Contribution of the Greenland and An-tarctica Ice Sheets to Future Sea-level Rise

In recent years, the rates of thinning and flow of ice sheet<!–[if supportFields]>XE “ice sheet”<![endif]–><!–[if supportFields]><![endif]–>s have been increasing rapidly all around the world. Typically, researchers use surface mass balance estimates to measure this mass loss and predict future trends in ice sheet mass balance. However, Rignot et al. (2011) claim that surface mass balance calculations do not accurately represent the ice sheet’s contribution to sea level rise. In their paper, the authors used the rate of change of mass loss coupled with surface mass balance calculations to study the contribution of the ice sheets of Greenland and Antarctica<!–[if supportFields]> XE “Antarctica” <![endif]–><!–[if supportFields]><![endif]–> to sea-level rise<!–[if supportFields]> XE “sea-level rise (SLR)” <![endif]–><!–[if supportFields]><![endif]–>. Their results revealed that over the last 8 years, the Greenland and Antarctica ice sheet loss has accelerated by 36.3 ± 2 Gt/yr, 3 times more than the acceleration from mountain glaciers<!–[if supportFields]> XE “glaciers” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “glacier” <![endif]–><!–[if supportFields]><![endif]–> and icecaps. Given the magnitude of this acceleration, the authors conclude that ice sheets will be the major contributor to sea-level rise in the 21st century. —Sachi Singh
Rignot, E., I. Velicogna, M. R.van den Broeke, A. Monaghan, and J. Lenaerts. 2011. Acceleration of the contribution of the Greenland and Antarctic<!–[if supportFields]> XE “Antarctic” <![endif]–><!–[if supportFields]><![endif]–> ice sheet<!–[if supportFields]> XE “ice sheet” <![endif]–><!–[if supportFields]><![endif]–>s to sea level rise, Geophys. Res. Lett., 38, L05503, doi:10.1029/2011GL046583

Multi-decadal mass balance observations are required to estimate long term trends in ice sheet<!–[if supportFields]> XE “ice sheet” <![endif]–><!–[if supportFields]><![endif]–> mass balance. While the mass balance estimates have improved significantly in the last decade, they are not sufficient to predict the non-linear contributions of the ice sheet to rise in sea-level. In this paper, the authors used the mass budget method (MBM) and the gravity method to estimate the temporal variations in the mass balance of the Greenland and Antarctica<!–[if supportFields]>XE “Antarctica” <![endif]–><!–[if supportFields]><![endif]–> ice sheets. The MBM calculates the ice sheet’s rate of mass change by comparing the surface mass balance from regional atmospheric models to the ice discharge—which is calculated using glacier<!–[if supportFields]> XE “glacier” <![endif]–><!–[if supportFields]><![endif]–> velocities and ice thickness data. The gravity method uses data from the Gravity Recovery and Climate Experiment (GRACE)<!–[if supportFields]> XE “Gravity Recovery and Climate Experiment (GRACE)” <![endif]–><!–[if supportFields]><![endif]–> to estimate the rate of change of mass as a function of time. Using both these models, the authors estimated that in 2006, the Greenland and Antarctic<!–[if supportFields]> XE “Antarctic” <![endif]–><!–[if supportFields]><![endif]–> ice sheets had a combined mass loss of 475 ± 158 Gt/year, equivalent to 1.3 ± 0.4 mm/year rise in sea-level. They also estimated in the last 18 years, the acceleration in mass loss was 21.9 ± 1 Gt/year for Greenland and 14.5 ± 2 Gt/year for Antarctica, with a combined total of 36.3 ± 2 Gt/year acceleration in mass loss. Since this acceleration is 3 times larger than for mountain glaciers<!–[if supportFields]> XE “glaciers” <![endif]–><!–[if supportFields]><![endif]–> and ice caps, the authors concluded that the mass loss from ice sheets will be the biggest contributor to sea-level in the forthcoming decade. Thus, the IPPC’s projections for the contribution of ice sheets to sea-level rise<!–[if supportFields]> XE “sea-level rise (SLR)” <![endif]–><!–[if supportFields]><![endif]–> may be too conservative.

