Improved modelling of the present-day Greenland firn layer

Recent studies indicate that a declining surface mass balance will dominate the Greenland Ice Sheet&#8217;s (GrIS) contribution to 21st century sea level rise. It is therefore crucial to understand the liquid water balance of the ice sheet and its response to increasing temperatures and surface melt if we want to accurately predict future sea level rise. The ice sheet firn layer covers ~90% of the GrIS and provides pore space for storage and refreezing of meltwater. Because of this, the firn layer can retain up to ~45% of the surface meltwater and thus act as an efficient buffer to ice sheet mass loss. However, in a warming climate this buffer capacity of the firn layer is expected to decrease, amplifying meltwater runoff and sea-level rise. Dedicated firn models are used to understand how firn layers evolve and affect runoff. Additionally, firn models are used to estimate the changing thickness of the firn layer, which is necessary in altimetry to convert surface height change into ice sheet mass loss.Here, we present the latest version of our firn model IMAU-FDM. With respect to the previous version, changes have been made to the handling of the freshly fallen snow, the densification rate of the firn and the conduction of heat. These changes lead to an improved representation of firn density and temperature. The results have been thoroughly validated using an extensive dataset of density and temperature measurements that we have compiled covering 126 different locations on the GrIS. Meltwater behaviour in the model is validated with upward-looking GPR measurements at Dye-2. Lastly, we present an in-depth look at the evolution firn characteristics at some typical locations in Greenland.Dedicated, stand-alone firn models offer various benefits to using a regional climate model with an embedded firn model. Firstly, the vertical resolution for buried snow and ice layers can be larger, improving accuracy. Secondly, a stand-alone firn model allows for spinning up the model to a more accurate equilibrium state. And thirdly, a stand-alone model is more cost- and time-effective to use. Firn models are increasingly capable of simulating the firn layer, but areas with large amounts of melt still pose the greatest challenge.

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Estimating the Greenland ice sheet surface mass balance contribution to future sea level rise using the regional atmospheric climate model MAR

The Cryosphere ◽

10.5194/tc-7-469-2013 ◽

2013 ◽

Vol 7 (2) ◽

pp. 469-489 ◽

Cited By ~ 224

Author(s):

X. Fettweis ◽

B. Franco ◽

M. Tedesco ◽

J. H. van Angelen ◽

J. T. M. Lenaerts ◽

...

Keyword(s):

Mass Balance ◽

Sea Level Rise ◽

Sea Level ◽

Climate Model ◽

Regional Climate ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Mass Balance ◽

Near Surface ◽

Surface Mass

Abstract. To estimate the sea level rise (SLR) originating from changes in surface mass balance (SMB) of the Greenland ice sheet (GrIS), we present 21st century climate projections obtained with the regional climate model MAR (Modèle Atmosphérique Régional), forced by output of three CMIP5 (Coupled Model Intercomparison Project Phase 5) general circulation models (GCMs). Our results indicate that in a warmer climate, mass gain from increased winter snowfall over the GrIS does not compensate mass loss through increased meltwater run-off in summer. Despite the large spread in the projected near-surface warming, all the MAR projections show similar non-linear increase of GrIS surface melt volume because no change is projected in the general atmospheric circulation over Greenland. By coarsely estimating the GrIS SMB changes from GCM output, we show that the uncertainty from the GCM-based forcing represents about half of the projected SMB changes. In 2100, the CMIP5 ensemble mean projects a GrIS SMB decrease equivalent to a mean SLR of &plus;4 &pm; 2 cm and &plus;9 &pm; 4 cm for the RCP (Representative Concentration Pathways) 4.5 and RCP 8.5 scenarios respectively. These estimates do not consider the positive melt–elevation feedback, although sensitivity experiments using perturbed ice sheet topographies consistent with the projected SMB changes demonstrate that this is a significant feedback, and highlight the importance of coupling regional climate models to an ice sheet model. Such a coupling will allow the assessment of future response of both surface processes and ice-dynamic changes to rising temperatures, as well as their mutual feedbacks.

