My current position is as a climate modeller at the Danish Meteorological Institute, where I run the Hirham5 model over Greenland for future climate projections. I am particularly intereste din climate/ice sheet interactions.
Previously I worked at GEUS (Geological Survey of Denmark and Greenland) on a range of mainly commercially focused projects in Greenland.
My PhD was carried out at the University of St Andrews in Scotland where I studied models of crevasse formation with regard to calving glaciers. My field work was carried out at Breidamerkurjokull, an outlet glacier ofthe Vatnajokull ice cap in Iceland. Supervisors: Prof D. Benn and Dr. C. Warren
To improve Greenland Ice Sheet surface mass balance (SMB) simulation, the subsurface scheme of th... more To improve Greenland Ice Sheet surface mass balance (SMB) simulation, the subsurface scheme of the HIRHAM5 regional climate model was extended to include snow densification, varying hydraulic conductivity, irreducible water saturation and other effects on snow liquid water percolation and retention. Sensitivity experiments to investigate the effects of the additions and the impact of different parameterization choices are presented. Compared with 68 accumulation area ice cores, the simulated mean annual net accumulation bias is −5% (correlation coefficient of 0.90). Modeled SMB in the ablation area compares favorably with 1041 PROMICE observations with regression slope of 0.95–0.97 (depending on model configuration), correlation coefficient of 0.75–0.86 and mean bias −3%. Weighting ablation area SMB biases at low-and high-elevation with the amount of runoff from these areas, we estimate ice sheet-wide mass loss biases in the ablation area at −5 and −7% using observed (MODIS-derived) and internally calculated albedo, respectively. Comparison with observed melt day counts shows that patterns of spatial (correlation ∼0.9) and temporal (correlation coefficient of ∼0.9) variability are realistically represented in the simulations. However, the model tends to underestimate the magnitude of inter-annual variability (regression slope ∼0.7) and overestimate that of spatial variability (slope ∼1.2). In terms of subsurface temperature structure and occurrence of perennial firn aquifers and perched ice layers, the most important model choices are the albedo implementation and irreducible water saturation parameterization. At one percolation area location, for instance, the internally calculated albedo yields too high subsurface temperatures below 5 m, but when using an implementation of irreducible saturation allowing higher values, an ice layer forms in 2011, reducing the deep warm bias in subsequent years. On the other hand, prior to the formation of the ice layer, observed albedos combined with lower irreducible saturation give the smallest bias. Perennial firn aquifers and perched ice layers occur in varying thickness and area for
Surface mass balance (SMB) is the builder of the Greenland ice sheet and the driver of ice dynami... more Surface mass balance (SMB) is the builder of the Greenland ice sheet and the driver of ice dynamics. Quantifying the past, present and future state of SMB is important to understand the drivers and climatic processes that control SMB, and to both initialize and run ice sheet models which will help clarify sea level rise, and how likely changes in ice sheet extent feedback within the climate system. Regional climate models (RCMs) and climate reanalysis are used to quantify SMB estimates. Although different models have different spatial and temporal biases and may include different processes giving significant uncertainty in both SMB and the ice sheet dynamic response to it, all RCMs show a recent declining trend in SMB from the Greenland ice sheet, driven primarily by enhanced melt rates. Here, we present new simulations of the Greenland ice sheet SMB at 5 km resolution from the RCM HIRHAM5. The RCM is driven by the ERA-Interim reanalysis and the global climate model (GCM) EC-Earth v2.3 to make future projections for climate scenarios RCP8.5 and RCP4.5. Future estimates of SMB are affected by biases in driving global climate models, and feedbacks between the ice sheet surface and the global and regional climate system are neglected, likely resulting in significant underestimates of melt and precipitation over the ice sheet. These challenges will need to be met to better estimate the role climate change will have in modulating the surface mass balance of the Greenland ice sheet.
During two exceptionally large July 2012 multiday Greenland ice sheet melt episodes, nonradiative... more During two exceptionally large July 2012 multiday Greenland ice sheet melt episodes, nonradiative energy fluxes (sensible, latent, rain, and subsurface collectively) dominated the ablation area surface energy budget of the southern and western ice sheet. On average the nonradiative energy fluxes contributed up to 76% of daily melt energy at nine automatic weather station sites in Greenland. Comprising 6% of the ablation period, these powerful melt episodes resulted in 12–15% of the south and west Greenland automatic weather station annual ablation totals. Analysis of high resolution (~5 km) HIRHAM5 regional climate model output indicates widespread dominance of nonradiative energy fluxes across the western ablation area during these episodes. Yet HIRHAM5 still underestimates melt by up to 56% during these episodes due to a systematic underestimation of turbulent energy fluxes typical of regional climate models. This has implications for underestimating future melt, when exceptional melt episodes are expected to occur more frequently.
Terrestrial freshwater runoff strongly influences physical and biogeochemical processes at the fj... more Terrestrial freshwater runoff strongly influences physical and biogeochemical processes at the fjord scale and can have global impacts when considered at the Greenland scale. We investigate the performance of the HIRHAM5 regional climate model over the catchments delivering freshwater to Tyrolerfjord and Young Sound by comparing to the unique Greenland Ecological Monitoring database of in situ observations from this region. Based on these findings, we estimate and discuss the fraction of runoff originating from glacierized and non-glacierized land delivered at the daily scale between 1996 and 2008. We find that glaciers contributed on average 50–80% of annual terrestrial runoff when considering different sections of Tyrolerfjord–Young Sound, but snowpack depletion on land and consequently runoff happens about one month earlier in the model than observed in the field. The temporal shift in the model is a likely explanation why summer surface salinity in the inner fjord did not correlate to modelled runoff.