Ocean Stratification Causes Submarine Melting in a Major Greenland Outlet Glacier

Submarine melting is said to be a potential cause for the widespread acceleration and mass loss of outlet glaciers<!–[if supportFields]> XE “glaciers” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “glacier” <![endif]–><!–[if supportFields]><![endif]–>. Since 2000, the retreat and acceleration of outlet glaciers accounts for 50% of Greenland’s net mass loss. To further explore this phenomenon, Straneo et al. (2011) conducted surveys of the Sermilik Fjord, which is a major outlet of the GrIS, in August 2009 and in March 2010. Their data revealed that both the cold, fresh Arctic<!–[if supportFields]> XE “Arctic” <![endif]–><!–[if supportFields]><![endif]–> waters (PW) and the warm, salty subtropical waters from the North Atlantic (STW) cause submarine melt in the summer, while only the STW drives the melt in the winter. Thus, due to the stratification of the waters, the ice edge is organized into multiple overturning cells, which increases the rate of fjord melting with depth. —Sachi Singh
Straneo, F., Curry, R. G., Sutherland, D. A., Hamilton, G. S., Cenedese, C., Våge, K., Stearns, L. A. 2011. Impact of ocean stratification on submarine melting of a major Greenland outlet glacier<!–[if supportFields]> XE “glacier” <![endif]–><!–[if supportFields]><![endif]–>. <http: 10101="""" npre.2011.5670.1

Ocean-driven melting has important implications for ice sheet<!–[if supportFields]>XE “ice sheet”<![endif]–><!–[if supportFields]><![endif]–> variability and sea-level rise<!–[if supportFields]> XE “sea-level rise (SLR)” <![endif]–><!–[if supportFields]><![endif]–>. Typically, submarine melt rates are estimated from mass-balance calculations using ice-flow and ice-thickness data, however, these calculations do not provide any information on the water mass and circulation responsible for the melting. In this study, the authors surveyed a major outlet glacier<!–[if supportFields]> XE “glacier” <![endif]–><!–[if supportFields]><![endif]–> of the Greenland ice sheet and collected measurements for the summer and winter conditions in the fjord. They determined that the thermal forcing of ambient waters (the heat available to melt the ice) and the circulation at the ice edge are the main factors that drive submarine circulation. They found that in the summer, both PW and STW cause submarine melting in the fjord, leading to large seasonal changes in stratification of the waters. Temperature and salinity<!–[if supportFields]> XE “salinity” <![endif]–><!–[if supportFields]><![endif]–> data along the fjord show that the PW and STW layering is preserved even at the glacier’s terminus. Analysis of the summer data reveals that there are multiple overturning cells along the fjord, which give rise to non-uniform heat transport and increase the rates of melting with depth. These two observations reinforce the hypothesis that the PW and STW strongly influence circulation and melting at the ice edge.
Thus, the authors conclude that the single overturning cell model is not sufficient to understand the submarine melting rates of the Greenland ice sheet<!–[if supportFields]> XE “ice sheet” <![endif]–><!–[if supportFields]><![endif]–>. They believe that the Greenland glacier<!–[if supportFields]> XE “glacier” <![endif]–><!–[if supportFields]><![endif]–> dynamics are the complex consequence of the interface of the Atlantic/Arctic<!–[if supportFields]>XE “Arctic” <![endif]–><!–[if supportFields]><![endif]–> waters, as well as the changes in large-scale ocean circulation. 

Ocean Regulation Controls Ice Sheet Mass Loss in the Southeast Greenland Ice Sheet

During the years 2003 and 2005, the southeast outlet glaciers of the GrIS underwent dramatic thinning and the ice sheet significantly retreated; surprisingly in 2006, the acceleration rates of the two largest outlet glaciers of the ice sheet decreased, causing the ice sheet to cease thinning and re-advance. In this paper, Murray et al. (2010) explore this synchronous acceleration and thinning of the ice sheet, and propose that regional factors have the greatest impact on these ice sheet-ocean interactions. They conclude that the ice sheet mass loss in the southeast GrIS is primarily caused by the warming and cooling of the coastal waters around the coastal glaciers. —Sachi Singh
Murray, K., Scharrer, K. , James, T. D., Dye, S. R., Hanna, E., Booth, A. D., Selmes, N., Luckman, A., Hughes, A. L. C., Cook, S., Huybrechts, P. 2010. Ocean regulation hypothesis for glacier dynamics in southeast Greenland and implications for ice sheet mass changes. doi:10.1029/2009JF001522.

