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See detailHigh-resolution modelling of the Antarctic surface mass balance, application for the twentieth, twenty first and twenty second centuries
Agosta, Cécile ULg; Favier, Vincent; Krinner, Gerhard et al

in Climate Dynamics (2013), 41(11-12), 3247-3260

About 75% of the Antarctic surface mass gain occurs over areas below 2000 m asl, which cover 40% of the grounded ice-sheet. As the topography is complex in many of these regions, SMB modelling is highly ... [more ▼]

About 75% of the Antarctic surface mass gain occurs over areas below 2000 m asl, which cover 40% of the grounded ice-sheet. As the topography is complex in many of these regions, SMB modelling is highly dependent on resolution, and studying the impact of Antarctica on the future rise in sea level requires physical approaches. We have developed a low time consuming, physical downscaling model for high-resolution (15 km) long-term surface mass balance (SMB) projections. Here, we present results of this model, called SMHiL (surface mass balance high-resolution downscaling), which was forced with the LMDZ4 atmospheric general circulation model to assess SMB variation in the 21st and the 22nd centuries under two different scenarios. The higher resolution of SMHiL better reproduces the geographical patterns of SMB and increase significantly the averaged SMB over the grounded ice-sheet for the end of the 20th century. A comparison with more than 3200 quality-controlled field data shows that LMDZ4 and SMHiL compare the observed values equally well. Nevertheless, field data below 2000 m asl are too scarce to efficiency show the interest of SMHiL and measuring the SMB in these undocumented areas should be then a future scientific priority. Our results suggest that running LMDZ4 at a finer resolution (15km) may give a future increase in SMB in Antarctica about 30% higher than by using its standard resolution (60 km) due to higher increase in precipitation in the coastal areas at 15 km. However, a part (~ 15%) of these discrepancies could be an artefact from SMHiL since it neglects the foehn effect and then likely overestimates the precipitation increase. Future changes in the Antarctic SMB at low elevations will result from the conflict between higher snow accumulation and runoff. For this reason, developing downscaling models is crucial to represent processes in sufficient detail and correctly model the SMB in the coastal areas. [less ▲]

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See detailA comparison of supraglacial lake observations derived from MODIS imagery at the western margin of the Greenland ice sheet
Leeson, A.; Shepherd, A.; Sundal, A. et al

in Journal of Glaciology (2013), 59(208), 1179-1188

Supraglacial lakes (SGLs) affect the dynamics of the Greenland ice sheet by storing runoff and draining episodically. We investigate the evolution of SGLs as reported in three datasets, each based on ... [more ▼]

Supraglacial lakes (SGLs) affect the dynamics of the Greenland ice sheet by storing runoff and draining episodically. We investigate the evolution of SGLs as reported in three datasets, each based on automated classification of satellite imagery. Although the datasets span the period 2001–10, there are differences in temporal sampling, and only the years 2005–07 are common. By subsampling the most populous dataset, we recommend a sampling frequency of one image per 6.5 days in order to minimize uncertainty associated with poor temporal sampling. When compared with manual classification of satellite imagery, all three datasets are found to omit a sizeable (29, 48 and 41%) fraction of lakes and are estimated to document the average size of SGLs to within 0.78, 0.48 and 0.95 km2 . We combine the datasets using a hierarchical scheme, producing a single, optimized, dataset. This combined record reports up to 67% more lakes than a single dataset. During 2005–07, the rate of SGL growth tends to follow the rate at which runoff increases in each year. In 2007, lakes drain earlier than in 2005 and 2006 and remain absent despite continued runoff. This suggests that lakes continue to act as open surface–bed conduits following drainage. [less ▲]

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See detailContribution future du bilan de masse de surface Antarctique au niveau des mers par la modélisation régionale
Agosta, Cécile ULg; Fettweis, Xavier ULg; Gallée, Hubert

Scientific conference (2013, October 15)

