Mid-Term Monitoring of Glacier’s Variations with UAVs: The Example of the Belvedere Glacier

Recently, Unmanned Aerial Vehicles (UAV) have opened up unparalleled opportunities for alpine glacier monitoring, as they allow for reconstructing extensive and high-resolution 3D models. In order to evaluate annual ice flow velocities and volume variations, six yearly measurements were carried out between 2015 and 2020 on the debris-covered Belvedere Glacier (Anzasca Valley, Italian Alps) with low-cost fixed-wing UAVs and quadcopters. Every year, ground control points and check points were measured with GNSS. Images acquired from UAV were processed with Structure-from-Motion and Multi-View Stereo algorithms to build photogrammetric models, orthophotos and digital surface models, with decimetric accuracy. Annual glacier velocities were derived by combining manually-tracked features on orthophotos with GNSS measurements. Velocities ranging between 17 m y−1 and 22 my−1 were found in the central part of the glacier, whereas values between 2 m y−1 and 7 my−1 were found in the accumulation area and at the glacier terminus. Between 2 × 106 m3 and 3.5 × 106m3 of ice volume were lost every year. A pair of intra-year measurements (October 2017–July 2018) highlighted that winter and spring volume reduction was ∼1/4 of the average annual ice loss. The Belvedere monitoring activity proved that decimetric-accurate glacier models can be derived with low-cost UAVs and photogrammetry, limiting in-situ operations. Moreover, UAVs require minimal data acquisition costs and allow for great surveying flexibility, compared to traditional techniques. Information about annual flow velocities and ice volume variations of the Belvedere Glacier may have great value for further understanding glacier dynamics, compute mass balances, or it might be used as input for glacier flow modelling.

Drone Surveying of Volumetric Ice Growth in a Steep River

Representative ice thickness data is essential for accurate hydraulic modelling, assessing the potential for ice induced floods, understanding environmental conditions during winter and estimation of ice-run forces. Steep rivers exhibit complex freeze-up behaviour combining formation of columnar ice with successions of anchor ice dams to build a complete ice cover, resulting in an ice cover with complex geometry. For such ice covers traditional single point measurements are unrepresentative. Gathering sufficiently distributed measurements for representativeness is labour intensive and at times impossible with hard to access ice. Structure from Motion (SfM) software and low-cost drones have enabled river ice mapping without the need to directly access the ice, thereby reducing both the workload and the potential danger in accessing the ice. In this paper we show how drone-based photography can be used to efficiently survey river ice and how these photographic surveys can be processed into digital elevation models (DEMs) using Structure from Motion. We also show how DEMs of the riverbed, riverbanks and ice conditions can be used to deduce ice volume and ice thickness distributions. A QGIS plugin has been implemented to automate these tasks. These techniques are demonstrated with a survey of a stretch of the river Sokna in Trøndelag, Norway. The survey was carried out during the winter 2020–2021 at various stages of freeze-up using a simple quadcopter with camera. The 500 m stretch of river studied was estimated to have an ice volume of up to 8.6 × 103 m3 (This corresponds to an average ice thickness of ∼67 cm) during the full ice cover condition of which up to 7.2 × 103 m3 (This corresponds to an average ice thickness of ∼57 cm) could be anchor ice. Ground Control Points were measured with an RTK-GPS and used to determine that the accuracy of these ice surface geometry measurements lie between 0.03 and 0.09 m. The ice thicknesses estimated through the SfM methods are on average 18 cm thicker than the manual measurements. Primarily due to the SfM methods inability to detect suspended ice covers. This paper highlights the need to develop better ways of estimating the volume of air beneath suspended ice covers.

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

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 on the long-term, mainly as a result of surface melting and acceleration of ice flow. These ice losses can be mitigated, however, in some cases with strong GIA feedback even promoting the partial 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 recovery, via quasi-periodic oscillations in ice volume to ice-sheet collapse. In the 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 of tens to hundreds of thousands of 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 time scales 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.