Understanding the drivers of recent change at Greenlandic tidewater glaciers is of great importan... more Understanding the drivers of recent change at Greenlandic tidewater glaciers is of great importance if we are to predict how these glaciers will respond to climatic warming. A poorly constrained component of tidewater glacier processes is the near-terminus subglacial hydrology. Here we present a novel method for constraining near-terminus subglacial hydrology with application to marine-terminating Kangiata Nunata Sermia in Southwest Greenland. By simulating proglacial plume dynamics using buoyant plume theory and a general circulation model, we assess the critical subglacial discharge, if delivered through a single compact channel, required to generate a plume that reaches the fjord surface. We then compare catchment runoff to a time series of plume visibility acquired from a time-lapse camera. We identify extended periods throughout the 2009 melt season where catchment runoff significantly exceeds the discharge required for a plume to reach the fjord surface, yet we observe no plume. We attribute these observations to spatial spreading of runoff across the grounding line. Persistent distributed drainage near the terminus would lead to more spatially homogeneous submarine melting and may promote more rapid basal sliding during warmer summers, potentially providing a mechanism independent of ocean forcing for increases in atmospheric temperature to drive tidewater glacier acceleration.
Meltwater running off the Greenland ice sheet yield significant hydropower potentials in catchmen... more Meltwater running off the Greenland ice sheet yield significant hydropower potentials in catchments bordering the ice sheet, especially in West and South Greenland. Hydropower has been chosen as the most desired source of energy by the Greenland Home Rule, but recent changes in the Greenland ice sheet has emphasized the risk of sudden changes in catchment supply. In this study, we present a thorough investigation of hydropower feasibility at the Paakitsoq basin, near Ilulissat in West Greenland. The catchment is completely dominated by the Greenland ice sheet which provides large quantities of meltwater during the summer season. However, geometrical changes in the ice sheet, for example due to a retreat or an advance of the ice sheet margin, could change the hydrological catchment within the ice sheet. Such a change would have a devastating economical impact as a hydropower plant is a significant long-term investment for an Arctic community of modest population. Here we present a ne...
ABSTRACT Observations show that the Greenland Ice Sheet is sensitive to changes in the climate fo... more ABSTRACT Observations show that the Greenland Ice Sheet is sensitive to changes in the climate forcing which primarily manifests itself as the surface mass balance (SMB). Therefore, getting a realistic SMB is a key issue when an ice sheet model is coupled to a global or regional climate model. In this study, we assess the Greenland SMB computed using three schemes that are commonly applied in ice sheet modelling, i.e., 1) a positive degree-day scheme using temperature and precipitation from the climate model, 2) an offline SMB calculation using precipitation and radiative fluxes from the climate model, 3) an in-line SMB calculation within the climate model. In these three SMB calculations, the climate model outputs are from simulations using the global climate model EC-Earth at a horizontal resolution of about 125x125 km and forced by the ERA-Interim reanalysis as the boundary conditions for the period 1989-2009. The model output is then downscaled to the ice sheet model grid (20x20 km) using simple elevation corrections with standard lapse rates. To evaluate the impact of downscaling due to the coarse resolution of the global model, a fourth way of computing SMB is applied, using the regional climate model HIRHAM5 that dynamically downscales the same ERA-Interim reanalysis data for 1989-2009 to a resolution of 20x20 km. The SMBs calculated using the four different methods are evaluated using the ERA-Interim reanalysis and available observations of the Greenland SMB. Furthermore, the sensitivity of the response of the Greenland ice sheet to different SMBs is investigated by comparing the modelled equilibrium states obtained using the four sets of forcing fields. Finally, we discuss if the parameter selection and model assumptions are appropriate for modelling climates different from present-day.
Intense rainfall events significantly aect Alpine and Alaskan glaciers through enhanced melting,... more Intense rainfall events significantly aect Alpine and Alaskan glaciers through enhanced melting, ice-flow acceleration and subglacial sediment erosion, yet their impact on the Greenland ice sheet has not been assessed. Here we present measurements of ice velocity, subglacial water pressure and meteorological variables from the western margin of the Greenland ice sheet during a week of warm, wet cyclonic weather in late August and early September 2011. We find that extreme surface runo from melt and rainfall led to a widespread acceleration in ice flow that extended 140 km into the ice-sheet interior. We suggest that the late-season timing was critical in promoting rapid runo across an extensive bare ice surface that overwhelmed a subglacial hydrological system in transition to a less-e cient winter mode. Reanalysis data reveal that similar cyclonic weather conditions prevailed across southern and western Greenland during this time, and we observe a corresponding ice-flow response at all land- and marine-terminating glaciers in these regions for which data are available. Given that the advection of warm, moist air masses and rainfall over Greenland is expected to become more frequent in the coming decades, our findings portend a previously unforeseen vulnerability of the Greenland ice sheet to climate change.
A short overview presentation of work currently (2015) ongoing in the research department, climat... more A short overview presentation of work currently (2015) ongoing in the research department, climate and Arctic section.
Freshwater runoff to fjords with marine-terminating glaciers along the Greenland Ice Sheet margin... more Freshwater runoff to fjords with marine-terminating glaciers along the Greenland Ice Sheet margin has an impact on fjord circulation and, potentially, ice sheet mass balance through increasing heat transport to the glacier front. Here we use the high resolution (5.5 km) HIRHAM5 regional climate model, allowing high detail in topography and surface types, to estimate freshwater input to Godthåbsfjord in Southwest Greenland. Model output is compared to hydro-meteorological observations, and while simulated daily variability in temperature and downwelling radiation shows high correlation with observations (typically >0.9), there are biases that impact our results. In particular, overestimated albedo leads to underestimation of melt and runoff at low-elevations.