Le bilan de masse de surface (BMS) Antarctique est encore mal connu, bien qu’on sache qu’il contribue de façon significative à l’évolution actuelle du niveau des mers et que sa contribution soit ... [more ▼]

Le bilan de masse de surface (BMS) Antarctique est encore mal connu, bien qu’on sache qu’il contribue de façon significative à l’évolution actuelle du niveau des mers et que sa contribution soit supposée s’intensifier au cours des prochains siècles. Outre son effet direct sur le niveau des mers, le BMS est également un champs de forçage primordial pour les modèles de calotte. Enfin, alors qu’il existe des mesures directes de l’écoulement de la glace vers l’océan et des variations de masse totales (surface+écoulement) de la calotte, il n’existe pas de mesure directe du bilan de masse de surface à l’échelle du continent. La climatologie actuelle du BMS Antarctique est donc estimée principalement à partir de résultats de modélisation. Par ailleurs, le BMS est le résultat de processus complexes. Afin de le modéliser correctement, il est nécessaire de bien représenter la circulation atmosphérique et les processus physiques spécifiques aux régions polaires. Or les modèles de circulation générale présentent une résolution trop grossière et une physique peu adaptée pour modéliser correctement ces processus. Nous présentons ici des résultats de simulations réalisées le modèle atmosphérique régional MAR, qui fait référence pour la modélisation de l’atmosphère et des processus de surface en région polaire, à une résolution de 50 km pour la fin du 20ème et du 21ème siècle. Nous connaissons la qualité du modèle MAR, cependant, comme tout modèle atmosphérique régional, ses performances sont fortement liées à la qualité des forçages aux limites provenant des Modèles de Circulation Générale (MCG). Nous avons donc sélectionné le MCG le plus apte à simuler le climat présent parmi la nouvelle génération des MCGs provenant de la base de données CMIP5 (http://cmip- pcmdi.llnl.gov/cmip5/), qui seront utilisés dans le prochain rapport du GIEC. Cela est une étape cruciale car les MCGs ne représentant pas correctement le climat présent ne pourront pas donner de résultats probants pour les simulations futures. Nous nous penchons enfin sur l’épineux problème de l’évaluation du BMS modélisé à partir de données de terrain. En effet, un effort important a été réalisé pour répertorier les données de BMS de qualité en Antarctique, cependant nous montrons que ces données ne permettent pas d’évaluer les performances des modèles de façon suffisamment contraignante. L’utilisation d’autres types de données, satellites ou aéroportées par exemple, semble nécessaire, ce qui constitue un volet important de mes recherches en cours. [less ▲]

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See detailEnhanced basal lubrication and the contribution of the Greenland ice sheet to future sea-level rise
Shannon, S.; Payne, A.; Bartholomew, I. et al

in Proceedings of the National Academy of Sciences of the United States of America (2013), 110(49), 19719-19724

We assess the effect of enhanced basal sliding on the flow and mass budget of the Greenland ice sheet, using a newly developed parameterization of the relation between meltwater runoff and ice flow. A ... [more ▼]

We assess the effect of enhanced basal sliding on the flow and mass budget of the Greenland ice sheet, using a newly developed parameterization of the relation between meltwater runoff and ice flow. A wide range of observations suggest that water generated by melt at the surface of the ice sheet reaches its bed by both fracture and drainage through moulins. Once at the bed, this water is likely to affect lubrication, although current observations are insufficient to determine whether changes in subglacial hydraulics will limit the potential for the speedup of flow. An uncertainty analysis based on our best-fit parameterization admits both possibilities: continuously increasing or bounded lubrication. We apply the parameterization to four higher-order ice-sheet models in a series of experiments forced by changes in both lubrication and surface mass budget and determine the additional mass loss brought about by lubrication in comparison with experiments forced only by changes in surface mass balance. We use forcing from a regional climate model, itself forced by output from the European Centre Hamburg Model (ECHAM5) global climate model run under scenario A1B. Although changes in lubrication generate widespread effects on the flow and form of the ice sheet, they do not affect substantial net mass loss; increase in the ice sheet’s contribution to sea-level rise from basal lubrication is projected by all models to be no more than 5% of the contribution from surface mass budget forcing alone. [less ▲]

