70 research outputs found

    Reconstructing the history of the Antarctic ice sheet using internal reflecting horizons from radio-echo sounding

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    Understanding the contribution of the Antarctic Ice Sheet (AIS) to past and future sea-level rise has emerged as a scientific priority over the last four decades. Whilst our knowledge of ice-dynamical changes occurring as a result of current anthropogenic forcing has improved considerably since the start of the satellite era, significantly less is known about the evolution of the AIS during the pre-industrial Holocene (the last ~11.7 thousand years; ka). Quantifying these changes is crucial, however, as this time period corresponds to a time when the ice sheet was retreating from its maximal extent at the Last Glacial Maximum (LGM; ~20 ka) and environmental conditions were similar to today. Therefore, improving our understanding of this period may provide a long-term context to the decadal changes observed in recent times and how these may evolve in the future. Whilst point-based geochronological measurements of ice and sediment cores, or surface exposure dating, can be used to assess past ice-sheet changes over the AIS, it remains unclear how representative they are of a wider region. A complementary and spatially extensive resource across the ice sheet are Internal Reflecting Horizons (IRHs) as imaged by Radio-Echo Sounding (RES) techniques, which provide a cumulative record of accumulation, basal melt and ice dynamics that, if dated precisely at ice cores, can be used to inform numerical ice-sheet models projecting past and future changes on large spatial scales. The aim of this thesis is therefore to develop and extend age-depth models from IRHs across the AIS to assess the past stability of the ice sheet. In this thesis, an age-depth model of Pine Island Glacier spanning the LGM and Holocene periods is derived from spatially extensive IRHs. The connection between RES profiles and the WAIS Divide ice core enables the direct dating of the IRHs, and reveals that they match large peaks in sulphate concentrations which are unparalleled in the 68,000 year-old record, thus suggesting that the cause of these IRHs is from past explosive volcanic eruptions. By connecting this IRH stratigraphy with a previously developed age-depth model across the Institute and Möller Ice Streams (IMIS), I show that a precisely dated age-depth model now exists over 20% of the West Antarctic Ice Sheet (WAIS). One of these IRHs, precisely dated at ~4.7 ka, is then used as input into a one-dimensional ice-flow model to estimate past accumulation rates during the mid-Holocene over the catchments encompassing Pine Island Glacier, Thwaites Glacier, and IMIS, together representing 30% of the WAIS. The inferred mid-Holocene accumulation estimates are then compared with modern rates derived from climate models and observational measurements to show that accumulation rates were 18% greater during the last five millennia compared to the present over the Amundsen-Weddell-Ross Divide. These results match previous findings from isolated ice-core measurements and spatially targeted studies over the divide, and correspond to periods of grounding line retreat and readvance during the Holocene over the WAIS. Together, these show the potential for extracting further IRH information from other sectors of the AIS in order to build an age-depth model of the ice sheet. However, the underlying RES data necessary for this work were, until recently, relatively inaccessible to the wider scientific community, thus restricting the extraction and interpretation of age-depth models across the AIS. This motivated the release of ~300,000 line-km of RES profiles acquired by the British Antarctic Survey between 2004 and 2020. In addition to standardising and releasing these data, this thesis shows that large sections of continuous englacial layering exist widely across both East and West Antarctica, suggesting that, together with previously developed age-depth models of both regions and nearby ice-core stratigraphies, these newly released RES datasets will be critical in our aim to build an ice-sheet wide age-depth model of Antarctica, as motivated by the AntArchitecture Initiative. Together, the findings from this thesis reveal the spatially extensive nature of IRHs across West and East Antarctica and demonstrate how these can be used to infer past ice-sheet changes. This thesis also highlights the need to extract further age-depth models, particularly across East Antarctica, in order to provide important boundary conditions such as past accumulation rates and ice-elevation change which can be used by numerical ice-sheet models to help improve predictions of past and future ice-sheet change and ensuing sea-level rise contributions

    Historical and future trends in global snow conditions – observed by remote sensing and forecasted by spatio-temporal modelling