In the model simulation (1991-2012), the ice sheet experiences increasing energy input from the surface turbulent heat flux (up to elevations of 2000 m) and shortwave radiation (at all elevations). Southerly wind anomalies and declining cloudiness due to an increase in atmospheric pressure over North Greenland contribute to increased summer melt. This results in declining surface mass balance (SMB), increasing surface runoff and upward shift of the equilibrium line altitude.
SMB is reconstructed back to 1890 though regression between simulated SMB and observed temperature and precipitation, with added uncertainty in the period 1890-1952 due to possible inhomogeneity in the precipitation record. SMB as low as in recent years appears to have occurred before, most notably around 1930, 1950 and 1960. While previous low SMBs were mainly caused by low accumulation, those around 1930 and in the 2000s are mainly due to warming.
The wild card for reliable sea level rise prediction is the contribution of the Greenland Ice She... more The wild card for reliable sea level rise prediction is the contribution of the Greenland Ice Sheet. There is an urgent need to determine the predictability of models that simulate the response of Greenland Ice Sheet to rising temperatures and the amount of freshwater flux that can be expected into the ocean. Modelling efforts have been limited by poorly known boundary and initial conditions, low resolution and lack of presentation of fast flowing ice streams. We address these limitations by building a model system consisting of a high resolution regional climate model (HIRHAM4), that has been run for the period 1950-2080 at 25 km, and Parallel Ice Sheet Model (PISM), which simulates spatially and temporally varying ice streams by combining the solutions of the Shallow Shelf and Shallow Ice Approximations. The surface mass balance is simulated with a positive-degree-day method. The important and poorly constrained model component is the past climate forcing, which serves the purpose of initializing the model by simulating the present ice sheet and observed rate of mass changes. Simulated gradients of mass loss due to warming trends of past decade and prediction for the future are presented as well as estimated sensitivities due to the various model component uncertainties.
Advanced Climate Dynamics Course 2010: Ice Sheet-Ocean Interactions; Lyngen, Norway, 8-19 June 20... more Advanced Climate Dynamics Course 2010: Ice Sheet-Ocean Interactions; Lyngen, Norway, 8-19 June 2010; Sea level rise is one of many expected consequences of climate change, with accompanying complex social and economic challenges. Major uncertainties in sea level rise projections relate to the response of ice sheets to sea level rise and the key role that interactions with the ocean may play. Recognizing that probably no comprehensive curriculum currently exists at any single university that covers this novel and interdisciplinary subject, the Advanced Climate Dynamics Courses (ACDC) team brought together a group of 40 international students, postdocs, and lecturers from diverse backgrounds to provide an overview and discussion of state-of-the-art research into ocean-ice sheet interactions and to propose research priorities for the next decade. Among the key issues addressed were small-scale processes near the Antarctic ice shelves and Greenland outlet glaciers. These are fast changing components in the climate system, often related to large-scale forcings (atmospheric teleconnections and oceanic circulation). Progress in understanding and modeling is hampered by the range of scales involved, the lack of observations, and the difficulties in constraining, initializing, and providing adequate boundary conditions for ice sheet and ocean models.
Predicting future hydrological regimes with regard to climate change is an increasingly important... more Predicting future hydrological regimes with regard to climate change is an increasingly important task for hydrologists. In polar regions the task is more difficult due to the lack of datasets and long term monitoring as well as logistical difficulties in remote and inaccessible basins. Here, we demonstrate a case study predicting the future run-off in a difficult to model hydrological basin by integrating a range of data, methods and numerical models. A study, evaluating the future conditions in the Pakitsup Akuliarusersua basin near Ilulissat, West Greenland, was initiated to determine the viability of a small hydropower scheme based around two lakes adjacent to the ice-sheet margin. This basin is mainly supplied by meltwater from the ice-sheet margin and the position of the ice sheet relative to the lakes makes them sensitive to changes in drainage pathways. We combined glaciological and hydrological models with data from climate models in order to resolve these issues. An ice dynamic model (Reeh, 1988), incorporating new digital terrain models for the ice sheet surface and basal topographies (Mottram and other, 2009), was driven by climate data from a combined global/regional climate model (HIRHAM4) for the period 1950-2080 (Stendel and others, 2007). The climate data was downscaled to catchment scale and corrected using observational data from the local area. The corrected HIRHAM4 output was used as input to a temperature-index mass-balance model (Reeh, 1991) and used to force the ice-dynamic model in order to predict the future ice sheet geometry and to drive meltwater production at the ice sheet surface. These ice sheet geometries were used to predict the size of the ice-sheet part of the hydrological basin for a range of different levels of ice sheet basal water pressure every 5 years from present day to 2080. Thus, the present analysis takes into account global and regional climate change, ice dynamical response and changes in the internal drainage system of the ice sheet. We predict that the ice-sheet margin will continue its present thinning trend, causing a thinning of approximately 80 metres over the next 70 years, corresponding roughly to the current rate of ice-sheet thinning in the region, approximately 1 metre per year over the last 40 years. The predicted retreat rates are realistic from a glaciological point of view and cause only minor changes in the basin delineation for high (realistic) basal water pressures. Our analysis shows that the HIRHAM4 output needs substantial adjustment to reproduce observations on catchment scale and ice-dynamic models do not yet capture all the important processes of the ice sheet, in particular its dynamic response to an increase in the surface meltwater input to its internal drainage system. Nevertheless, the predictions from this study are realistic and take into account the most important processes occurring at the ice sheet margin.