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See detailSensitivity of Greenland ice sheet projections to model formulations
Goelzer, H.; Huybrechts, P.; Furst, J. et al

in Journal of Glaciology (2013), 59(216), 733749

Physically based projections of the Greenland ice sheet contribution to future sea-level change are subject to uncertainties of the atmospheric and oceanic climatic forcing and to the formulations within ... [more ▼]

Physically based projections of the Greenland ice sheet contribution to future sea-level change are subject to uncertainties of the atmospheric and oceanic climatic forcing and to the formulations within the ice flow model itself. Here a higher-order, three-dimensional thermomechanical ice flow model is used, initialized to the present-day geometry. The forcing comes from a high-resolution regional climate model and from a flowline model applied to four individual marine-terminated glaciers, and results are subsequently extended to the entire ice sheet. The experiments span the next 200 years and consider climate scenario SRES A1B. The surface mass-balance (SMB) scheme is taken either from a regional climate model or from a positive-degree-day (PDD) model using temperature and precipitation anomalies from the underlying climate models. Our model results show that outlet glacier dynamics only account for 6–18% of the sea-level contribution after 200 years, confirming earlier findings that stress the dominant effect of SMB changes. Furthermore, interaction between SMB and ice discharge limits the importance of outlet glacier dynamics with increasing atmospheric forcing. Forcing from the regional climate model produces a 14–31% higher sea-level contribution compared to a PDD model run with the same parameters as for IPCC AR4. [less ▲]

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See detailAtmospheric and oceanic climate forcing of the exceptional Greenland ice sheet surface melt in summer 2012
Hanna, E.; Fettweis, Xavier ULg; Mernild, S. et al

in International Journal of Climatology (2013), online

The NASA announcement of record surface melting of the Greenland ice sheet in July 2012 led us to examine the atmospheric and oceanic climatic anomalies that are likely to have contributed to these ... [more ▼]

The NASA announcement of record surface melting of the Greenland ice sheet in July 2012 led us to examine the atmospheric and oceanic climatic anomalies that are likely to have contributed to these exceptional conditions and also to ask the question of how unusual these anomalies were compared to available records. Our analysis allows us to assess the relative contributions of these two key influences to both the extreme melt event and ongoing climate change. In 2012, as in recent warm summers since 2007, a blocking high pressure feature, associated with negative NAO conditions, was present in the mid-troposphere over Greenland for much of the summer. This circulation pattern advected relatively warm southerly winds over the western flank of the ice sheet, forming a ‘heat dome’ over Greenland that led to the widespread surface melting. Both sea-surface temperature and sea-ice cover anomalies seem to have played a minimal role in this record melt, relative to atmospheric circulation. Two representative coastal climatological station averages and several individual stations in south, west and north-west Greenland set new surface air temperature records for May, June, July and the whole (JJA) summer. The unusually warm summer 2012 conditions extended to the top of the ice sheet at Summit, where our reanalysed (1994–2012) DMI Summit weather station summer (JJA) temperature series set new record high mean and extreme temperatures in 2012; 3-hourly instantaneous 2-m temperatures reached an exceptional value of 2.2°C at Summit on 11 July 2012. These conditions translated into the record observed ice-sheet wide melt during summer 2012. However, 2012 seems not to be climatically representative of future ‘average’ summers projected this century. [less ▲]

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See detailIce-sheet mass balance and climate change
Hanna, E.; Navarro, F.; Pattyn, F. et al

in Nature (2013), 498

Since the 2007 Intergovernmental Panel on Climate Change Fourth Assessment Report, new observations of ice-sheet mass balance and improved computer simulations of ice-sheet response to continuing climate ... [more ▼]

