Anomalous acceleration of mass loss in the Greenland ice sheet drainage basins and its contribution to the sea level fingerprints during 2010–2012
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Abstract. The sea level rise contributed from ice sheet melting has been accelerating due to global warming. Continuous melting of the Greenland ice sheet (GrIS) is a major contributor to sea level rise, which impacts directly on the surface mass balance and the instantaneous elastic response of the solid Earth. To study the sea level fingerprints (SLF) caused by the anomalous acceleration of the mass loss in GrIS can help us to understand drivers of sea level changes due to global warming and the frequently abnormal climate events. In this study, we focus on the anomalous acceleration of the mass loss in GrIS at the drainage basins from 2010 to 2012 and on its contributions to SLF and relative sea level (RSL) changes based on self-attraction and loading effects. Using GRACE monthly gravity fields and surface mass balance (SMB) data spanning 13 years between 2003 and 2015, the spatial and temporal distribution of the ice sheet balance in Greenland is estimated by mascons fitting based on six extended drainage basins and matrix scaling factors. Then the SLF spatial variations are computed by solving the sea level equation. Our results indicate that the total ice sheet mass loss is contributed from few regions only in Greenland, i.e., from the northwest, central west, southwestern and southeastern parts. Especially along the north-west coast and the south-east coast, ice was melting significantly during 2010–2012. The total mass loss rates during 2003–2015 are −288±7 Gt/yr and −275±1 Gt/yr as derived from scaled GRACE data and SMB respectively; and the magnitude of the trend increased to −456±30 Gt/yr and to −464±38 Gt/yr correspondingly over the period 2010–2012. The residuals obtained by GRACE after removing SMB show a good agreement with the surface elevation change rates derived from pervious ICESat results, which reflect a contribution from glacial dynamics to the total ice mass changes. Melting of GrIS results in decreased RSL in Scandinavia and North Europe, up to about −0.6 cm/yr. The far-field peak increase is less dependent on the precise pattern of self-attraction and loading; and the average global RSL was raised by 0.07 cm/yr only. Greenland contributes about 31 % of the total terrestrial water storage transferring to the sea level rise from 2003 to 2015. We also found that variations of the GrIS contribution to sea level have an opposite V shape (i.e., from rising to falling) during 2010–2012, while a clear global mean sea level drop also took place (i.e., from falling to rising).Keywords:
Greenland ice sheet
Future sea level
Glacier mass balance
The Programme for Monitoring of the Greenland Ice Sheet (PROMICE) has measured ice-sheet elevation and thickness via repeat airborne surveys circumscribing the ice sheet at an average elevation of 1708 ± 5 m (Sørensen et al. 2018). We refer to this 5415 km survey as the ‘PROMICE perimeter’. Here, we assess ice-sheet mass balance following the input-output approach of Andersen et al. (2015). We estimate ice-sheet output, or the ice discharge across the ice-sheet grounding line, by applying downstream corrections to the ice flux across the PROMICE perimeter. We subtract this ice discharge from ice-sheet input, or the area-integrated, ice sheet surface mass balance, estimated by a regional climate model. While Andersen et al. (2015) assessed ice-sheet mass balance in 2007 and 2011, this updated input-output assessment now estimates the annual sea-level rise contribution from eighteen sub-sectors of the Greenland ice sheet over the 1995–2015 period.
Greenland ice sheet
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In the 21st century, polar land ice melting became one of the driving factors of global sea level rise, which is discussed widely by the media and the public. Although the fact of the shrinking ice caps and accompanying changes in the sea level is established, the actual amount of polar ice melting still needs to be quantified in separate regions. Sitting on top of bedrock, the Greenland ice sheet (GrIS) is the second largest ice sheet on Earth. With traditional glaciological methods the change of the Greenland ice sheet is difficult to measure directly, however with the GRACE (Gravity Recovery and Climate Experiment) satellite system the mass changes can be measured directly. There are several sub-drainage areas within the Greenland Ice Sheet. Some of the subsystems may contribute differently to the overall mass changes of GrIS. For instance, while the mass loss in the GrIS ablation zone is enhanced during the last decades, the central high altitude areas experienced increased mass accumulation (Krabill et al., 2000, Thomas et al., 2001, Colgan et al., 2015, Xu et al., 2016). It is important to quantify the regional mass changes because it gives us insight what is going on beyond the realization that the GrIS is shrinking...
