for interpretation of altimetry and assessing changes in these processes. The four key processes are surface mass balance (SMB), firn densification, ice dynamics, and isostatic adjustment. We quantified these processes at Summit, Greenland, where monthly Global Navigation Satellite System (GNSS) snowmobile traverses measured elevation change from 2008 to 2018. We find an elevation increase of 0.019 m a−1. The sum of the effects of the four processes reproduces the measured elevation time series, in linear trend and in intra‐annual variability. The short‐term variability in elevation is primarily explained by the variability in SMB. Since SMB has not changed significantly over the last century, and the other processes change over longer time scales, the elevation change likely has been ongoing for at least the last 100 years.

The Community Firn Model (CFM) v1.0

Abstract. Models that simulate the evolution of polar firn are important for several applications in glaciology, including converting ice-sheet elevation change measurements to mass change and interpreting climate records in ice cores. We have developed the Community Firn Model (CFM), an open-source, modular model framework designed to simulate numerous physical processes in firn. The modules include firn densification, heat transport, meltwater percolation and refreezing, water isotope diffusion, and firn-air diffusion. The CFM is designed so that new modules can be added with ease. In this paper, we first describe the CFM and its modules. We then demonstrate the CFM's usefulness in two model applications that utilize two of its novel aspects. The CFM currently has the ability to run any of 13 previously published firn densification models, and in the first application we compare those models' results when they are forced with regional climate model outputs for Summit, Greenland. The results show that the models do not agree well (spread greater than 10 %) when predicting depth-integrated porosity, firn age, or the trend in surface elevation change. In the second application, we show that the CFM's coupled firn-air and firn densification models can simulate noble gas records from an ice core better than a firn-air model alone.