Since the 2007 Intergovernmental Panel on Climate Change Fourth Assessment Report, new observations of ice-sheet mass balance and improved computer simulations of ice-sheet response to continuing climate change have been published. Whereas Greenland is losing ice mass at an increasing pace, current Antarctic ice loss is likely to be less than some recently published estimates. It remains unclear whether East Antarctica has been gaining or losing ice mass over the past 20 years, and uncertainties in ice-mass change for West Antarctica and the Antarctic Peninsula remain large. We discuss the past six years of progress and examine the key problems that remain. [less ▲]

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See detailRapid loss of firn pore space accelerates 21st century Greenland mass loss
van Angelen, J.; Lenaerts, J.; van den Broeke, M. et al

in Geophysical Research Letters (2013), 40

Mass loss from the two major ice sheets and their contribution to global sea level rise is accelerating. In Antarctica, mass loss is dominated by increased flow velocities of outlet glaciers, following ... [more ▼]

Mass loss from the two major ice sheets and their contribution to global sea level rise is accelerating. In Antarctica, mass loss is dominated by increased flow velocities of outlet glaciers, following the thinning or disintegration of coastal ice shelves into which they flow. In contrast, ∼55% of post‒1992 Greenland ice sheet (GrIS) mass loss is accounted for by surface processes, notably increased meltwater runoff. A subtle process in the surface mass balance of the GrIS is the retention and refreezing of meltwater, currently preventing ∼40% of the meltwater to reach the ocean. Here we force a high‒resolution atmosphere/snow model with a mid‒range warming scenario (RCP4.5, 1970–2100), to show that rapid loss of firn pore space, by >50% at the end of the 21st century, quickly reduces this refreezing buffer. As a result, GrIS surface mass loss accelerates throughout the 21st century and its contribution to global sea level rise increases to 1.7 ±0.5 mm yr−1, more than four times the current value. [less ▲]

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See detailThe future sea-level rise contribution of Greenland's glaciers and ice caps
Machguth, H.; Rastner, P.; Bolch, T. et al

in Environmental Research Letters (2013), 8(025005), 14

We calculate the future sea-level rise contribution from the surface mass balance of all of Greenland's glaciers and ice caps (GICs, ~90 000 km2) using a simplified energy balance model which is driven by ... [more ▼]

We calculate the future sea-level rise contribution from the surface mass balance of all of Greenland's glaciers and ice caps (GICs, ~90 000 km2) using a simplified energy balance model which is driven by three future climate scenarios from the regional climate models HIRHAM5, RACMO2 and MAR. Glacier extent and surface elevation are modified during the mass balance model runs according to a glacier retreat parameterization. Mass balance and glacier surface change are both calculated on a 250 m resolution digital elevation model yielding a high level of detail and ensuring that important feedback mechanisms are considered. The mass loss of all GICs by 2098 is calculated to be 2016 ± 129 Gt (HIRHAM5 forcing), 2584 ± 109 Gt (RACMO2) and 3907 ± 108 Gt (MAR). This corresponds to a total contribution to sea-level rise of 5.8 ± 0.4, 7.4 ± 0.3 and 11.2 ± 0.3 mm, respectively. Sensitivity experiments suggest that mass loss could be higher by 20–30% if a strong lowering of the surface albedo were to take place in the future. It is shown that the sea-level rise contribution from the north-easterly regions of Greenland is reduced by increasing precipitation while mass loss in the southern half of Greenland is dominated by steadily decreasing summer mass balances. In addition we observe glaciers in the north-eastern part of Greenland changing their characteristics towards greater activity and mass turnover. [less ▲]

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See detailEstimation of the Greenland ice sheet surface mass balance contribution to future sea level rise using the regional climate model MAR
Fettweis, Xavier ULg; Gallée, H.; van den Broeke, M. et al

Conference (2013, April 10)

With the aim of estimating the sea level rise (SLR) coming from Surface Mass Balance (SMB) changes over the Greenland ice sheet (GrIS), we report future projections obtained with the regional climate ... [more ▼]