Pliocene glacial-interglacial sea-level change

<p>The mid- to late Pliocene (3.3-2.6 Ma) spans one of the most significant climatic transitions of the Cenozoic. It is characterised by global cooling from a climate with an atmospheric CO2 concentration of ~400 ppm and temperatures of 2-3°C warmer-than-present, to one marked by the progressive expansion of ice sheets on northern hemisphere. Consequently, the mid-Pliocene warm period (MPWP; 3.3-3.0 Ma) provides the most accessible and recent geological analogue for global sea-level variability relevant to future warming. Global mean sea level has been estimated at 22 ± 10 m above present-day for MPWP. However, recent re-evaluations of this estimate suggest that spatially-varying visco-elastic responses of the crust, local gravitational changes and dynamic topography from mantle processes may preclude ever being able to reconstruct peak Pliocene mean sea level. The Whanganui Basin, New Zealand, contains a ~5 km thick stratigraphic succession of Pliocene-Pleistocene (last 5 Ma), shallow-marine, cyclical sedimentary sequences demonstrated to record orbitally-paced, glacial-interglacial global sea-level fluctuations. A limitation of the Whanganui sea level record, to date, has been an inability to resolve the full amplitude of glacial-interglacial water depth change due to the occurrence of cycle bounding unconformities representing sub-aerial erosion during glacial lowstands. This thesis analyses a new ~900 m-thick, mid- (3.3-3.0 Ma) to late Pliocene (3.0-2.6 Ma), shallow-marine, cyclical sedimentary succession from a remote and relatively understudied part of Whanganui Basin. Unlike previous studies, these shelf sediments were continuously deposited, and were not eroded during sea-level lowstands, and thus provide the potential to reconstruct the full amplitude of glacial-interglacial sea-level change. On orbital timescales the influence of mantle dynamic processes is minimal. The approach taken applies lithofacies, sequence stratigraphy, and benthic foraminiferal analyses and a novel depth-dependent sediment grain size method to reconstruct the paleowater depths for, two continuously-cored drill holes, which are integrated with studies of outcropping sections. The thesis presents a new record of the amplitude and frequency of orbitally-paced, global sea-level changes from a wave-graded continental shelf, that is independent of the benthic δ¹⁸O proxy record of global ice-volume change. Paleobathymetric interpretations are underpinned by analysis of extant benthic foraminiferal census data and a statistical correlation with the distribution of modern taxa. In general, water depths derived from foraminiferal modern analogue technique are consistent with variability recorded by lithofacies. The inferred sea-level cycles co-vary with a qualitative climate record reconstructed from a census of extant pollen and spores, and a modern temperature relationship. A high-resolution age model is established using magnetostratigraphy constrained by biostratigraphy, and the dating and correlation of tephra. This integrated chronostratigraphy allows the recognition of 23 individual sedimentary cycles, that are correlated “one-to-one” across the paleo-shelf and are compared to the deep-ocean benthic oxygen isotope (δ ¹⁸O) record. A grain size-water depth technique was developed to quantify the paleobathymetry with more precision than the relatively insensitive benthic foraminifera approach. The method utilises a water depth threshold relationship between wave-induced near bed velocity and the velocity required to transport sand. The resulting paleobathymetric records of the most sensitive sites, the mid-Pliocene Siberia-1 drill core and the late Pliocene Rangitikei River section, were selected to compile a composite paleobathymetry. A one-dimensional backstripping method was then applied to remove the effects of tectonic subsidence, sediment and water loading on the record, to derive a relative sea level (RSL) curve. The contribution of glacio-hydro-isostatic (GIA) processes to the RSL record was evaluated using a process-based forward numerical solid Earth model for a range of plausible meltwater scenarios. The Whanganui Basin RSL record approximates eustatic sea level (ESL) in all scenarios when variability is dominated by Antarctic Ice Sheet meltwater source during the mid-Pliocene, but overestimates ESL once Northern Hemisphere ice sheet variability dominates in the late Pliocene. The RSL record displays 20 kyr precession-paced sea level variability during the MPWP with an average amplitude of ~15 ± 8 m, in-phase with southern high-latitude summer insolation. These are interpreted as ~20 m Antarctic Ice Sheet contributions, offset by ~ 5 m anti-phased Greenland Ice Sheet contribution, in the absence of a significant Northern Hemisphere ice sheets. This interpretation is supported by a previously published ice-proximal precession-paced, ice-berg-rafted debris record recovered off the coast of Wilkes Land. The Whanganui RSL record is not consistent with a dominant 40 kyr pacing observed the benthic oxygen isotope stack at this time. While the deep ocean benthic δ¹⁸O stack is of varying temporal and spatial resolution, during this time interval, the Whanganui RSL record implies a more complex relationship between ice-volume and oxygen isotope composition of sea water (δ¹⁸Oseawater). The relative influences of varying composition of the polar ice sheets, marine versus land based ice, the out-of-phase behaviour of polar ice sheet growth and retreat, and a potential decoupling of ocean bottom water temperature and δ¹⁸Oseawater are explored. The late Pliocene relative sea level record exhibits increasing ~40 kyr obliquity-paced amplitudes of ~20 ± 8 m. This is interpreted as a response to the expansion of Northern Hemisphere ice sheets after ~2.9 Ma. During this time the Antarctic proximal ice-berg rafted debris records display continuing precession-paced ice-volume fluctuations, but with decreasing amplitude suggesting cooling and stabilisation of the East Antarctic Ice Sheet. With the bipolar glaciation, the ocean δ¹⁸O signal became increasingly dominated by northern hemisphere ice-volume. However, the RSL record implies relatively limited ice-volume contributions (up to ~25 m sea level equivalent) prior to ~2.6 Ma. The large amplitude contribution of Antarctic Ice Sheets to global sea level during the MPWP has significant implications for the sensitivity of the Antarctica Ice Sheet to global temperatures 2-3°C above preindustrial levels, and atmospheric CO₂ forecast for the coming decades.</p>