The occurrence of tors within glaciated regions has been widely cited as evidence for the preserv... more The occurrence of tors within glaciated regions has been widely cited as evidence for the preservation of relic pre-Quaternary landscapes beneath protective covers of non-erosive dry-based ice. Here, we test for the preservation of pre-Quaternary landscapes with cosmogenic surface exposure dating of tors. Numerous granite tors are present on summit plateaus in the Cairngorm Mountains of Scotland where they were covered by local ice caps many times during the Pleistocene. Cosmogenic 10Be and 26Al data together with geomorphic relationships reveal that these landforms are more dynamic and younger than previously suspected. Many Cairngorm tors have been bulldozed and toppled along horizontal joints by ice motion, leaving event surfaces on tor remnants and erratics that can be dated with cosmogenic nuclides. As the surfaces have been subject to episodic burial by ice, an exposure model based upon ice and marine sediment core proxies for local glacial cover is necessary to interpret the cosmogenic nuclide data. Exposure ages and weathering characteristics of tors are closely correlated. Glacially modified tors and boulder erratics with slightly weathered surfaces have 10Be exposure ages of about 15 to 43 ka. Nuclide inheritance is present in many of these surfaces. Correction for inheritance indicates that the eastern Cairngorms were deglaciated at 15.6 ± 0.9 ka. Glacially modified tors with moderate to advanced weathering features have 10Be exposure ages of 19 to 92 ka. These surfaces were only slightly modified during the last glacial cycle and gained much of their exposure during the interstadial of marine Oxygen Isotope Stage 5 or earlier. Tors lacking evidence of glacial modification and exhibiting advanced weathering have 10Be exposure ages between 52 and 297 ka. Nuclide concentrations in these surfaces are probably controlled by bedrock erosion rates instead of discrete glacial events. Maximum erosion rates estimated from 10Be range from 2.8 to 12.0 mm/ka, with an error weighted mean of 4.1 ± 0.2 mm/ka. Three of these surfaces yield model exposure-plus-burial ages of 295 - 71 + 84 , 520 - 141 + 178 , and 626 - 85 + 102 ka. A vertical cosmogenic nuclide profile across the oldest sampled tor indicates a long-term emergence rate of 31 ± 2 mm/ka. These findings show that dry-based ice caps are capable of substantially eroding tors by entraining blocks previously detached by weathering processes. Bedrock surfaces and erratic boulders in such settings are likely to have nuclide inheritance and may yield erroneous (too old) exposure ages. While many Cairngorm tors have survived multiple glacial cycles, rates of regolith stripping and bedrock erosion are too high to permit the widespread preservation of pre-Quaternary rock surfaces.
Calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers... more Calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world. Calving rates can increase dramatically in response to increases in velocity and/or retreat of the glacier margin, with important implications for sea level change. Despite their importance, calving and related dynamic processes are poorly represented in the current generation of ice sheet models. This is largely because understanding the 'calving problem' involves several other long-standing problems in glaciology, combined with the difficulties and dangers of field data collection. In this paper, we systematically review different aspects of the calving problem, and outline a new framework for representing calving processes in ice sheet models. We define a hierarchy of calving processes, to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events. The first-order control on calving is the strain rate arising from spatial variations in velocity (particularly sliding speed), which determines the location and depth of surface crevasses. Superimposed on this first-order process are second-order processes that can further erode the ice margin. These include: fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front; undercutting of the glacier terminus by melting at or below the waterline; and bending at the junction between grounded and buoyant parts of an ice tongue. Calving of projecting, submerged 'ice feet' can be regarded as a third-order process, because it is paced by first- or second-order calving above the waterline. First-order calving can be represented in glacier models using a calving criterion based on crevasse depth, which is a function of longitudinal strain rate. Modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution. Realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law. Models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini, but an important limitation is that they cannot be used to model floating glacier termini or ice shelves. Alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models. Such models can explain a remarkable range of observed phenomena within a simple, unifying framework, including: downglacier increases in velocity and strain rates where basal and/or lateral drag diminishes; flow acceleration in response to thinning through time; the tendency for glaciers to stabilise at 'pinning points' in relatively shallow water or fjord narrowings; the constraints on ice shelf stability; and the contrasts in calving rates between tidewater and freshwater calving glaciers. Many unresolved issues remain, however, including the role played by the removal of backstress in the acceleration of retreating calving glaciers, and the controls on melting at and below the waterline.