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An Integrated View of Greenland Ice Sheet Mass Changes Based on Models and Satellite Observations

Remote Sensing ◽

10.3390/rs11121407 ◽

2019 ◽

Vol 11 (12) ◽

pp. 1407 ◽

Cited By ~ 10

Author(s):

Ruth Mottram ◽

Sebastian B. Simonsen ◽

Synne Høyer Svendsen ◽

Valentina R. Barletta ◽

Louise Sandberg Sørensen ◽

...

Keyword(s):

Mass Balance ◽

Sea Level Rise ◽

Sea Level ◽

Climate Model ◽

Regional Climate ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Mass ◽

Ice Shelves ◽

Ice Velocity

The Greenland ice sheet is a major contributor to sea level rise, adding on average 0.47 ± 0.23 mm year − 1 to global mean sea level between 1991 and 2015. The cryosphere as a whole has contributed around 45% of observed global sea level rise since 1993. Understanding the present-day state of the Greenland ice sheet is therefore vital for understanding the processes controlling the modern-day rates of sea level change and for making projections of sea level rise into the future. Here, we provide an overview of the current state of the mass budget of Greenland based on a diverse range of remote sensing observations to produce the essential climate variables (ECVs) of ice velocity, surface elevation change, grounding line location, calving front location, and gravimetric mass balance as well as numerical modelling that together build a consistent picture of a shrinking ice sheet. We also combine these observations with output from a regional climate model and from an ice sheet model to gain insight into existing biases in ice sheet dynamics and surface mass balance processes. Observations show surface lowering across virtually all regions of the ice sheet and at some locations up to −2.65 m year − 1 between 1995 and 2017 based on radar altimetry analysis. In addition, calving fronts at 28 study sites, representing a sample of typical glaciers, have retreated all around Greenland since the 1990s and in only two out of 28 study locations have they remained stable. During the same period, two of five floating ice shelves have collapsed while the locations of grounding lines at the remaining three floating ice shelves have remained stable over the observation period. In a detailed case study with a fracture model at Petermann glacier, we demonstrate the potential sensitivity of these floating ice shelves to future warming. GRACE gravimetrically-derived mass balance (GMB) data shows that overall Greenland has lost 255 ± 15 Gt year − 1 of ice over the period 2003 to 2016, consistent with that shown by IMBIE and a marked increase compared to a rate of loss of 83 ± 63 Gt year − 1 in the 1993–2003 period. Regional climate model and ice sheet model simulations show that surface mass processes dominate the Greenland ice sheet mass budget over most of the interior. However, in areas of high ice velocity there is a significant contribution to mass loss by ice dynamical processes. Marked differences between models and observations indicate that not all processes are captured accurately within models, indicating areas for future research.

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An Integrated View of Greenland Ice Sheet Mass Changes Based on Models and Satellite Observations

10.20944/preprints201810.0337.v1 ◽

2018 ◽

Cited By ~ 1

Author(s):

Ruth Mottram ◽

Sebastian B. Simonsen ◽

Synne Høyer Svendsen ◽

Valentina R. Barletta ◽

Louise Sandberg Sørensen ◽

...

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Climate Model ◽

Regional Climate ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Future Climate Change ◽