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    Snow resources worldwide are undergoing extensive changes in response to widespread and rapid changing of the global climate. These resources are vital in many areas and changes to them have and will continue to impact human societies and ecosystems in cold regions. The research presented in this dissertation entails the assessment and comparison of historical trends in the climate and snow regimes and the projection of these trends until the end of the 21st century, under different emission scenarios. The results show that extensive changes have occurred to the frequency of Northern Hemisphere (NH) snow cover since the beginning of the 21st century, as estimated based on remote sensing data from the MODIS satellite instrument. The future evolution of NH snow resources was modelled for the period 1950-2100 for each of the 21 downscaled and bias corrected CMIP5 climate models for two emission scenarios (RCP45 and RCP85) using the Snow17 model. The simulations show that the Snow Cover Frequency (SCF) is in general projected to diminish substantially across the NH. However, the NH 1st April Snow Water Equivalent (SWE) is projected to increase slightly at the beginning of the period, driven by increased snowfall at high latitudes in the Arctic and then decline back to 1950-1975 levels under RCP45 and 10% under those given RCP85. These trends were analyzed specifically for Icelandic circumstances revealing a trend of increasing SCF in many parts of the country over the period 1930-2021, whereas the simulated results project a decrease in SCF across Iceland between 1950 to 2100.Snjóauðlindir víðsvegar um heiminn eru nú breytingum undiropnar í kjölfar hnattrænna loftlagsbreytinga. Þessar auðlindir eru mikilvægar víðsvegar og breytingar á eðli þeirra hafa haft og munu halda áfram að hafa áhrif á mannleg samfélög og vistkerfi á kvöldum svæðum. Rannsókn sú er birt er í þessari ritgerð fjallar um greiningu og samanburð á sögulegri þróun loftlags og snjós og gerð forspár um það hvernig væntar loftlagsbreytingar munu hafa áhrif á snjóauðlindir út 21 öldina miðað við mismunandi sviðsmyndir í hlýnun. Niðurstöður rannsóknarinnar sýna fram á að víðtækar breytingar hafa þegar orðið á snjóþekju á Norður Hveli jarðar (NH) frá byrjun 21 aldarinnar útfrá fjarkönnunargögnum frá MODIS gervihnattamælinum. Spáð var fyrir um framtíðarþróun snjóauðlinda á NH fyrir tímabilið 1950-2100 með Snow17 snjólíkaninu útfrá 21 CMIP5 loflagslíkönum fyrir tvö hlýnunartilvik (RCP45 og RCP85). Niðurstöður líkansins gefa til kynna að tíðni snjóhulu (SCF) muni almennt minnka verulega um allt NH en að hinsvegar, muni meðal rúmál vatns sem geymt er í snjóalögum NH aukast lítillega í byrjun tímabilsins, aðallega vegna aukinnar snjókomu og norðlægum breiddargráðum innan norðurheimskautsbaugs, en minnka svo aftur að því sem var um 1950 fyrir RCP45 en 10% neðar en svo fyrir RCP85.. Þróun í loftlagi og snjóauðlindum var rannsökuð sérstaklega á Íslandi, sem leiddi í ljós tölfræðilega marktækta aukningu á SCF stórum svæðum frá aldamótum, spá um þróun snjóauðlinda út 21 öldina gerir hinsvegar ráð fyrir verulegri minnkun á SCF í öllum hæðarbilum á Íslandi.Doktorsstyrkjasjóður Háskóla Ísland

    Spatio-temporal variability in Southern Hemisphere glacier snowline altitudes from 2000-2020

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    The glacierised Southern Hemisphere is vulnerable to continued shrinkage under climate change, but representation of these mountainous regions in climate research is limited by hemispheric and altitudinal scarcity of meteorological observations. End-of-summer snowline altitude (SLAEOS) indicates glacier response to climatic forcing, though has been estimated with low spatio-temporal coverage for the Southern Hemisphere. This study presents the first Southern Hemisphere-wide quantification of SLAEOS, with analysis of regional and intra-regional trends. An automated approach was implemented in Google Earth Engine, in which glacier snow cover was classified in Landsat scenes using Otsu image segmentation and SLAEOS was estimated as the lowest altitude from which snow cover ratio was continuously > 0.5. Results encompassed 6485 glaciers of the Southern Alps, Andes, and Antarctic Peninsula, with trends calculated from 2000-2020. Snowlines underwent widespread retreat in this period; mean rates of SLAEOS rise were between 2.19 and 6.28 m yr-1 for regions, between 1.63 and 7.55 m yr-1 for east/west sub-regions, and were mostly accelerated for the recent decade (2010-2020). Mean SLAEOS lowering (-30 to -1 m yr-1) indicated stability in the southernmost Andes, contrasting to rapid SLAEOS rise (10 to 30 m yr-1) in the southern Central Chilean Andes, and eastern slopes generally experienced increased rates of SLAEOS rise compared to western slopes. SLAEOS variability was reflected in periods of summer warming and reductions in summer snowfall, though correlation with these variables was not consistently identified. East-west and north-south disparities in absolute SLAEOS and rates of SLAEOS change were linked to spatial variability in terrain elevation and prevailing moisture transport, with the latter evidencing the variability and impact of large-scale climatic modes. Given implications of observed trends for glacier mass loss, continued research may involve developing an annually-updated global dataset, investigating additional drivers of SLAEOS variability, and estimating glacier response times

