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

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 ▲]

Surface mass balance model intercomparison for the Greenland ice sheet

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 ▲]

High-resolution modelling of the Antarctic surface mass balance, application for the 20th, 21st and 22nd centuries

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 ▲]

Effect of uncertainty in surface mass balance elevation feedback on projections of the future sea level contribution of the Greenland ice sheet – Part 2: Projections

in Cryosphere Discussions (The) (2013), 7

We apply a new parameterisation of the Greenland ice sheet (GrIS) feedback between surface mass balance (SMB: the sum of surface accumulation and surface ablation) and surface elevation in the MAR ... [more ▼]

We apply a new parameterisation of the Greenland ice sheet (GrIS) feedback between surface mass balance (SMB: the sum of surface accumulation and surface ablation) and surface elevation in the MAR regional climate model (Edwards et al., 2013) to projections of future climate change using five ice sheet models (ISMs). The MAR climate projections are for 2000–2199, forced by the ECHAM5 and HadCM3 global climate models (GCMs) under the SRES A1B emissions scenario. The additional sea level contribution due to the SMB-elevation feedback averaged over five ISM projections for ECHAM5 and three for HadCM3 is 4.3% (best estimate; 95% credibility interval 1.8–6.9%) at 2100, and 9.6% (best estimate; 95% credibility interval 3.6–16.0%) at 2200. In all results the elevation feedback is significantly positive, amplifying the GrIS sea level contribution relative to the MAR projections in which the ice sheet topography is fixed: the lower bounds of our 95% credibility intervals (CIs) for sea level contributions are larger than the "no feedback" case for all ISMs and GCMs. Our method is novel in sea level projections because we propagate three types of modelling uncertainty – GCM and ISM structural uncertainties, and elevation feedback parameterisation uncertainty – along the causal chain, from SRES scenario to sea level, within a coherent experimental design and statistical framework. The relative contributions to uncertainty depend on the timescale of interest. At 2100, the GCM uncertainty is largest, but by 2200 both the ISM and parameterisation uncertainties are larger. We also perform a~perturbed parameter ensemble with one ISM to estimate the shape of the projected sea level probability distribution; our results indicates that the probability density is slightly skewed towards higher sea level contributions. [less ▲]

Important role of the mid-tropospheric atmospheric circulation in the recent surface melt increase over the Greenland ice sheet

in Cryosphere (The) (2013), 7

Since 2007, there has been a series of surface melt records over the Greenland ice sheet (GrIS), continuing the trend towards increased melt observed since the end of the 1990's. The last two decades are ... [more ▼]

Since 2007, there has been a series of surface melt records over the Greenland ice sheet (GrIS), continuing the trend towards increased melt observed since the end of the 1990's. The last two decades are characterized by an increase of negative phases of the North Atlantic Oscillation (NAO) favouring warmer and drier summers than normal over GrIS. In this context, we use a circulation type classification based on daily 500 hPa geopotential height to evaluate the role of atmospheric dynamics in this surface melt acceleration for the last two decades. Due to the lack of direct observations, the interannual melt variability is gauged here by the summer (June–July–August) mean temperature from reanalyses at 700 hPa over Greenland; analogous atmospheric circulations in the past show that ~70% of the 1993–2012 warming at 700 hPa over Greenland has been driven by changes in the atmospheric flow frequencies. Indeed, the occurrence of anticyclones centred over the GrIS at the surface and at 500 hPa has doubled since the end of 1990's, which induces more frequent southerly warm air advection along the western Greenland coast and over the neighbouring Canadian Arctic Archipelago (CAA). These changes in the NAO modes explain also why no significant warming has been observed these last summers over Svalbard, where northerly atmospheric flows are twice as frequent as before. Therefore, the recent warmer summers over GrIS and CAA cannot be considered as a long-term climate warming but are more a consequence of NAO variability affecting atmospheric heat transport. Although no global model from the CMIP5 database projects subsequent significant changes in NAO through this century, we cannot exclude the possibility that the observed NAO changes are due to global warming. [less ▲]

Future projections of the Greenland ice sheet energy balance driving the surface melt

in Cryosphere (The) (2013), 7

In this study, simulations at 25 km resolution are performed over the Greenland ice sheet (GrIS) throughout the 20th and 21st centuries, using the regional climate model MAR forced by four RCP scenarios ... [more ▼]

In this study, simulations at 25 km resolution are performed over the Greenland ice sheet (GrIS) throughout the 20th and 21st centuries, using the regional climate model MAR forced by four RCP scenarios from three CMIP5 global circulation models (GCMs), in order to investigate the projected changes of the surface energy balance (SEB) components driving the surface melt. Analysis of 2000–2100 melt anomalies compared to melt results over 1980–1999 reveals an exponential relationship of the GrIS surface melt rate simulated by MAR to the near-surface air temperature (TAS) anomalies, mainly due to the surface albedo positive feedback associated with the extension of bare ice areas in summer. On the GrIS margins, the future melt anomalies are preferentially driven by stronger sensible heat fluxes, induced by enhanced warm air advection over the ice sheet. Over the central dry snow zone, the surface albedo positive feedback induced by the increase in summer melt exceeds the negative feedback of heavier snowfall for TAS anomalies higher than 4 °C. In addition to the incoming longwave flux increase associated with the atmosphere warming, GCM-forced MAR simulations project an increase of the cloud cover decreasing the ratio of the incoming shortwave versus longwave radiation and dampening the albedo feedback. However, it should be noted that this trend in the cloud cover is contrary to that simulated by ERA-Interim–forced MAR for recent climate conditions, where the observed melt increase since the 1990s seems mainly to be a consequence of more anticyclonic atmospheric conditions. Finally, no significant change is projected in the length of the melt season, which highlights the importance of solar radiation absorbed by the ice sheet surface in the melt SEB. [less ▲]