Future Research ◽

Surface Mass ◽

Mass Budget

The Greenland ice sheet is a major contributor to sea level rise, adding an estimated 0.47 +/− 0.23 mm/yr to global mean sea level between 1991 and 2015 (van den Broeke et al., 2016). Making sea level rise projections for the future and understanding the processes controlling current observed rates of sea level rise are crucially dependent on understanding the present-day state of the ice sheet. Here, we provide an overview of the current state of the mass budget of Greenland based on satellite gravimetry and remote sensing observations of surface elevation change, ice sheet velocity and calving front positions. We also combine these essential climate variables with a regional climate model (RCM) output from an ice sheet model (ISM) to gain insight into poorly understood ice sheet dynamical and surface mass processes. On average from 1992 to 2017 the ice sheet in some locations has lost up −2.65 m/yr in elevation based on ESA Radar altimetry analysis. Calving fronts have retreated all around Greenland since the 1990s and in only two out of 28 study locations have they remained stable. The locations of grounding lines at 5 key glaciers with floating ice tongues have remained stable over the observation period. However a detailed case study at Petermann glacier with an ice fracture model shows the sensitivity of these floating ice shelves to future climate change. GRACE gravimetric mass balance (GMB) data allows us to tie together disparate lines of evidence showing that Greenland has lost about 265 +/− 25 Gt/yr of ice over the period 2002 to 2015. RCM and ISM simulations show that surface mass processes dominate the overall Greenland ice sheet mass budget except for areas of fast ice sheet flow but marked differences between models and between models and observations indicate that not all processes are captured accurately, indicating areas of greater uncertainty and directions of future research for future sea level rise projections.

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On the recent contribution of the Greenland ice sheet to sea level change

The Cryosphere ◽

10.5194/tc-10-1933-2016 ◽

2016 ◽

Vol 10 (5) ◽

pp. 1933-1946 ◽

Cited By ~ 212

Author(s):

Michiel R. van den Broeke ◽

Ellyn M. Enderlin ◽

Ian M. Howat ◽

Peter Kuipers Munneke ◽

Brice P. Y. Noël ◽

...

Keyword(s):

Mass Loss ◽

Mass Balance ◽

Sea Level ◽

Pore Space ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Sea Level Change ◽

Recent Contribution ◽

Surface Mass ◽

Level Change

Abstract. We assess the recent contribution of the Greenland ice sheet (GrIS) to sea level change. We use the mass budget method, which quantifies ice sheet mass balance (MB) as the difference between surface mass balance (SMB) and solid ice discharge across the grounding line (D). A comparison with independent gravity change observations from GRACE shows good agreement for the overlapping period 2002–2015, giving confidence in the partitioning of recent GrIS mass changes. The estimated 1995 value of D and the 1958–1995 average value of SMB are similar at 411 and 418 Gt yr−1, respectively, suggesting that ice flow in the mid-1990s was well adjusted to the average annual mass input, reminiscent of an ice sheet in approximate balance. Starting in the early to mid-1990s, SMB decreased while D increased, leading to quasi-persistent negative MB. About 60 % of the associated mass loss since 1991 is caused by changes in SMB and the remainder by D. The decrease in SMB is fully driven by an increase in surface melt and subsequent meltwater runoff, which is slightly compensated by a small ( <  3 %) increase in snowfall. The excess runoff originates from low-lying ( <  2000 m a.s.l.) parts of the ice sheet; higher up, increased refreezing prevents runoff of meltwater from occurring, at the expense of increased firn temperatures and depleted pore space. With a 1991–2015 average annual mass loss of ∼  0.47 ± 0.23 mm sea level equivalent (SLE) and a peak contribution of 1.2 mm SLE in 2012, the GrIS has recently become a major source of global mean sea level rise.

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On the recent contribution of the Greenland ice sheet to sea level change

10.5194/tc-2016-123 ◽

2016 ◽

Cited By ~ 2

Author(s):

Michiel van den Broeke ◽

Ellyn Enderlin ◽

Ian Howat ◽

Peter Kuipers Munneke ◽

Brice Noël ◽

...