    ALOS-2/PALSAR-2 Calibration, Validation, Science and Applications

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    Twelve edited original papers on the latest and state-of-art results of topics ranging from calibration, validation, and science to a wide range of applications using ALOS-2/PALSAR-2. We hope you will find them useful for your future research

    Expeditions to Antarctica: ANT-Land 2021/22 Neumayer Station III, Kohnen Station, Flight Operations and Field Campaigns

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    Ocean remote sensing techniques and applications: a review (Part II)

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    As discussed in the first part of this review paper, Remote Sensing (RS) systems are great tools to study various oceanographic parameters. Part I of this study described different passive and active RS systems and six applications of RS in ocean studies, including Ocean Surface Wind (OSW), Ocean Surface Current (OSC), Ocean Wave Height (OWH), Sea Level (SL), Ocean Tide (OT), and Ship Detection (SD). In Part II, the remaining nine important applications of RS systems for ocean environments, including Iceberg, Sea Ice (SI), Sea Surface temperature (SST), Ocean Surface Salinity (OSS), Ocean Color (OC), Ocean Chlorophyll (OCh), Ocean Oil Spill (OOS), Underwater Ocean, and Fishery are comprehensively reviewed and discussed. For each application, the applicable RS systems, their advantages and disadvantages, various RS and Machine Learning (ML) techniques, and several case studies are discussed.Peer ReviewedPostprint (published version

    Quad polarimetric synthetic aperture radar analysis of icebergs in Greenland and Svalbard