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Abstract. The Greenland ice sheet contributes increasingly to global sea level rise. Its history during past warm intervals is a valuable reference for future sea level projections. We present ice sheet simulations for the Eemian interglacial period (∼130 000 to 115 000 years ago), a period with warmer-than-present summer climate over Greenland. The evolution of the Eemian Greenland ice sheet is simulated with a 3-D higher-order ice sheet model, forced with a surface mass balance derived from regional climate simulations. Sensitivity experiments with various surface mass balances, basal friction, and ice flow approximations are discussed. The surface mass balance forcing is identified as the controlling factor setting the minimum in Eemian ice volume, emphasizing the importance of a reliable surface mass balance model. Furthermore, the results indicate that the surface mass balance forcing is more important than the representation of ice flow for simulating the large-scale ice sheet evolution. This implies that modeling of the future contribution of the Greenland ice sheet to sea level rise highly depends on an accurate surface mass balance.
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A more accurate assessment of the contemporary evolution of the Greenland ice sheet and its major drainage basins requires a close interaction between observational data and modeling. The main challenge when interpreting satellite and observational data is to separate the ice mass contribution from the contribution of postglacial isostatic rebound, to separate ice-sheet dynamic changes from interannual surface mass balance changes, and to separate long-term ice-dynamic changes from short-term flow fluctuations. Here we report from recent progress towards these goals within the DFG SPP 1257 project 'Assessing the current evolution of the Greenland ice sheet' from studies combining observational data with glaciological modeling. This comprises studies to reconstruct the surface mass balance of the Greenland ice sheet between 1866 and 2006, optical satellite data from ASTER to obtain surface velocities, modelled balance velocities, and simulations with a three-dimensional thermomechanical ice-sheet model. In combination with GRACE data, these studies are expected to contribute to an improved estimate of the present-day contribution of the Greenland ice sheet to global sea-level change and a better understanding of the various contributions to current ice mass changes and their associated uncertainties.
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It is often suggested that the Greenland ice sheet is a relict ice mass left over from the Ice Ages. In that case the ice sheet would not regrow to its present configuration under present-day climatic conditions if it were to melt away in a warmer climate, except for some residual mountain glaciers in the east. The most cited reasons for this alleged hysteresis are the height-mass balance feedback and the albedo-temperature feedback. Future climatic warming is expected to cause more melting on the Greenland ice sheet, resulting in an overall shrinking of the ice sheet. For a warming in excess of some 3°C, models predict that the surface mass balance will become negative such that the ice sheet can no longer be sustained and starts to gradually disintegrate. An intriguing question is whether there exist points-of-no-return once such a disintegration has set in beyond which complete removal of the ice sheet were to become irreversible, even if climatic conditions were to revert to present-day conditions. To investigate these issues, we have inserted a 3-D thermomechanical model of the Greenland ice sheet in the HadCM3 atmosphere-ocean general circulation model. Under constant 4xCO2 conditions, the Greenland ice sheet is found to disintegrate to less than 5% of its current volume within 3000 years. At different moments in time, we interrupt the melting process by inserting a 1xCO2 climate and let the ice sheet evolve to a new steady state. We also investigate the end member in which the ice sheet is entirely removed and bedrock topography has been uplifted. The original contribution of our approach is that precipitation and temperature patterns are allowed to fully interact with the changing topography and surface type of the Greenland continent.
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The Greenland ice sheet's mass loss is increasing and so is its impact to the climate system. Yet, Earth System models mostly keep ice sheets at a constant extent or treat interactions with the ice sheets fairly simple.Here, we present the first simulations of NorESM2 coupled to the ice sheet model CISM over Greenland. We compare NorESM2 simulations from 1850 to 2300 with and without an evolving ice sheet over Greenland based on the ssp585 scenario and its extension to 2300. Ocean and atmosphere horizontal resolution are on 1deg, while the coupled ice sheet module CISM is running on 4km. The coupling setup is based on CESM2. Ice extent and elevation are provided to the atmosphere every 5years and the land model every year. Whereas the ice sheet receives updated surface mass balance every year.We show the evolution of the Greenland ice sheet and changes in atmosphere, ocean and sea ice.Overall global mean surface air temperatures (SAT) change from 14°C to 24°C by 2300 with the steepest increase between 2070-2200.Over the Southern ocean and Antarctica, SAT are increasing by 10°C, while over the Northern hemisphere we see a change of 15-28°C by 2300. At the end of the simulations (year 2300), SAT over Greenland are 6°C warmer when including an evolving ice sheet. In contrast, the ocean surrounding Greenland shows SAT that are 2°C colder in the coupled system, compared to the simulation with a fixed Greenland ice sheet. Sea surface temperatures show the same ~2°C difference around Greenland in coupled and uncoupled simulation. The overall change in sea surface temperatures is 12°C.Minimum and maximum sea ice extent differs only slightly with and without the coupling, indicating that the overall warming seems to dictate speed of the sea ice retreat.