High-resolution modelling of the Antarctic surface mass balance, application for the 20th, 21st and 22nd centuries

E-print/Working paper (2013)

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 ▲]

Greenland ice sheet surface mass balance: evaluating simulations and making projections with regional climate models

in Cryosphere (The) (2012), 6

Four high-resolution regional climate models (RCMs) have been set up for the area of Greenland, with the aim of providing future projections of Greenland ice sheet surface mass balance (SMB), and its ... [more ▼]

Four high-resolution regional climate models (RCMs) have been set up for the area of Greenland, with the aim of providing future projections of Greenland ice sheet surface mass balance (SMB), and its contribution to sea level rise, with greater accuracy than is possible from coarser-resolution general circulation models (GCMs). This is the first time an intercomparison has been carried out of RCM results for Greenland climate and SMB. Output from RCM simulations for the recent past with the four RCMs is evaluated against available observations. The evaluation highlights the importance of using a detailed snow physics scheme, especially regarding the representations of albedo and meltwater refreezing. Simulations with three of the RCMs for the 21st century using SRES scenario A1B from two GCMs produce trends of between −5.5 and −1.1 Gt yr−2 in SMB (equivalent to +0.015 and +0.003 mm sea level equivalent yr−2), with trends of smaller magnitude for scenario E1, in which emissions are mitigated. Results from one of the RCMs whose present-day simulation is most realistic indicate that an annual mean near-surface air temperature increase over Greenland of ~ 2°C would be required for the mass loss to increase such that it exceeds accumulation, thereby causing the SMB to become negative, which has been suggested as a threshold beyond which the ice sheet would eventually be eliminated. [less ▲]

Greenland ice-sheet contribution to sea-level rise buffered by meltwater storage in firn

in Nature (2012), 491

Surface melt on the Greenland ice sheet has shown increasing trends in areal extent and duration since the beginning of the satellite era. Records for melt were broken in 2005, 2007, 2010 and 2012. Much ... [more ▼]

Surface melt on the Greenland ice sheet has shown increasing trends in areal extent and duration since the beginning of the satellite era. Records for melt were broken in 2005, 2007, 2010 and 2012. Much of the increased surface melt is occurring in the percolation zone, a region of the accumulation area that is perennially covered by snow and firn (partly compacted snow). The fate of melt water in the percolation zone is poorly constrained: some may travel away from its point of origin and eventually influence the ice sheet’s flow dynamics and mass balance and the global sea level, whereas some may simply infiltrate into cold snow or firn and refreeze with none of these effects. Here we quantify the existing water storage capacity of the percolation zone of the Greenland ice sheet and show the potential for hundreds of gigatonnes of meltwater storage. We collected in situ observations of firn structure and meltwater retention along a roughly 85-kilometre-long transect of the melting accumulation area. Our data show that repeated infiltration events in which melt water penetrates deeply (more than 10 metres) eventually fill all pore space with water. As future surface melt intensifies under Arctic warming, a fraction of melt water that would otherwise contribute to sea-level rise will fill existing pore space of the percolation zone. We estimate the lower and upper bounds of this storage sink to be 322 ± 44 gigatonnes and 1.289 gigatonnes, respectively. Furthermore, we find that decades are required to fill this pore space under a range of plausible future climate conditions. Hence, routing of surface melt water into filling the pore space of the firn column will delay expansion of the area contributing to sea-level rise, although once the pore space is filled it cannot quickly be regenerated. [less ▲]

Simulating the growth of supra-glacial lakes at the western margin of the Greenland ice sheet

in Cryosphere (The) (2012), 6

We present a new method of modelling the growth of supraglacial lakes at the western margin of the Greenland ice sheet, based on routing runoff estimated by a regional climate model across a digital ... [more ▼]

We present a new method of modelling the growth of supraglacial lakes at the western margin of the Greenland ice sheet, based on routing runoff estimated by a regional climate model across a digital elevation model (DEM) of the ice sheet surface. Using data acquired during the 2003 melt season, we demonstrate that the model is 19 times more likely to correctly predict the presence (or absence) of lakes than it is to make incorrect predictions, within an elevation range of 1100 to 1700 metres above sea level (m a.s.l.), when compared with MODIS satellite imagery. Of the 66% of observed lake locations which the model correctly reproduces, the simulated lake onset day is found to be correlated with that observed with a Pearson correlation coefficient of 0.76. Our model accurately simulates maximum cumulative lake area with only a 1.5% overestimate. However, because our model does not simulate processes leading to lake stagnation or decay, such as refreezing or drainage, at present we do not simulate absolute daily lake area. We find that the maximum potential lake-covered ice sheet area is limited by topography to 6.4%. We estimate that this corresponds to a volume of 1.49 km3, 12% of the runoff produced in 2003. This can be taken as an upper bound given uncertainty in the DEM. This study has proved a good first step towards capturing the variability of supraglacial lake evolution with a numerical model. These initial results are promising and suggest that the model is a useful tool for use in analysing the behaviour of supraglacial lakes on the Greenland ice sheet in the present day and potentially beyond. [less ▲]