To improve Greenland Ice Sheet surface mass balance (SMB) simulation, the subsurface scheme of th... more To improve Greenland Ice Sheet surface mass balance (SMB) simulation, the subsurface scheme of the HIRHAM5 regional climate model was extended to include snow densification, varying hydraulic conductivity, irreducible water saturation and other effects on snow liquid water percolation and retention. Sensitivity experiments to investigate the effects of the additions and the impact of different parameterization choices are presented. Compared with 68 accumulation area ice cores, the simulated mean annual net accumulation bias is −5% (correlation coefficient of 0.90). Modeled SMB in the ablation area compares favorably with 1041 PROMICE observations with regression slope of 0.95–0.97 (depending on model configuration), correlation coefficient of 0.75–0.86 and mean bias −3%. Weighting ablation area SMB biases at low-and high-elevation with the amount of runoff from these areas, we estimate ice sheet-wide mass loss biases in the ablation area at −5 and −7% using observed (MODIS-derived) and internally calculated albedo, respectively. Comparison with observed melt day counts shows that patterns of spatial (correlation ∼0.9) and temporal (correlation coefficient of ∼0.9) variability are realistically represented in the simulations. However, the model tends to underestimate the magnitude of inter-annual variability (regression slope ∼0.7) and overestimate that of spatial variability (slope ∼1.2). In terms of subsurface temperature structure and occurrence of perennial firn aquifers and perched ice layers, the most important model choices are the albedo implementation and irreducible water saturation parameterization. At one percolation area location, for instance, the internally calculated albedo yields too high subsurface temperatures below 5 m, but when using an implementation of irreducible saturation allowing higher values, an ice layer forms in 2011, reducing the deep warm bias in subsequent years. On the other hand, prior to the formation of the ice layer, observed albedos combined with lower irreducible saturation give the smallest bias. Perennial firn aquifers and perched ice layers occur in varying thickness and area for
Surface mass balance (SMB) is the builder of the Greenland ice sheet and the driver of ice dynami... more Surface mass balance (SMB) is the builder of the Greenland ice sheet and the driver of ice dynamics. Quantifying the past, present and future state of SMB is important to understand the drivers and climatic processes that control SMB, and to both initialize and run ice sheet models which will help clarify sea level rise, and how likely changes in ice sheet extent feedback within the climate system. Regional climate models (RCMs) and climate reanalysis are used to quantify SMB estimates. Although different models have different spatial and temporal biases and may include different processes giving significant uncertainty in both SMB and the ice sheet dynamic response to it, all RCMs show a recent declining trend in SMB from the Greenland ice sheet, driven primarily by enhanced melt rates. Here, we present new simulations of the Greenland ice sheet SMB at 5 km resolution from the RCM HIRHAM5. The RCM is driven by the ERA-Interim reanalysis and the global climate model (GCM) EC-Earth v2.3 to make future projections for climate scenarios RCP8.5 and RCP4.5. Future estimates of SMB are affected by biases in driving global climate models, and feedbacks between the ice sheet surface and the global and regional climate system are neglected, likely resulting in significant underestimates of melt and precipitation over the ice sheet. These challenges will need to be met to better estimate the role climate change will have in modulating the surface mass balance of the Greenland ice sheet.
During two exceptionally large July 2012 multiday Greenland ice sheet melt episodes, nonradiative... more During two exceptionally large July 2012 multiday Greenland ice sheet melt episodes, nonradiative energy fluxes (sensible, latent, rain, and subsurface collectively) dominated the ablation area surface energy budget of the southern and western ice sheet. On average the nonradiative energy fluxes contributed up to 76% of daily melt energy at nine automatic weather station sites in Greenland. Comprising 6% of the ablation period, these powerful melt episodes resulted in 12–15% of the south and west Greenland automatic weather station annual ablation totals. Analysis of high resolution (~5 km) HIRHAM5 regional climate model output indicates widespread dominance of nonradiative energy fluxes across the western ablation area during these episodes. Yet HIRHAM5 still underestimates melt by up to 56% during these episodes due to a systematic underestimation of turbulent energy fluxes typical of regional climate models. This has implications for underestimating future melt, when exceptional melt episodes are expected to occur more frequently.
Terrestrial freshwater runoff strongly influences physical and biogeochemical processes at the fj... more Terrestrial freshwater runoff strongly influences physical and biogeochemical processes at the fjord scale and can have global impacts when considered at the Greenland scale. We investigate the performance of the HIRHAM5 regional climate model over the catchments delivering freshwater to Tyrolerfjord and Young Sound by comparing to the unique Greenland Ecological Monitoring database of in situ observations from this region. Based on these findings, we estimate and discuss the fraction of runoff originating from glacierized and non-glacierized land delivered at the daily scale between 1996 and 2008. We find that glaciers contributed on average 50–80% of annual terrestrial runoff when considering different sections of Tyrolerfjord–Young Sound, but snowpack depletion on land and consequently runoff happens about one month earlier in the model than observed in the field. The temporal shift in the model is a likely explanation why summer surface salinity in the inner fjord did not correlate to modelled runoff.
Understanding the drivers of recent change at Greenlandic tidewater glaciers is of great importan... more Understanding the drivers of recent change at Greenlandic tidewater glaciers is of great importance if we are to predict how these glaciers will respond to climatic warming. A poorly constrained component of tidewater glacier processes is the near-terminus subglacial hydrology. Here we present a novel method for constraining near-terminus subglacial hydrology with application to marine-terminating Kangiata Nunata Sermia in Southwest Greenland. By simulating proglacial plume dynamics using buoyant plume theory and a general circulation model, we assess the critical subglacial discharge, if delivered through a single compact channel, required to generate a plume that reaches the fjord surface. We then compare catchment runoff to a time series of plume visibility acquired from a time-lapse camera. We identify extended periods throughout the 2009 melt season where catchment runoff significantly exceeds the discharge required for a plume to reach the fjord surface, yet we observe no plume. We attribute these observations to spatial spreading of runoff across the grounding line. Persistent distributed drainage near the terminus would lead to more spatially homogeneous submarine melting and may promote more rapid basal sliding during warmer summers, potentially providing a mechanism independent of ocean forcing for increases in atmospheric temperature to drive tidewater glacier acceleration.
Meltwater running off the Greenland ice sheet yield significant hydropower potentials in catchmen... more Meltwater running off the Greenland ice sheet yield significant hydropower potentials in catchments bordering the ice sheet, especially in West and South Greenland. Hydropower has been chosen as the most desired source of energy by the Greenland Home Rule, but recent changes in the Greenland ice sheet has emphasized the risk of sudden changes in catchment supply. In this study, we present a thorough investigation of hydropower feasibility at the Paakitsoq basin, near Ilulissat in West Greenland. The catchment is completely dominated by the Greenland ice sheet which provides large quantities of meltwater during the summer season. However, geometrical changes in the ice sheet, for example due to a retreat or an advance of the ice sheet margin, could change the hydrological catchment within the ice sheet. Such a change would have a devastating economical impact as a hydropower plant is a significant long-term investment for an Arctic community of modest population. Here we present a ne...