With the aim of estimating the sea level rise (SLR) coming from Surface Mass Balance (SMB) changes over the Greenland ice sheet (GrIS), we report future projections obtained with the regional climate model MAR, forced by outputs of three CMIP5 General Circulation Models (GCMs). Our results indicate that in warmer climates, the mass gained due to increased winter snowfall over GrIS does not compensate the mass lost through increased meltwater run-off in summer. All the MAR projections shows similar non-linear melt increases with rising temperatures as a result of the positive surface albedo feedback, because no change is projected in the general atmospheric circulation over Greenland. Nevertheless, MAR exhibits a large range in its future projections. By coarsely estimating the GrIS SMB changes from CMIP5 GCMs outputs, we show that the uncertainty coming from the GCM-based forcing represents about half of projected SMB changes. In 2100, the CMIP5 ensemble mean projects a SLR, resulting from a GrIS SMB decrease, estimated to be 4 2 cm and 9 4 cm for the RCP 4.5 and RCP 8.5 scenarios, respectively. However, these future projections do not consider the positive melt-elevation feedback. Sensitivity MAR experiments using perturbed ice sheet topographies consistent with the projected SMB changes highlight the importance of coupling climate models to an ice sheet model. Such a coupling will allow to consider the future response of both surface processes and ice-dynamic changes, and their mutual feedbacks to rising temperatures. [less ▲]

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See detailEvidence and analysis of 2012 Greenland records from spaceborne observations, a regional climate model and reanalysis data
Tedesco, M.; Fettweis, Xavier ULg; Mote, T. et al

in Cryosphere (The) (2013), 7

A combined analysis of remote sensing observations, regional climate model (RCM) outputs and reanalysis data over the Greenland ice sheet provides evidence that multiple records were set during summer ... [more ▼]

A combined analysis of remote sensing observations, regional climate model (RCM) outputs and reanalysis data over the Greenland ice sheet provides evidence that multiple records were set during summer 2012. Melt extent was the largest in the satellite era (extending up to ∼97% of the ice sheet) and melting lasted up to ∼2 months longer than the 1979–2011 mean. Model results indicate that near surface temperature was ∼3 standard deviations (σ) above the 1958–2011 mean, while surface mass balance (SMB) was ∼3σ below the mean and runoff was 3.9σ above the mean over the same period. Albedo, exposure of bare ice and surface mass balance also set new records, as did the total mass balance with summer and annual mass changes of, respectively, −627 Gt and −574 Gt, 2σ below the 2003–2012 mean. We identify persistent anticyclonic conditions over Greenland associated with anomalies in the North Atlantic Oscillation (NAO), changes in surface conditions (e.g., albedo, surface temperature) and preconditioning of surface properties from recent extreme melting as major driving mechanisms for the 2012 records. Less positive if not increasingly negative SMB will likely occur should these characteristics persist. [less ▲]

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See detailSurface mass balance model intercomparison for the Greenland ice sheet
Vernon, C.L.; Bamber, J.L.; Box, J.E. et al

in Cryosphere (The) (2013), 7

A number of high resolution reconstructions of the surface mass balance (SMB) of the Greenland ice sheet (GrIS) have been produced using global re-analyses data extending back to 1958. These ... [more ▼]

