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The large ice sheets in Greenland and Antarctica are losing mass due to global warming. In particular, the acceleration of ice streams and thus the increased discharge into the ocean contributes significantly to global sea-level rise. The floating extensions of the ice streams counteract this, but intense basal melting can destabilise the ice shelves. In this thesis, a contribution is made to determine the melt rates of two ice shelves, which are crucial for the future mass losses of the respective ice sheets. In the north, the focus is on the Northeast Greenland Ice Stream (NEGIS) that feeds the Nioghalvfjerdsbrae (79°N Glacier). My analysis of phase-sensitive radar measurements indicates high melt rates near the onset of the ice stream and thus the presence of subglacial melt water, which is associated with the formation of the ice flow. An extensive study in my thesis reveals that the 79°N Glacier has been thinned out considerably in recent years due to extreme melt rates and that large channels have been formed. Melt rates of the Filchner Ice Shelf, Antarctica, which I also determined using phase-sensitive radar measurements, are comparatively low. I was able to attribute significant deviations from remote sensing-derived melt rates to inaccuracies in the used ice flow velocity field. Furthermore, I show that the use of newer velocity fields improves the determination of the melt rates from remote sensing. My analysis of melt rate time series in the vicinity of a channel indicates higher melt rates in the summer as well as several melt events spread over the entire measurement period. Another study combines measurements and numerical modelling and shows that higher melt rates must have occurred in the past than those that were measured. These would lead to the closure of the channel within 250 years. Thus, neither the channel itself nor the present day melt rates endanger the stability of one of the largest Antarctic ice shelves at present.

The large ice sheets on Earth respond to changes in the global climate. Ice mass loss increases with rising global mean temperature and thus is a major contributor to sea-level rise. In order to reduce the uncertainty to predict the contributions to sea-level rise of ice sheets, it is crucial to study how the ice sheets‘ fast-flowing drainage pathways (so-called ice streams) have evolved over the last thousand to millions of years. In this thesis, a contribution to the understanding of the flow characteristics of large ice streams in Greenland and Antarctica is performed by an analysis of (ice-penetrating) radar reflections within the ice column and at the ice base. My focus lies on the question how these data can be used to obtain information about present and paleo ice flow regimes. I concentrate on radar data acquired in Northeast Greenland in the upstream regions of the North East Greenland Ice Stream (NEGIS) and in the upstream catchment of the Nioghalvfjerdsbrae (79°N Glacier) as well as on data recorded at the onset of the Jutulstraumen Glacier in Antarctica. In my studies, I show that the NEGIS in its present form is a relatively young feature and that its geometry and flow characteristics are intertwined with the subglacial topography. I also found indications for a re-organization of ice stream activity in the NEGIS catchment during the Holocene. This suggests that ice streams are probably are less persistent than previously thought and adapt in their entire length to the changing geometry of the ice sheet on short time scales. In Antarctica, I investigate past ice flow patterns over a period of millions of years, as in the example of the Jutulstraumen Glacier basin in Antarctica. Many of the glacial and fluvial landscapes, which developed since the glaciation of Antarctica, have been mostly preserved under the contemporary thick ice sheet, and some even serve as basins for active subglacial lakes today.

Quaternary interglacials provide key observations of the Earth system's responses to orbital and greenhouse gas forcing. They also inform on the capabilities of Earth system models, used for projecting the polar ice-sheet and sea-level responses to a regional warmth comparable to that expected by 2100 C.E. However, a number of uncertainties remain regarding the processes and feedbacks linking climate, ice-sheet and sea-level changes during past warm intervals. Here, we delineate the major research questions that need to be resolved and future research directions that should be taken by the paleoclimate, sea-level and ice-sheet research communities in order to increase confidence in the use of past interglacial climate, ice-sheet and sea-level reconstructions to constrain future predictions. These questions were formulated during a joint workshop held by the PAGES-INQUA PALSEA (PALeo constraints on SEA level rise) and the PAGES-PMIP QUIGS (QUaternary InterGlacialS) Working Groups in September 2018.