ABSTRACT Observations show that the Greenland Ice Sheet is sensitive to changes in the climate fo... more ABSTRACT Observations show that the Greenland Ice Sheet is sensitive to changes in the climate forcing which primarily manifests itself as the surface mass balance (SMB). Therefore, getting a realistic SMB is a key issue when an ice sheet model is coupled to a global or regional climate model. In this study, we assess the Greenland SMB computed using three schemes that are commonly applied in ice sheet modelling, i.e., 1) a positive degree-day scheme using temperature and precipitation from the climate model, 2) an offline SMB calculation using precipitation and radiative fluxes from the climate model, 3) an in-line SMB calculation within the climate model. In these three SMB calculations, the climate model outputs are from simulations using the global climate model EC-Earth at a horizontal resolution of about 125x125 km and forced by the ERA-Interim reanalysis as the boundary conditions for the period 1989-2009. The model output is then downscaled to the ice sheet model grid (20x20 km) using simple elevation corrections with standard lapse rates. To evaluate the impact of downscaling due to the coarse resolution of the global model, a fourth way of computing SMB is applied, using the regional climate model HIRHAM5 that dynamically downscales the same ERA-Interim reanalysis data for 1989-2009 to a resolution of 20x20 km. The SMBs calculated using the four different methods are evaluated using the ERA-Interim reanalysis and available observations of the Greenland SMB. Furthermore, the sensitivity of the response of the Greenland ice sheet to different SMBs is investigated by comparing the modelled equilibrium states obtained using the four sets of forcing fields. Finally, we discuss if the parameter selection and model assumptions are appropriate for modelling climates different from present-day.
Intense rainfall events significantly aect Alpine and Alaskan glaciers through enhanced melting,... more Intense rainfall events significantly aect Alpine and Alaskan glaciers through enhanced melting, ice-flow acceleration and subglacial sediment erosion, yet their impact on the Greenland ice sheet has not been assessed. Here we present measurements of ice velocity, subglacial water pressure and meteorological variables from the western margin of the Greenland ice sheet during a week of warm, wet cyclonic weather in late August and early September 2011. We find that extreme surface runo from melt and rainfall led to a widespread acceleration in ice flow that extended 140 km into the ice-sheet interior. We suggest that the late-season timing was critical in promoting rapid runo across an extensive bare ice surface that overwhelmed a subglacial hydrological system in transition to a less-e cient winter mode. Reanalysis data reveal that similar cyclonic weather conditions prevailed across southern and western Greenland during this time, and we observe a corresponding ice-flow response at all land- and marine-terminating glaciers in these regions for which data are available. Given that the advection of warm, moist air masses and rainfall over Greenland is expected to become more frequent in the coming decades, our findings portend a previously unforeseen vulnerability of the Greenland ice sheet to climate change.
A short overview presentation of work currently (2015) ongoing in the research department, climat... more A short overview presentation of work currently (2015) ongoing in the research department, climate and Arctic section.
Freshwater runoff to fjords with marine-terminating glaciers along the Greenland Ice Sheet margin... more Freshwater runoff to fjords with marine-terminating glaciers along the Greenland Ice Sheet margin has an impact on fjord circulation and, potentially, ice sheet mass balance through increasing heat transport to the glacier front. Here we use the high resolution (5.5 km) HIRHAM5 regional climate model, allowing high detail in topography and surface types, to estimate freshwater input to Godthåbsfjord in Southwest Greenland. Model output is compared to hydro-meteorological observations, and while simulated daily variability in temperature and downwelling radiation shows high correlation with observations (typically >0.9), there are biases that impact our results. In particular, overestimated albedo leads to underestimation of melt and runoff at low-elevations.
In the model simulation (1991-2012), the ice sheet experiences increasing energy input from the surface turbulent heat flux (up to elevations of 2000 m) and shortwave radiation (at all elevations). Southerly wind anomalies and declining cloudiness due to an increase in atmospheric pressure over North Greenland contribute to increased summer melt. This results in declining surface mass balance (SMB), increasing surface runoff and upward shift of the equilibrium line altitude.
SMB is reconstructed back to 1890 though regression between simulated SMB and observed temperature and precipitation, with added uncertainty in the period 1890-1952 due to possible inhomogeneity in the precipitation record. SMB as low as in recent years appears to have occurred before, most notably around 1930, 1950 and 1960. While previous low SMBs were mainly caused by low accumulation, those around 1930 and in the 2000s are mainly due to warming.
The wild card for reliable sea level rise prediction is the contribution of the Greenland Ice She... more The wild card for reliable sea level rise prediction is the contribution of the Greenland Ice Sheet. There is an urgent need to determine the predictability of models that simulate the response of Greenland Ice Sheet to rising temperatures and the amount of freshwater flux that can be expected into the ocean. Modelling efforts have been limited by poorly known boundary and initial conditions, low resolution and lack of presentation of fast flowing ice streams. We address these limitations by building a model system consisting of a high resolution regional climate model (HIRHAM4), that has been run for the period 1950-2080 at 25 km, and Parallel Ice Sheet Model (PISM), which simulates spatially and temporally varying ice streams by combining the solutions of the Shallow Shelf and Shallow Ice Approximations. The surface mass balance is simulated with a positive-degree-day method. The important and poorly constrained model component is the past climate forcing, which serves the purpose of initializing the model by simulating the present ice sheet and observed rate of mass changes. Simulated gradients of mass loss due to warming trends of past decade and prediction for the future are presented as well as estimated sensitivities due to the various model component uncertainties.