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    Polarimetric synthetic aperture radar (PolSAR) has been widely used in ocean and cryospheric applications. This is because, PolSAR can be used in all-day operations and in areas of cloud cover, and therefore can provide valuable large-scale monitoring in polar regions, which is very helpful to shipping and offshore maritime operations. In the last decades, attention has turned to the potential of PolSAR to detect icebergs in the Arctic since they are a major hazard to vessels. However, there is a substantial lack of literature exploring the potentialities of PolSAR and the understanding of iceberg scattering mechanisms. Additionally, it is not known if high resolution PolSAR can be used to detect icebergs smaller than 120 metres. This thesis aims to improve the knowledge of the use of PolSAR scattering mechanisms of icebergs, and detection of small icebergs. First, an introduction to PolSAR is outlined in chapter two, and monitoring of icebergs is presented in chapter three. The first data chapter (Chapter 4) is focused on developing a multi-scale analysis of icebergs using parameters from the Cloude-Pottier and the Yamaguchi decompositions, the polarimetric span and the Pauli scattering vector. This method is carried out using ALOS-2 PALSAR quad polarimetric L-band SAR on icebergs in Greenland. This approach outlines the good potential for using PolSAR for future iceberg classification. One of the main important outcomes is that icebergs are composed by a combination of single targets, which therefore may require a more complex way of processing SAR data to properly extract physical information. In chapter five, the problem of detecting icebergs is addressed by introducing six state-of-the-art detectors previously applied to vessel monitoring. These detectors are the Dual Intensity Polarisation Ratio Anomaly Detector (iDPolRAD), Polarimetric Notch Filter (PNF), Polarimetric Matched Filter (PMF), reflection symmetry (sym), Optimal Polarimetric Detector (OPD) and the Polarimetric Whitening Filter (PWF). Cloude-Pottier entropy, and first and third eigenvalues (eig1 and eig3) of the coherency matrix are also utilised as parameters for comparison. This approach uses the same ALOS-2 dataset, but also evaluates detection performance in two scenarios: icebergs in open ocean, and in sea ice. Polarimetric modes (quad-pol, dual-pol, and single intensities) are also considered for comparison. Currently it is very difficult to detect icebergs less than 120 metres in length using this approach, due to the scattering mechanisms of icebergs and sea ice being very similar. However, it was possible to obtain detection performances of the OPD and PWF, which both showed a Probability of Detection (PF) of 0.99 when the Probability of False Alarms (PF) was set to 10-5 in open ocean. Similarly, in dual pol images, the PWF gave the best performance with a PD of 0.90. Results in sea ice found eig3 to be the best detector with a PD of 0.90 while in dual-pol mode, iDPolRAD gave a PD of 0.978. Single intensity detector performance found the HV channel gave the best detection with a PD of 0.99 in open ocean and 0.87 in sea ice. In the previous two approaches, only satellite data is used. However, in chapter six, data from a ground-based Ku-band Gamma Portable Radio Interferometer (GPRI) instrument is introduced, providing images that are synchronised with the satellite acquisitions. In this approach, the same six detectors are applied to three multitemporal RADARSAT-2 quad pol C-band SAR images on icebergs in Kongsfjorden, Svalbard to evaluate the detection performance within a changing fjord environment. As before, we also make use of Cloude-Pottier entropy, eig1 and eig3. Finally, we evaluate the target-to-clutter ratio (TCR) of the icebergs and check for correlation between the backscattering coefficients and the iceberg dimension. The results obtained from this thesis present original additions to the literature that contributes to the understanding of PolSAR in cryospheric applications. Although these methods are applied to PolSAR and ground-based radar on vessels, they have been applied for the first time on icebergs in this thesis. To summarise, the main findings are that icebergs cannot be represented as single or partial targets, but they do exhibit a collection of single targets clustered together. This result leads to the fact that entropy is not sufficient as a parameter to detect icebergs. Detection results show that the OPD and PWF detectors perform best in an open ocean setting and using quad-pol mode. These results are degraded in dual-pol mode, while single intensity detection is best in the HV cross polarisation channel. When these detectors are applied to the RADARSAT-2 in Svalbard, the OPD and PWF detectors also perform best with PD values ranging between 0.5-0.75 for a PF of 0.01-0.05. However, the sea ice present in the fjord degrades performance across all detectors. Correlation plots with iceberg size show that a regression is not straightforward and Computer Vision methodologies may work best for this

    Snow observations from Arctic Ocean Soviet drifting stations: legacy and new directions

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    The Arctic Ocean is one of the most rapidly changing regions on the planet. Its warming climate has driven reductions in the region's sea ice cover which are likely unprecedented in recent history, with many of the environmental impacts being mediated by the overlying snow cover. As well as impacting energetic and material fluxes, the snow cover also obscures the underlying ice from direct satellite observation. While the radar waves emitted from satellite-mounted altimeters have some ability to penetrate snow cover, an understanding of snow geophysical properties remains critical to remote sensing of sea ice thickness. The paucity of Arctic Ocean snow observations was recently identified as a key knowledge gap and uncertainty by the Intergovernmental Panel on Climate Change's Special Report on Oceans and Cryosphere in a Changing Climate. This thesis aims to address that knowledge gap. Between 1937 and 1991 the Soviet Union operated a series of 31 crewed stations which drifted around the Arctic Ocean. During their operation, scientists took detailed observations of the atmospheric conditions, the physical oceanography, and the snow cover on the sea ice. This thesis contains four projects that feature these observations. The first two consider a well known snow depth and density climatology that was compiled from observations at the stations between 1954 & 1991. Specifically, Chapter two considers the role of seasonally evolving snow density in sea ice thickness retrievals, and Chapter three considers the impact of the climatological treatment itself on satellite estimates of sea ice thickness variability and trends. Chapter four presents a statistical model for the sub-kilometre distribution of snow depth on Arctic sea ice through analysis of snow depth transect data. Chapter five then compares the characteristics of snow melt onset at the stations with satellite observations and results from a recently developed model

    Generation of a Land Cover Atlas of environmental critic zones using unconventional tools

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    L'abstract è presente nell'allegato / the abstract is in the attachmen

    Retrieval of soil physical properties:Field investigations, microwave remote sensing and data assimilation

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