Keyword(s):

Mass Loss ◽

Mass Balance ◽

Sea Level ◽

Pore Space ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Sea Level Change ◽

Recent Contribution ◽

Surface Mass ◽

Level Change

Abstract. We assess the recent contribution of the Greenland ice sheet (GrIS) to sea level change. We use the mass budget method, which quantifies ice sheet mass balance (MB) as the difference between surface mass balance (SMB) and solid ice discharge across the grounding line (D). A comparison with independent gravity change observations from GRACE shows good agreement for the overlapping period 2002–2015, giving confidence in the partitioning of recent GrIS mass changes. The estimated 1995 value of D and the 1958–1995 average value of SMB are similar at 411 and 418 Gt yr-1, respectively, suggesting that ice flow in the mid-nineties was well adjusted to the average annual mass input, reminiscent of an ice sheet in approximate balance. Starting in the early to mid-1990's, SMB decreased while D increased, leading to quasi-persistent negative MB. About 60 % of the associated mass loss since 1991 is caused by changes in SMB and the remainder by D. The decrease in SMB is fully driven by an increase in surface melt and subsequent meltwater runoff, which is slightly compensated by a small (< 3 %) increase in snowfall. The excess runoff originates from low-lying (< 2000 m a.s.l.) parts of the ice sheet; higher up, increased refreezing prevents runoff of meltwater to occur, at the expense of increased firn temperatures and depleted pore space. With a 1991–2015 average annual mass loss of ~ 0.47 ± 0.23 mm sea level equivalent (SLE) and a peak contribution of 1.2 mm SLE in 2012, the GrIS has recently become a major source of global mean sea level rise.

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Contrasting regional variability of buried meltwater extent over two years across the Greenland Ice Sheet

10.5194/tc-2021-3 ◽

2021 ◽

Author(s):

Devon Dunmire ◽

Alison F. Banwell ◽

Jan T. M. Lenaerts ◽

Rajashree Tri Datta

Keyword(s):

Mass Loss ◽

Climate Model ◽

Regional Climate ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Late Summer ◽

Shortwave Radiation ◽

Regional Variability ◽

Meltwater Runoff ◽

Lake Formation

Abstract. The Greenland Ice Sheet (GrIS) rapid mass loss is primarily driven by an increase in meltwater runoff, which highlights the importance of understanding the formation, evolution and impact of meltwater features on the ice sheet. Buried lakes are meltwater features that contain liquid water and exist under layers of snow, firn, and/or ice. These lakes are invisible in optical imagery, challenging the analysis of their evolution and implication for larger GrIS dynamics and mass change. Here, we present a method that uses a convolutional neural network, a deep learning method, to automatically detect buried lakes across the GrIS. For the years 2018 and 2019, we compare total areal extent of both buried and surface lakes across six regions, and use a regional climate model to explain the spatial and temporal differences. We find that the total buried lake extent after the 2019 melt season is 56 % larger than after the 2018 melt season across the entire ice sheet. Northern Greenland observes the largest increase in buried lake extent after the 2019 melt season, which we attribute to late-summer surface melt and high autumn temperatures. We also provide evidence that different processes are responsible for buried lake formation in different regions of the ice sheet. For example, in western Greenland, buried lakes often appear on the surface during the previous melt season, indicating that these features form when surface lakes partially freeze and become insulated as snowfall buries them. In contrast, in southeast Greenland, most buried lakes never appear on the surface, signifying that these features may form due to subsurface penetration of shortwave radiation and/or downward percolation of meltwater. This study helps to provide additional perspective on the potential role of meltwater on GrIS dynamics and mass loss.

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Recent retreat of Greenland's marine terminating glaciers has a lasting impact on velocity and mass loss during the 21st Century

10.5194/egusphere-egu21-9908 ◽

2021 ◽

Author(s):

Isabel Nias ◽

Sophie Nowicki ◽

Denis Felikson

Keyword(s):