Advanced Climate Dynamics Course 2010: Ice Sheet-Ocean Interactions; Lyngen, Norway, 8-19 June 20... more Advanced Climate Dynamics Course 2010: Ice Sheet-Ocean Interactions; Lyngen, Norway, 8-19 June 2010; Sea level rise is one of many expected consequences of climate change, with accompanying complex social and economic challenges. Major uncertainties in sea level rise projections relate to the response of ice sheets to sea level rise and the key role that interactions with the ocean may play. Recognizing that probably no comprehensive curriculum currently exists at any single university that covers this novel and interdisciplinary subject, the Advanced Climate Dynamics Courses (ACDC) team brought together a group of 40 international students, postdocs, and lecturers from diverse backgrounds to provide an overview and discussion of state-of-the-art research into ocean-ice sheet interactions and to propose research priorities for the next decade. Among the key issues addressed were small-scale processes near the Antarctic ice shelves and Greenland outlet glaciers. These are fast changing components in the climate system, often related to large-scale forcings (atmospheric teleconnections and oceanic circulation). Progress in understanding and modeling is hampered by the range of scales involved, the lack of observations, and the difficulties in constraining, initializing, and providing adequate boundary conditions for ice sheet and ocean models.
Predicting future hydrological regimes with regard to climate change is an increasingly important... more Predicting future hydrological regimes with regard to climate change is an increasingly important task for hydrologists. In polar regions the task is more difficult due to the lack of datasets and long term monitoring as well as logistical difficulties in remote and inaccessible basins. Here, we demonstrate a case study predicting the future run-off in a difficult to model hydrological basin by integrating a range of data, methods and numerical models. A study, evaluating the future conditions in the Pakitsup Akuliarusersua basin near Ilulissat, West Greenland, was initiated to determine the viability of a small hydropower scheme based around two lakes adjacent to the ice-sheet margin. This basin is mainly supplied by meltwater from the ice-sheet margin and the position of the ice sheet relative to the lakes makes them sensitive to changes in drainage pathways. We combined glaciological and hydrological models with data from climate models in order to resolve these issues. An ice dynamic model (Reeh, 1988), incorporating new digital terrain models for the ice sheet surface and basal topographies (Mottram and other, 2009), was driven by climate data from a combined global/regional climate model (HIRHAM4) for the period 1950-2080 (Stendel and others, 2007). The climate data was downscaled to catchment scale and corrected using observational data from the local area. The corrected HIRHAM4 output was used as input to a temperature-index mass-balance model (Reeh, 1991) and used to force the ice-dynamic model in order to predict the future ice sheet geometry and to drive meltwater production at the ice sheet surface. These ice sheet geometries were used to predict the size of the ice-sheet part of the hydrological basin for a range of different levels of ice sheet basal water pressure every 5 years from present day to 2080. Thus, the present analysis takes into account global and regional climate change, ice dynamical response and changes in the internal drainage system of the ice sheet. We predict that the ice-sheet margin will continue its present thinning trend, causing a thinning of approximately 80 metres over the next 70 years, corresponding roughly to the current rate of ice-sheet thinning in the region, approximately 1 metre per year over the last 40 years. The predicted retreat rates are realistic from a glaciological point of view and cause only minor changes in the basin delineation for high (realistic) basal water pressures. Our analysis shows that the HIRHAM4 output needs substantial adjustment to reproduce observations on catchment scale and ice-dynamic models do not yet capture all the important processes of the ice sheet, in particular its dynamic response to an increase in the surface meltwater input to its internal drainage system. Nevertheless, the predictions from this study are realistic and take into account the most important processes occurring at the ice sheet margin.
The occurrence of tors within glaciated regions has been widely cited as evidence for the preserv... more The occurrence of tors within glaciated regions has been widely cited as evidence for the preservation of relic pre-Quaternary landscapes beneath protective covers of non-erosive dry-based ice. Here, we test for the preservation of pre-Quaternary landscapes with cosmogenic surface exposure dating of tors. Numerous granite tors are present on summit plateaus in the Cairngorm Mountains of Scotland where they were covered by local ice caps many times during the Pleistocene. Cosmogenic 10Be and 26Al data together with geomorphic relationships reveal that these landforms are more dynamic and younger than previously suspected. Many Cairngorm tors have been bulldozed and toppled along horizontal joints by ice motion, leaving event surfaces on tor remnants and erratics that can be dated with cosmogenic nuclides. As the surfaces have been subject to episodic burial by ice, an exposure model based upon ice and marine sediment core proxies for local glacial cover is necessary to interpret the cosmogenic nuclide data. Exposure ages and weathering characteristics of tors are closely correlated. Glacially modified tors and boulder erratics with slightly weathered surfaces have 10Be exposure ages of about 15 to 43 ka. Nuclide inheritance is present in many of these surfaces. Correction for inheritance indicates that the eastern Cairngorms were deglaciated at 15.6 ± 0.9 ka. Glacially modified tors with moderate to advanced weathering features have 10Be exposure ages of 19 to 92 ka. These surfaces were only slightly modified during the last glacial cycle and gained much of their exposure during the interstadial of marine Oxygen Isotope Stage 5 or earlier. Tors lacking evidence of glacial modification and exhibiting advanced weathering have 10Be exposure ages between 52 and 297 ka. Nuclide concentrations in these surfaces are probably controlled by bedrock erosion rates instead of discrete glacial events. Maximum erosion rates estimated from 10Be range from 2.8 to 12.0 mm/ka, with an error weighted mean of 4.1 ± 0.2 mm/ka. Three of these surfaces yield model exposure-plus-burial ages of 295 - 71 + 84 , 520 - 141 + 178 , and 626 - 85 + 102 ka. A vertical cosmogenic nuclide profile across the oldest sampled tor indicates a long-term emergence rate of 31 ± 2 mm/ka. These findings show that dry-based ice caps are capable of substantially eroding tors by entraining blocks previously detached by weathering processes. Bedrock surfaces and erratic boulders in such settings are likely to have nuclide inheritance and may yield erroneous (too old) exposure ages. While many Cairngorm tors have survived multiple glacial cycles, rates of regolith stripping and bedrock erosion are too high to permit the widespread preservation of pre-Quaternary rock surfaces.
Calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers... more Calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world. Calving rates can increase dramatically in response to increases in velocity and/or retreat of the glacier margin, with important implications for sea level change. Despite their importance, calving and related dynamic processes are poorly represented in the current generation of ice sheet models. This is largely because understanding the 'calving problem' involves several other long-standing problems in glaciology, combined with the difficulties and dangers of field data collection. In this paper, we systematically review different aspects of the calving problem, and outline a new framework for representing calving processes in ice sheet models. We define a hierarchy of calving processes, to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events. The first-order control on calving is the strain rate arising from spatial variations in velocity (particularly sliding speed), which determines the location and depth of surface crevasses. Superimposed on this first-order process are second-order processes that can further erode the ice margin. These include: fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front; undercutting of the glacier terminus by melting at or below the waterline; and bending at the junction between grounded and buoyant parts of an ice tongue. Calving of projecting, submerged 'ice feet' can be regarded as a third-order process, because it is paced by first- or second-order calving above the waterline. First-order calving can be represented in glacier models using a calving criterion based on crevasse depth, which is a function of longitudinal strain rate. Modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution. Realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law. Models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini, but an important limitation is that they cannot be used to model floating glacier termini or ice shelves. Alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models. Such models can explain a remarkable range of observed phenomena within a simple, unifying framework, including: downglacier increases in velocity and strain rates where basal and/or lateral drag diminishes; flow acceleration in response to thinning through time; the tendency for glaciers to stabilise at 'pinning points' in relatively shallow water or fjord narrowings; the constraints on ice shelf stability; and the contrasts in calving rates between tidewater and freshwater calving glaciers. Many unresolved issues remain, however, including the role played by the removal of backstress in the acceleration of retreating calving glaciers, and the controls on melting at and below the waterline.
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Papers by Ruth Mottram
and subglacial sediment erosion, yet their impact on the Greenland ice sheet has not been assessed. Here we present
measurements of ice velocity, subglacial water pressure and meteorological variables from the western margin of the
Greenland ice sheet during a week of warm, wet cyclonic weather in late August and early September 2011. We find that
extreme surface runo from melt and rainfall led to a widespread acceleration in ice flow that extended 140 km into the
ice-sheet interior. We suggest that the late-season timing was critical in promoting rapid runo across an extensive bare ice
surface that overwhelmed a subglacial hydrological system in transition to a less-e cient winter mode. Reanalysis data reveal
that similar cyclonic weather conditions prevailed across southern and western Greenland during this time, and we observe
a corresponding ice-flow response at all land- and marine-terminating glaciers in these regions for which data are available.
Given that the advection of warm, moist air masses and rainfall over Greenland is expected to become more frequent in the
coming decades, our findings portend a previously unforeseen vulnerability of the Greenland ice sheet to climate change.
With contributions from many colleagues
In the model simulation (1991-2012), the ice sheet experiences increasing energy input from the surface turbulent heat flux (up to elevations of 2000 m) and shortwave radiation (at all elevations). Southerly wind anomalies and declining cloudiness due to an increase in atmospheric pressure over North Greenland contribute to increased summer melt. This results in declining surface mass balance (SMB), increasing surface runoff and upward shift of the equilibrium line altitude.
SMB is reconstructed back to 1890 though regression between simulated SMB and observed temperature and precipitation, with added uncertainty in the period 1890-1952 due to possible inhomogeneity in the precipitation record. SMB as low as in recent years appears to have occurred before, most notably around 1930, 1950 and 1960. While previous low SMBs were mainly caused by low accumulation, those around 1930 and in the 2000s are mainly due to warming.
and subglacial sediment erosion, yet their impact on the Greenland ice sheet has not been assessed. Here we present
measurements of ice velocity, subglacial water pressure and meteorological variables from the western margin of the
Greenland ice sheet during a week of warm, wet cyclonic weather in late August and early September 2011. We find that
extreme surface runo from melt and rainfall led to a widespread acceleration in ice flow that extended 140 km into the
ice-sheet interior. We suggest that the late-season timing was critical in promoting rapid runo across an extensive bare ice
surface that overwhelmed a subglacial hydrological system in transition to a less-e cient winter mode. Reanalysis data reveal
that similar cyclonic weather conditions prevailed across southern and western Greenland during this time, and we observe
a corresponding ice-flow response at all land- and marine-terminating glaciers in these regions for which data are available.
Given that the advection of warm, moist air masses and rainfall over Greenland is expected to become more frequent in the
coming decades, our findings portend a previously unforeseen vulnerability of the Greenland ice sheet to climate change.
With contributions from many colleagues
In the model simulation (1991-2012), the ice sheet experiences increasing energy input from the surface turbulent heat flux (up to elevations of 2000 m) and shortwave radiation (at all elevations). Southerly wind anomalies and declining cloudiness due to an increase in atmospheric pressure over North Greenland contribute to increased summer melt. This results in declining surface mass balance (SMB), increasing surface runoff and upward shift of the equilibrium line altitude.
SMB is reconstructed back to 1890 though regression between simulated SMB and observed temperature and precipitation, with added uncertainty in the period 1890-1952 due to possible inhomogeneity in the precipitation record. SMB as low as in recent years appears to have occurred before, most notably around 1930, 1950 and 1960. While previous low SMBs were mainly caused by low accumulation, those around 1930 and in the 2000s are mainly due to warming.