Pliocene glacial-interglacial sea-level change

<p>The mid- to late Pliocene (3.3-2.6 Ma) spans one of the most significant climatic transitions of the Cenozoic. It is characterised by global cooling from a climate with an atmospheric CO2 concentration of ~400 ppm and temperatures of 2-3°C warmer-than-present, to one marked by the progressive expansion of ice sheets on northern hemisphere. Consequently, the mid-Pliocene warm period (MPWP; 3.3-3.0 Ma) provides the most accessible and recent geological analogue for global sea-level variability relevant to future warming. Global mean sea level has been estimated at 22 ± 10 m above present-day for MPWP. However, recent re-evaluations of this estimate suggest that spatially-varying visco-elastic responses of the crust, local gravitational changes and dynamic topography from mantle processes may preclude ever being able to reconstruct peak Pliocene mean sea level. The Whanganui Basin, New Zealand, contains a ~5 km thick stratigraphic succession of Pliocene-Pleistocene (last 5 Ma), shallow-marine, cyclical sedimentary sequences demonstrated to record orbitally-paced, glacial-interglacial global sea-level fluctuations. A limitation of the Whanganui sea level record, to date, has been an inability to resolve the full amplitude of glacial-interglacial water depth change due to the occurrence of cycle bounding unconformities representing sub-aerial erosion during glacial lowstands. This thesis analyses a new ~900 m-thick, mid- (3.3-3.0 Ma) to late Pliocene (3.0-2.6 Ma), shallow-marine, cyclical sedimentary succession from a remote and relatively understudied part of Whanganui Basin. Unlike previous studies, these shelf sediments were continuously deposited, and were not eroded during sea-level lowstands, and thus provide the potential to reconstruct the full amplitude of glacial-interglacial sea-level change. On orbital timescales the influence of mantle dynamic processes is minimal. The approach taken applies lithofacies, sequence stratigraphy, and benthic foraminiferal analyses and a novel depth-dependent sediment grain size method to reconstruct the paleowater depths for, two continuously-cored drill holes, which are integrated with studies of outcropping sections. The thesis presents a new record of the amplitude and frequency of orbitally-paced, global sea-level changes from a wave-graded continental shelf, that is independent of the benthic δ¹⁸O proxy record of global ice-volume change. Paleobathymetric interpretations are underpinned by analysis of extant benthic foraminiferal census data and a statistical correlation with the distribution of modern taxa. In general, water depths derived from foraminiferal modern analogue technique are consistent with variability recorded by lithofacies. The inferred sea-level cycles co-vary with a qualitative climate record reconstructed from a census of extant pollen and spores, and a modern temperature relationship. A high-resolution age model is established using magnetostratigraphy constrained by biostratigraphy, and the dating and correlation of tephra. This integrated chronostratigraphy allows the recognition of 23 individual sedimentary cycles, that are correlated “one-to-one” across the paleo-shelf and are compared to the deep-ocean benthic oxygen isotope (δ ¹⁸O) record. A grain size-water depth technique was developed to quantify the paleobathymetry with more precision than the relatively insensitive benthic foraminifera approach. The method utilises a water depth threshold relationship between wave-induced near bed velocity and the velocity required to transport sand. The resulting paleobathymetric records of the most sensitive sites, the mid-Pliocene Siberia-1 drill core and the late Pliocene Rangitikei River section, were selected to compile a composite paleobathymetry. A one-dimensional backstripping method was then applied to remove the effects of tectonic subsidence, sediment and water loading on the record, to derive a relative sea level (RSL) curve. The contribution of glacio-hydro-isostatic (GIA) processes to the RSL record was evaluated using a process-based forward numerical solid Earth model for a range of plausible meltwater scenarios. The Whanganui Basin RSL record approximates eustatic sea level (ESL) in all scenarios when variability is dominated by Antarctic Ice Sheet meltwater source during the mid-Pliocene, but overestimates ESL once Northern Hemisphere ice sheet variability dominates in the late Pliocene. The RSL record displays 20 kyr precession-paced sea level variability during the MPWP with an average amplitude of ~15 ± 8 m, in-phase with southern high-latitude summer insolation. These are interpreted as ~20 m Antarctic Ice Sheet contributions, offset by ~ 5 m anti-phased Greenland Ice Sheet contribution, in the absence of a significant Northern Hemisphere ice sheets. This interpretation is supported by a previously published ice-proximal precession-paced, ice-berg-rafted debris record recovered off the coast of Wilkes Land. The Whanganui RSL record is not consistent with a dominant 40 kyr pacing observed the benthic oxygen isotope stack at this time. While the deep ocean benthic δ¹⁸O stack is of varying temporal and spatial resolution, during this time interval, the Whanganui RSL record implies a more complex relationship between ice-volume and oxygen isotope composition of sea water (δ¹⁸Oseawater). The relative influences of varying composition of the polar ice sheets, marine versus land based ice, the out-of-phase behaviour of polar ice sheet growth and retreat, and a potential decoupling of ocean bottom water temperature and δ¹⁸Oseawater are explored. The late Pliocene relative sea level record exhibits increasing ~40 kyr obliquity-paced amplitudes of ~20 ± 8 m. This is interpreted as a response to the expansion of Northern Hemisphere ice sheets after ~2.9 Ma. During this time the Antarctic proximal ice-berg rafted debris records display continuing precession-paced ice-volume fluctuations, but with decreasing amplitude suggesting cooling and stabilisation of the East Antarctic Ice Sheet. With the bipolar glaciation, the ocean δ¹⁸O signal became increasingly dominated by northern hemisphere ice-volume. However, the RSL record implies relatively limited ice-volume contributions (up to ~25 m sea level equivalent) prior to ~2.6 Ma. The large amplitude contribution of Antarctic Ice Sheets to global sea level during the MPWP has significant implications for the sensitivity of the Antarctica Ice Sheet to global temperatures 2-3°C above preindustrial levels, and atmospheric CO₂ forecast for the coming decades.</p>