Mass Loss ◽

Sea Level Rise ◽

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Initial State ◽

Ice Dynamics ◽

Total Contribution ◽

Surface Mass ◽

Glacier Terminus

Mass loss from the Greenland Ice Sheet (GrIS) can be partitioned between surface mass balance (SMB) and discharge due to ice dynamics through its marine-terminating outlet glaciers. A perturbation to a glacier terminus (e.g. a calving event) results in an instantaneous response in velocity and mass loss, but also a diffusive response due to the evolution of ice thickness over time. This diffusive response means the total impact of a retreat event can take decades to be fully realised. Here we model the committed response of the GrIS to recent observed changes in terminus position, neglecting any future climate perturbations. Our simulations quantify the sea level contribution that is locked in due to the slow dynamic response of the ice. Using the Ice Sheet System Model (ISSM), we run forward simulations starting from an initial state representative of the 2007 ice sheet. We apply perturbations to the marine-terminating glacier termini that represent recent observed changes, and simulate the response over the 21st Century, holding the climate forcing constant. The sensitivity of the ice sheet response to model parameter uncertainty is explored with in an ensemble framework, and GRACE data is used to constrain the results. We find that terminus retreat observed between 2007 and 2015 results in approximately 6 mm of sea level rise by 2100, with retreat having a lasting impact on velocity and mass loss. Our results complement the ISMIP6 projections, which report the ice sheet response to future forcing, excluding the background committed response. In this way, we can obtain estimates of Greenland&#8217;s total contribution to sea level rise by 2100.

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The role of an interactive Greenland Ice Sheet in abrupt 4xCO2 forcing experiments.

10.5194/egusphere-egu21-13327 ◽

2021 ◽

Author(s):

Victoria Lee ◽

Robin S. Smith ◽

Antony J. Payne

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Ocean Model ◽

Surface Melting ◽

Surface Mass ◽

Meltwater Runoff ◽

Iceberg Calving

We compare the response of a coupled atmosphere-ocean-Greenland Ice Sheet (GrIS) model forced with an abrupt quadrupling of CO2 from greenhouse gas concentrations in 1970 with the response of the atmosphere-ocean model with a static GrIS . The model, UKESM1.ice.N96.ORCA1, consists of HadGEM GC3.1 coupled to the BISICLES ice sheet model with mean annual surface mass balance (SMB) passed to BISICLES and orography and cumulated iceberg flux passed back to the atmosphere and ocean, respectively, at the end of each year. The differences in the surface temperature and atmospheric fields between the two experiments are confined to Greenland, with no discernible global effects from the evolving orography. The volume of the GrIS decreases by 15 % in 330 years. The surface height decreases the most (over 800m in 330 years) in southwest&#160;GrIS due to surface melting enhanced by feedbacks between elevation, air temperature and albedo. The input of freshwater to the ocean from Greenland is enhanced due to increased meltwater runoff, but the flux from melting icebergs decays to zero as calving from glaciers declines. The resulting sea level rise is dominated by SMB, where the equivalent sea level rise is 1179 mm (5.0 mm/yr) for the static GrIS and 1120 mm (4.4 mm/yr) for the interactive ice sheet at 2300.&#160; There is less sea level rise in the interactive GrIS experiment, even though more mass is lost through surface melting, because the amount lost through iceberg calving decreases as the grounding line of marine-terminating glaciers retreat inland whereas calving in the static experiment is constant.&#160;&#160;&#160;&#160;

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A regional atmospheric warming threshold for irreversible Greenland ice sheet mass loss

10.5194/egusphere-egu2020-19032 ◽

2020 ◽

Author(s):

Michiel van den Broeke ◽

Brice Noël ◽

Leo van Kampenhout ◽

Willem-Jan van de Berg

Keyword(s):

Mass Loss ◽

Mass Balance ◽

Sea Level Rise ◽

Sea Level ◽

Climate Models ◽

Climate Model ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Global Climate Models ◽