Replacing estimates of C20 from the Gravity Recovery and Climate Experiment (GRACE) monthly gravity field solutions by those from satellite laser ranging (SLR) data and including degree one terms has become a standard procedure for proper science applications in the satellite gravimetry community. Here, we assess the impact of degree one terms, SLR-based C20 and C30 estimates on GRACE-derived polar ice sheet mass variations. We report that degree one terms recommended for GRACE Release 06 (RL06) data have an impact of 2.5 times more than those for GRACE RL05 data on the mass trend estimates over the Greenland and the Antarctic ice sheets. The latest recommended C20 solutions in GRACE Technical Note 14 (TN14) affect the mass trend estimates of ice sheets in absolute value by more than 50%, as compared to those in TN11 and TN07. The SLR-based C30 replacement has some impact on the Antarctic ice sheet mass variations, mainly depending on the length of the study period. This study emphasizes that reliable solutions of low degree spherical harmonics are crucial for accurately deriving ice sheet mass balance from satellite gravimetry.

Ice sheets and continental hydrology changes on decadal time scales are the dominant drivers of decadal scale polar motion., Earth’s spin axis has been wandering along the Greenwich meridian since about 2000, representing a 75° eastward shift from its long-term drift direction. The past 115 years have seen unequivocal evidence for a quasi-decadal periodicity, and these motions persist throughout the recent record of pole position, in spite of the new drift direction. We analyze space geodetic and satellite gravimetric data for the period 2003–2015 to show that all of the main features of polar motion are explained by global-scale continent-ocean mass transport. The changes in terrestrial water storage (TWS) and global cryosphere together explain nearly the entire amplitude (83 ± 23%) and mean directional shift (within 5.9° ± 7.6°) of the observed motion. We also find that the TWS variability fully explains the decadal-like changes in polar motion observed during the study period, thus offering a clue to resolving the long-standing quest for determining the origins of decadal oscillations. This newly discovered link between polar motion and global-scale TWS variability has broad implications for the study of past and future climate.

Ice sheet evolution is both driven by a complex flow dynamics and by physical mechanisms such as basal sliding, ice temperature or surface mass balance. In addition to those, many feedback loops are observed between the different implicated phenomena. That explains how complex is to model this evolution. However several models have been developed in that purpose. These models depend on influential parameters, which often are unfortunately poorly known. So they need to be correctly specified. Data assimilation can give a better estimation of these parameters thanks to observations which are quite rare in glaciology. In this thesis, we work on the setup of efficient data assimilation systems for two inverses problems involving ice sheet evolution. We work with a simplified ice sheet evolution model called Winnie in order to focus on the setup. Nevertheless Winnie takes into account the major complex processes of ice dynamics and can be used for studies with different time scales. The first part of the thesis focuses on developing a 4D-Var approach in order to retrieve the evolution of a climatic parameter for a typical time scale of 20 000 years. This approach require the implementation the adjoint code of the evolution model. In a second part, we focus on the spin-up problem. This calibration problem for short term (maximum 100 years) simulations involve retrieving jointly the initial state, the bedrock topography and basal sliding parameters. In order to solve this problem we develop an Ensemble Kalman Filter approach.; L'évolution des calottes polaires est régie à la fois par une dynamique d'écoulement complexe et par des mécanismes tel le glissement à la base, la température de la glace ou le bilan de masse en surface. De plus, de nombreuses boucles de rétroactions sont constatées entre les différents phénomènes impliquées. Tout ceci rend la modélisation de cette évolution complexe. Malgré tout, un certain nombre de modèles ont été développés dans cette optique. Ceux-ci font tous intervenir des paramètres influents qui dans certains cas sont peu ou pas connus. Ils nécessitent donc d'être correctement spécifiés. L'assimilation de données peut permettre une meilleure estimation de ces paramètres grâce à l'utilisation d'observations qui sont peu nombreuses en glaciologie. Dans cette thèse, nous nous intéressons à la mise en place de systèmes d'assimilation performants pour deux problèmes inverses concernant l'évolution des calottes polaires. Pour mieux nous concentrer sur ce point, nous avons travaillé avec un modèle d'évolution de calotte simplifié (appelé Winnie) qui, cependant, représente bien la plupart des processus complexes de la dynamique de la glace, et permet de travailler à différentes échelles de temps. Dans un premier temps, nous mettons en place une approche 4D-Var pour la reconstruction de l'évolution d'un paramètre climatique influant sur l'évolution d'une calotte sur une échelle de temps typique de 20 000 ans. Elle nécessite notamment l'écriture du code adjoint du modèle. Dans un second temps, nous nous intéressons au problème du spin-up. Ce problème de calibration du modèle pour des simulations à échelle de temps courtes (pas plus de 100 ans) consiste plus particulièrement en la reconstruction conjointe de l'état initial, de la topographie du socle rocheux et des paramètres de glissement basal. Nous développons ici une approche filtre de Kalman d'ensemble pour résoudre ce problème.