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The Greenland Ice Sheet is thinning at an accelerating pace and the ice sheet's contribution to sea‐level rise has doubled in less than a decade. New data show rapid and widespread changes in the behaviour of the ice sheet, particularly along the coastal margin. These changes coincide with a decade of sustained Arctic warming of up to 3 °C. Decay of the Greenland Ice Sheet in response to global warming will not only be governed by increased surface melting during longer and warmer summers but also by a speed‐up of coastal glaciers that drain the interior ice sheet. A precise estimate of sea‐level rise in the twenty‐first century relies on improved theoretical treatment of these glaciers in computer models.
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The Greenland Ice Sheet, which extends south of the Arctic Circle, is vulnerable to melt in a warming climate. Complete melt of the ice sheet would raise global sea level by about 7 meters. Prediction of how the ice sheet will react to climate change requires inputs with a high degree of spatial resolution and improved simulation of the ice-dynamical responses to evolving surface mass balance. No Greenland Ice Sheet model has yet met these requirements.A three-dimensional thermo-mechanical ice sheet model of Greenland was enhanced to address these challenges. First, it was modified to accept high-resolution surface mass balance forcings. Second, a parameterization for basal drainage (of the sort responsible for sustaining the Northeast Greenland Ice Stream) was incorporated into the model. The enhanced model was used to investigate the century to millennial-scale evolution of the Greenland Ice Sheet in response to persistent climate trends. During initial experiments, the mechanism of flow in the outlet glaciers was assumed to be independent of climate change, and the outlet glaciers' dominant behavior was to counteract changes in surface mass balance. Around much of the ice sheet, warming resulted in calving front retreat and reduction of total ice sheet discharge. Observations show, however, that the character of outlet glacier flow changes with the climate. The ice sheet model was further developed to simulate observed dynamical responses of Greenland's outlet glaciers. A phenomenological description of the relation between outlet glacier discharge and surface mass balance was calibrated against recent observations. This model was used to investigate the ice sheet's response to a hypothesized 21st century warming trend. Enhanced discharge accounted for a 60% increase in Greenland mass loss, resulting in a net sea level increment of 7.3 cm by year 2100. By this time, the average surface mass balance had become negative, and widespread marginal thinning had caused 30% of historically active calving fronts to retreat. Mass losses persisted throughout the century due to flow of dynamically responsive outlets capable of sustaining high calving rates. Thinning in these areas propagated upstream into higher elevation catchments. Large drainage basins with low-lying outlets, especially those along Greenland's west coast and those fed by the Northeast Greenland Ice Stream, were most susceptible to dynamic mass loss in the 21st century
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Accurate simulation of ice-sheet surface mass balance requires higher spatial resolution than is afforded by typical atmosphere-ocean general circulation models (AOGCMs), owing, in particular, to the need to resolve the narrow and steep margins where the majority of precipitation and ablation occurs. We have developed a method for calculating mass-balance changes by combining ice-sheet average time-series from AOGCM projections for future centuries, both with information from high-resolution climate models run for short periods and with a 20km ice-sheet mass-balance model. Antarctica contributes negatively to sea level on account of increased accumulation, while Greenland contributes positively because ablation increases more rapidly. The uncertainty in the results is about 20% for Antarctica and 35% for Greenland. Changes in ice-sheet topography and dynamics are not included, but we discuss their possible effects. For an annual- and area-average warming exceeding 4.5+/-0.9K in Greenland and 3.1+/-0.8K in the global average, the net surface mass balance of the Greenland ice sheet becomes negative, in which case it is likely that the ice sheet would eventually be eliminated, raising global-average sea level by 7m.
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