The Future

The mass balance of the Greenland ice sheet (GrIS, units Gt per year) equals the surface mass balance (SMB) minus solid ice discharge across the grounding line. As the latter is definite positive, an important threshold for irreversible GrIS mass loss occurs when long-term average SMB becomes negative. For this to happen, runoff (mainly meltwater, some rain) must exceed mass accumulation (mainly snowfall minus sublimation). Even for a single year, this threshold has not been passed since at least 1958, the first year with reliable estimates of SMB components, although recent years with warm summers (e.g. 2012 and 2019) came close. Simply extrapolating the recent (1991-present) negative SMB trend into the future suggests that the SMB = 0 threshold could be reached before ~2040, but such predictions are extremely uncertain given the very large interannual SMB variability, the relative brevity of the time series and the uncertainty in future warming. In this study we use a cascade of models, extensively evaluated with in-situ and remotely sensed (GRACE) SMB observations, to better constrain the future regional warming threshold for the 5-year average GrIS SMB to become negative. To this end, a 1950-2100 climate change run with the global model CESM2 (app. 100 km resolution) was dynamically downscaled using the regional climate model RACMO2 (app. 11 km), which in turn was statistically downscaled to 1 km resolution. The result is a threshold regional Greenland warming of close to 4 degrees. We then use a range of CMIP5 and CMIP6 global climate models to translate the regional value into a global warming threshold for various warming scenarios, including its timing this century. We find substantial differences, ranging from stabilization before the threshold is reached in the RCP/SSP2.6 scenarios with a limited but still significant sea-level rise contribution (< 5 cm by 2100) to an imminent crossing of the warming threshold for the RCP/SSP8.5 scenarios with substantial and ever-growing contributions to sea level rise (> 10 cm by 2100). These results stress the need for strong mitigation to avoid irreversible GrIS mass loss. We finish by discussing the caveats and uncertainties of our approach.

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The effect of overshooting 1.5 °C global warming on the mass loss of the Greenland ice sheet

Earth System Dynamics ◽

10.5194/esd-9-1169-2018 ◽

2018 ◽

Vol 9 (4) ◽

pp. 1169-1189 ◽

Cited By ~ 6

Author(s):

Martin Rückamp ◽

Ulrike Falk ◽

Katja Frieler ◽

Stefan Lange ◽

Angelika Humbert

Keyword(s):

Global Warming ◽

Mass Loss ◽

Mass Balance ◽

Sea Level Rise ◽

Sea Level ◽

21St Century ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Mass Balance ◽

Surface Mass

Abstract. Sea-level rise associated with changing climate is expected to pose a major challenge for societies. Based on the efforts of COP21 to limit global warming to 2.0 ∘C or even 1.5 ∘C by the end of the 21st century (Paris Agreement), we simulate the future contribution of the Greenland ice sheet (GrIS) to sea-level change under the low emission Representative Concentration Pathway (RCP) 2.6 scenario. The Ice Sheet System Model (ISSM) with higher-order approximation is used and initialized with a hybrid approach of spin-up and data assimilation. For three general circulation models (GCMs: HadGEM2-ES, IPSL-CM5A-LR, MIROC5) the projections are conducted up to 2300 with forcing fields for surface mass balance (SMB) and ice surface temperature (Ts) computed by the surface energy balance model of intermediate complexity (SEMIC). The projected sea-level rise ranges between 21–38 mm by 2100 and 36–85 mm by 2300. According to the three GCMs used, global warming will exceed 1.5 ∘C early in the 21st century. The RCP2.6 peak and decline scenario is therefore manually adjusted in another set of experiments to suppress the 1.5 ∘C overshooting effect. These scenarios show a sea-level contribution that is on average about 38 % and 31 % less by 2100 and 2300, respectively. For some experiments, the rate of mass loss in the 23rd century does not exclude a stable ice sheet in the future. This is due to a spatially integrated SMB that remains positive and reaches values similar to the present day in the latter half of the simulation period. Although the mean SMB is reduced in the warmer climate, a future steady-state ice sheet with lower surface elevation and hence volume might be possible. Our results indicate that uncertainties in the projections stem from the underlying GCM climate data used to calculate the surface mass balance. However, the RCP2.6 scenario will lead to significant changes in the GrIS, including elevation changes of up to 100 m. The sea-level contribution estimated in this study may serve as a lower bound for the RCP2.6 scenario, as the currently observed sea-level rise is not reached in any of the experiments; this is attributed to processes (e.g. ocean forcing) not yet represented by the model, but proven to play a major role in GrIS mass loss.

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