A number of high resolution reconstructions of the surface mass balance (SMB) of the Greenland ice sheet (GrIS) have been produced using global re-analyses data extending back to 1958. These reconstructions have been used in a variety of applications but little is known about their consistency with each other and the impact of the downscaling method on the result. Here, we compare four reconstructions for the period 1960–2008 to assess the consistency in regional, seasonal and integrated SMB components. Total SMB estimates for the GrIS are in agreement within 34% of the four model average when a common ice sheet mask is used. When models' native land/ice/sea masks are used this spread increases to 57%. Variation in the spread of components of SMB from their mean: runoff 42% (29% native masks), precipitation 20% (24% native masks), melt 38% (74% native masks), refreeze 83% (142% native masks) show, with the exception of refreeze, a similar level of agreement once a common mask is used. Previously noted differences in the models' estimates are partially explained by ice sheet mask differences. Regionally there is less agreement, suggesting spatially compensating errors improve the integrated estimates. Modelled SMB estimates are compared with in situ observations from the accumulation and ablation areas. Agreement is higher in the accumulation area than the ablation area suggesting relatively high uncertainty in the estimation of ablation processes. Since the mid-1990s each model estimates a decreasing annual SMB. A similar period of decreasing SMB is also estimated for the period 1960–1972. The earlier decrease is due to reduced precipitation with runoff remaining unchanged, however, the recent decrease is associated with increased precipitation, now more than compensated for by increased melt driven runoff. Additionally, in three of the four models the equilibrium line altitude has risen since the mid-1990s, reducing the accumulation area at a rate of approximately 60 000 km2 per decade due to increased melting. Improving process representation requires further study but the use of a single accurate ice sheet mask is a logical way to reduce uncertainty among models. [less ▲]

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See detailEstimating Antarctic ice sheet surface mass balance contribution to future sea level rise using the regional atmospheric climate model MAR
Agosta, Cécile ULg; Fettweis, Xavier ULg; Gallée, Hubert

Poster (2013, April)

We report future projections of Surface Mass Balance (SMB) over the Antarctic ice sheet obtained with the regional climate model MAR, for different warming scenarios. MAR forcing is carefully selected ... [more ▼]

We report future projections of Surface Mass Balance (SMB) over the Antarctic ice sheet obtained with the regional climate model MAR, for different warming scenarios. MAR forcing is carefully selected among the CMIP5 GCMs panel according to its ability to simulate the current climate over Antarctica. MAR includes blowing snow modeling, an important process in Antarctica. [less ▲]

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See detailHigh-resolution modelling of the Antarctic surface mass balance, application for the 20th, 21st and 22nd centuries
Agosta, Cécile ULg; Favier, Vincent; Krinner, Gerhard et al

Poster (2013, April)

Although areas below 2000 m above sea level (a.s.l.) cover 40% of the Antarctic grounded ice-sheet, they represent about 75% of the surface mass balance (SMB) of the continent. Because the topography is ... [more ▼]

Although areas below 2000 m above sea level (a.s.l.) cover 40% of the Antarctic grounded ice-sheet, they represent about 75% of the surface mass balance (SMB) of the continent. Because the topography is complex in many of these regions, SMB modelling is highly dependent on resolution, and studying the impact of Antarctica on the fu- ture rise in sea level requires high resolution physical approaches. We have developed a new, low time consuming, physical downscaling model for high-resolution (15 km) long-term SMB projections. Here, we present results of our SMHiL (surface mass balance high-resolution downscaling) model, which was forced with the LMDZ4 atmo- spheric general circulation model to assess SMB variation in the 21st and the 22nd centuries under two different scenarios. The higher resolution of SMHiL reproduces the geographical patterns of SMB better and induces a significantly higher averaged SMB over the grounded ice-sheet for the end of the 20th century. Our comparison of more than 2700 quality-controlled field data showed that LMDZ4 and SMHiL fit the observed values equally well. Never- theless, field data below 2000 m a.s.l. are too scarce to settle SMHiL efficiency. Measuring the SMB in these undocumented areas is a future scientific priority. Our results suggest that running LMDZ4 at a finer resolution may give a future increase in SMB in Antarctica between 15% to 30% higher than its standard resolution. Future changes in the Antarctic SMB at low elevations will result from the conflict between higher snow accumulation and runoff. For this reason, developing a downscaling model was crucial to represent processes in sufficient detail and correctly model the SMB in coastal areas. [less ▲]

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See detailEstimating the Greenland ice sheet surface mass balance contribution to future sea level rise using the regional atmospheric climate model MAR
Fettweis, Xavier ULg; Franco, Bruno ULg; Tedesco, M. et al

in Cryosphere (The) (2013), 7

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 ... [more ▼]

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 +4 ± 2 cm and +9 ± 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. [less ▲]

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