Reduced-complexity model for the impact of anthropogenic CO₂ emissions on future glacial cycles

We propose a reduced-complexity process-based model for the long-term evolution of the global ice volume, atmospheric CO2 concentration, and global mean temperature. The model's only external forcings are the orbital forcing and anthropogenic CO2 cumulative emissions. The model consists of a system of three coupled non-linear differential equations representing physical mechanisms relevant for the evolution of the climate–ice sheet–carbon cycle system on timescales longer than thousands of years. Model parameters are calibrated using paleoclimate reconstructions and the results of two Earth system models of intermediate complexity. For a range of parameters values, the model is successful in reproducing the glacial–interglacial cycles of the last 800 kyr, with the best correlation between modelled and global paleo-ice volume of 0.86. Using different model realisations, we produce an assessment of possible trajectories for the next 1 million years under natural and several fossil-fuel CO2 release scenarios. In the natural scenario, the model assigns high probability of occurrence of long interglacials in the periods between the present and 120 kyr after present and between 400 and 500 kyr after present. The next glacial inception is most likely to occur ∼50 kyr after present with full glacial conditions developing ∼90 kyr after present. The model shows that even already achieved cumulative CO2 anthropogenic emissions (500 Pg C) are capable of affecting the climate evolution for up to half a million years, indicating that the beginning of the next glaciation is highly unlikely in the next 120 kyr. High cumulative anthropogenic CO2 emissions (3000 Pg C or higher), which could potentially be achieved in the next 2 to 3 centuries if humanity does not curb the usage of fossil fuels, will most likely provoke Northern Hemisphere landmass ice-free conditions throughout the next half a million years, postponing the natural occurrence of the next glacial inception to 600 kyr after present or later.