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Tamisiea, Mark E. Review; Book Chapter 4139 EL CAMINO WAY, PO BOX 10139, PALO ALTO, CA 94303-0897 USA; Regional sea level changes can deviate substantially from those of the global mean, can vary on a broad range of timescales, and in some regions can even lead to a reversal of long-term global mean sea level trends. The underlying causes are associated with dynamic variations in the ocean circulation as part of climate modes of variability and with an isostatic adjustment of Earth's crust to past and ongoing changes in polar ice masses and continental water storage. Relative to the coastline, sea level is also affected by processes such as earthquakes and anthropogenically induced subsidence. Present-day regional sea level changes appear to be caused primarily by natural climate variability. However, the imprint of anthropogenic effects on regional sea level-whether due to changes in the atmospheric forcing or to mass variations in the system-will grow with time as climate change progresses, and toward the end of the twenty-first century, regional sea level patterns will be a superposition of climate variability modes and natural and anthropogenically induced static sea level patterns. Attribution and predictions of ongoing and future sea level changes require an expanded and sustained climate observing system.

Ice sheet evolution is both driven by a complex flow dynamics and by physical mechanisms such as basal sliding, ice temperature or surface mass balance. In addition to those, many feedback loops are observed between the different implicated phenomena. That explains how complex is to model this evolution. However several models have been developed in that purpose. These models depend on influential parameters, which often are unfortunately poorly known. So they need to be correctly specified. Data assimilation can give a better estimation of these parameters thanks to observations which are quite rare in glaciology. In this thesis, we work on the setup of efficient data assimilation systems for two inverses problems involving ice sheet evolution. We work with a simplified ice sheet evolution model called Winnie in order to focus on the setup. Nevertheless Winnie takes into account the major complex processes of ice dynamics and can be used for studies with different time scales. The first part of the thesis focuses on developing a 4D-Var approach in order to retrieve the evolution of a climatic parameter for a typical time scale of 20 000 years. This approach require the implementation the adjoint code of the evolution model. In a second part, we focus on the spin-up problem. This calibration problem for short term (maximum 100 years) simulations involve retrieving jointly the initial state, the bedrock topography and basal sliding parameters. In order to solve this problem we develop an Ensemble Kalman Filter approach., L'évolution des calottes polaires est régie à la fois par une dynamique d'écoulement complexe et par des mécanismes tel le glissement à la base, la température de la glace ou le bilan de masse en surface. De plus, de nombreuses boucles de rétroactions sont constatées entre les différents phénomènes impliquées. Tout ceci rend la modélisation de cette évolution complexe. Malgré tout, un certain nombre de modèles ont été développés dans cette optique. Ceux-ci font tous intervenir des paramètres influents qui dans certains cas sont peu ou pas connus. Ils nécessitent donc d'être correctement spécifiés. L'assimilation de données peut permettre une meilleure estimation de ces paramètres grâce à l'utilisation d'observations qui sont peu nombreuses en glaciologie. Dans cette thèse, nous nous intéressons à la mise en place de systèmes d'assimilation performants pour deux problèmes inverses concernant l'évolution des calottes polaires. Pour mieux nous concentrer sur ce point, nous avons travaillé avec un modèle d'évolution de calotte simplifié (appelé Winnie) qui, cependant, représente bien la plupart des processus complexes de la dynamique de la glace, et permet de travailler à différentes échelles de temps. Dans un premier temps, nous mettons en place une approche 4D-Var pour la reconstruction de l'évolution d'un paramètre climatique influant sur l'évolution d'une calotte sur une échelle de temps typique de 20 000 ans. Elle nécessite notamment l'écriture du code adjoint du modèle. Dans un second temps, nous nous intéressons au problème du spin-up. Ce problème de calibration du modèle pour des simulations à échelle de temps courtes (pas plus de 100 ans) consiste plus particulièrement en la reconstruction conjointe de l'état initial, de la topographie du socle rocheux et des paramètres de glissement basal. Nous développons ici une approche filtre de Kalman d'ensemble pour résoudre ce problème.

In relation to Arctic warming, the possible occurrence of methane hydrate degassing events has attracted an increasing interest in recent years. We evaluate the atmospheric impact of rapid and massive emissions of methane and how they are imprinted in ice core records, by combining for the first time models of atmospheric chemistry and trace gas transport in firn. Different emission characteristics as well as climatic conditions (present, pre-industrial, glacial) are considered. The δ isotopic signatures characterizing stable isotopologues of methane DCH 3 and 13CH 4 are also analysed. Our results suggest little effect of clathrate degassing on the main methane oxidant: OH radicals. Due to the relatively short atmospheric lifetime of methane, the simulated clathrate-induced perturbations last for less than a century. This time scale is comparable to or shorter than the duration of air bubble closure in polar ice sheets. As a consequence, rapid methane perturbations in the atmosphere are strongly smoothed in ice core records. This smoothing mostly depends on the snow accumulation rate at the site of ice core drilling. We propose a methodology to identify a potential clathrate degassing event in ice core records. Continuous CH 4 records from high accumulation rate sites could allow to decipher short time scale events δD of CH 4 should reveal a typical "lying S" shape at high accumulation rate sites, reflecting the combined effects of the clathrate source signature (negative excursion) and subsequent OH fractionation in the atmosphere (positive excursion). The amplitude ratio of the negative and positive δD swings recorded in Greenland and Antarctica under similar accumulation rate conditions could also indicate the latitude of a clathrate degassing event. © 2012 Elsevier B.V.

In order to estimate the impact of climate change it is necessary to monitor the surface mass balance of the ice sheets. Much effort has gone into developing accurate methods for surface mass balance estimation over the past years. Mass balance is defined as the sum of mass gain and mass loss. Still, it remains difficult to quantify surface accumulation, which is the net gain term of the balance equation. This is due to the fact that the polar ice sheets are difficult to access, so in-situ measurements are sparse. Therefore, it is important to employ remote sensing techniques to obtain accumulation data with a better spatial and temporal resolution. However, the resulting datasets still exhibit large amounts of uncertainty. This thesis seeks to improve the currently available methods to derive snow accumulation rates from microwave remote sensing data. For this purpose, different types of microwave data are systematically evaluated with respect to their suitability for accumulation rate retrieval. The approach taken here makes use of the fact that microwaves interact with a volume of dry polar firn and are sensitive to accumulation rate-dependent firn characteristics. For this reason, firn microstructure is examined in more detail. On the basis of this analysis, an improved parameterization of firn properties (grain size and density) is developed, which is valid for the layers of the firn column that interact with the microwave radiation. The improved microstructure model is used in conjunction with a simple radiative transfer model to simulate firn-microwave interaction, resulting in a synthetic microwave signal. Accumulation rates can subsequently be inverted by matching the signal from the model to microwave data measured by satellite sensors. A number of assumptions made in the radiative transfer model have proven to be invalid. In this work, the radiative transfer model is improved by including Mie scattering instead of the widely-used Rayleigh approximation for the combination of scatterer sizes and microwave frequencies investigated. The results from the accumulation retrieval algorithm developed in this work are validated with in-situ data, and limits in its applicability are evaluated. Accumulation rates inverted from the method introduced in this work are found to agree with field data as well as with accumulation maps from external sources within the range of the model's validity.

The transition from the Last Glacial to the Holocene is a key period for understanding the mechanisms of global climate change. Ice cores from the large polar ice sheets provide a wealth of information with good time resolution for this period. However, interactions between the two hemispheres can only be investigated if ice core records from Greenland and Antarctica can be synchronised accurately and reliably. The atmospheric methane concentration shows large and very fast changes during this period. These variations are well suited for a synchronisation of the age scales of ice cores from Greenland and Antarctica. Here we confirm the proposed lead of the Antarctic Cold Reversal on the Younger Dryas cold event. The Antarctic cooling precedes the Younger Dryas by at least l.8 kyr. This suggests that northern and southern hemispheres were in anti-phase during the Younger Dryas cold event. A further result of the synchronisation is that the long-term glacial-interglacial increase of atmospheric CO2 was not interrupted during the Younger Dryas event and that atmospheric CO2 changes are not necessarily dominated by changes in the North Atlantic circulation. Copyright 1997 by the American Geophysical Union.

Ice sounding radar from high altitudes is compromised by off-nadir ice surface reflections (clutter), which overlays the nadir signal of interest originating from the bedrock and/or internal ice layers. Multi-aperture antenna radar systems were developed to mitigate the impact of surface clutter allowing spatially variant digital beamforming (DBF) processing of the data received from the individual spatially separated apertures. This paper investigates four different beamforming algorithms: conventional beamsteering, nulling of clutter angles, optimum beamformer and the MVDR beamformer. Comparisons are performed based on simulations for an airborne ice sounding scenarios. In addition, real echograms are computed from data acquired by the P-band POLARIS sensor. Relevant conclusions are drawn with respect to the implementation of a future space-based ice sounding mission.

The large ice sheets on Earth respond to changes in the global climate. Ice mass loss increases with rising global mean temperature and thus is a major contributor to sea-level rise. In order to reduce the uncertainty to predict the contributions to sea-level rise of ice sheets, it is crucial to study how the ice sheets‘ fast-flowing drainage pathways (so-called ice streams) have evolved over the last thousand to millions of years. In this thesis, a contribution to the understanding of the flow characteristics of large ice streams in Greenland and Antarctica is performed by an analysis of (ice-penetrating) radar reflections within the ice column and at the ice base. My focus lies on the question how these data can be used to obtain information about present and paleo ice flow regimes. I concentrate on radar data acquired in Northeast Greenland in the upstream regions of the North East Greenland Ice Stream (NEGIS) and in the upstream catchment of the Nioghalvfjerdsbrae (79°N Glacier) as well as on data recorded at the onset of the Jutulstraumen Glacier in Antarctica. In my studies, I show that the NEGIS in its present form is a relatively young feature and that its geometry and flow characteristics are intertwined with the subglacial topography. I also found indications for a re-organization of ice stream activity in the NEGIS catchment during the Holocene. This suggests that ice streams are probably are less persistent than previously thought and adapt in their entire length to the changing geometry of the ice sheet on short time scales. In Antarctica, I investigate past ice flow patterns over a period of millions of years, as in the example of the Jutulstraumen Glacier basin in Antarctica. Many of the glacial and fluvial landscapes, which developed since the glaciation of Antarctica, have been mostly preserved under the contemporary thick ice sheet, and some even serve as basins for active subglacial lakes today.

Quaternary interglacials provide key observations of the Earth system's responses to orbital and greenhouse gas forcing. They also inform on the capabilities of Earth system models, used for projecting the polar ice-sheet and sea-level responses to a regional warmth comparable to that expected by 2100 C.E. However, a number of uncertainties remain regarding the processes and feedbacks linking climate, ice-sheet and sea-level changes during past warm intervals. Here, we delineate the major research questions that need to be resolved and future research directions that should be taken by the paleoclimate, sea-level and ice-sheet research communities in order to increase confidence in the use of past interglacial climate, ice-sheet and sea-level reconstructions to constrain future predictions. These questions were formulated during a joint workshop held by the PAGES-INQUA PALSEA (PALeo constraints on SEA level rise) and the PAGES-PMIP QUIGS (QUaternary InterGlacialS) Working Groups in September 2018.