Atmospheric warming and ocean/fjord temperatures both significantly affect glacier dynamics. Atmospheric warming can lead to increased sea-surface temperatures. which enhance basal sliding and increase glacier speeds. Consequently, the meltwater can enter the crevasses of the glaciers and increase the rate of glacier calving. Similarly, warm surface waters can reduce the extent of fjord ice and undermine the integrity of the ice melange (which are small pieces of calved ice and sea ice), which can eventually lead to higher rates of calving and faster flow conditions. In the paper, the authors analyzed the flow speeds, surface elevation, and calving front positions for tidewater terminating glaciers in southeast Greenland to determine the relationship between oceanic processes and glacier dynamics. From the analysis of the sea-surface temperatures, they found the the glacier speed-up of 2003 was not caused by the increase in surface meltwater and basal sliding; instead, it was caused by the warm surface waters that were brought to the southeast coast of Greenland by the Irminger Current (IC). The IC—which brings warm, high salinity water from the Atlantic—bifurcates to the west of Iceland, causing one of its branches to flow southwards along the southeast coast of Greenland; the East Greenland Coastal Current (EGCC)—which brings fresh, cold water to the coast—flows right by the IC, along the landward side of the continental shelf. The authors found that the 2003 speed-up of glaciers—associated with warm IC and weak EGCC—led to a large deposition of cold water into the EGCC; this increased ice discharge into the EGCC strengthened the cold current, decreased the sea-surface temperatures and subsequently, slowed down the glaciers in 2006. Thus, the authors conclude that the ice sheet‘s input provided a negative feedback to the EGCC, which controlled the ice sheet mass loss and re-stabilized the coastal glaciers. 

Debris Cover Affects Himalayan Glaciers’ Response to Climate Change

The Himalayan glaciers<!–[if supportFields]> XE “glaciers” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “glacier” <![endif]–><!–[if supportFields]><![endif]–> are an important source of drinking water, agriculture<!–[if supportFields]> XE “agriculture” <![endif]–><!–[if supportFields]><![endif]–> and hydropower<!–[if supportFields]> XE “hydropower” <![endif]–><!–[if supportFields]><![endif]–> for central and south Asia, however, the remoteness of these glaciers makes ground-based data collection tricky. Thus, scientists are forced to use glacier retreats and advances to measure the impact of climate change on the glaciers. However, Scherler et al. (2011) claim these approaches are not entirely accurate as supraglacial debris can affect the the glacier’s response to climate change. In order to asses this further, the authors analyzed 286 glaciers in the greater Himalayan Range between 2000 and 2008. They discovered that glaciers that are heavily covered with debris and have stagnant (not moving), low-gradient terminus regions have stable fronts while the monsoon<!–[if supportFields]> XE “monsoon” <![endif]–><!–[if supportFields]><![endif]–>-driven glaciers are retreating. Their results indicate that Himalayan glaciers dynamics seem to be heavily dependent on the debris cover, and show no uniform response to climate change. —Sachi Singh

Supraglacial debris are said to influence the terminus dynamics of glaciers<!–[if supportFields]> XE “glaciers” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “glacier” <![endif]–><!–[if supportFields]><![endif]–> and modify their response to climate change. To further study the terminus dynamics of the glaciers in the greater Himalaya region, the authors studied the frontal changes and surface velocities of 286 glaciers between 2000 and 2008. They also mapped debris cover to test whether regional disparities in debris cover accounted for the spatial variations in glacier terminus dynamics. The authors found that the regional distribution of stagnant glaciers with stable fronts varied significantly in the greater Himalayan region: they were most common in the Hindukush, southern, and northern central Himalayas<!–[if supportFields]> XE “Himalayas” <![endif]–><!–[if supportFields]><![endif]–> and were completely absent in the Karakoram region. Since accumulation areas in Karakoram are relatively steep, stagnant glaciers are absent and cannot account for the stable or advancing glaciers in the region. The authors claim that the westerly-derived winter precipitation could explain the positive mass balance disturbance in Karakoram.

In all other places, the formation of stagnant ice relies on low-gradient slopes and is confined to the terminus region of debris-covered glaciers<!–[if supportFields]> XE “glaciers” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “glacier” <![endif]–><!–[if supportFields]><![endif]–> with shallow gradients. The authors claim that debris-cover—which is almost always a few centimeters thick—leads to a reduction in melt rates and slows down the glacier’s response to climate change. Debris-cover also influences anthropogenic and natural radiative heat transfer. Thus, the authors conclude that topographic factors, which usually vary with terrain, have significant effects on the glacier’s response to climate change and should be accounted for in future mas-balance calculations. 

Reduced Albedo and Accumulation Contribute to Negative Surface Mass Balance of the Greenland Ice Sheet

The year 2010 saw a large increase in near-surface temperatures along the coast of the Greenland ice sheet (GrIS); consequently, these unusually warm surface temperatures led to a huge increase in surface melting over the GrIS. To further explore this phenomenon, Tedesco et al. (2011) used data from satellite sensors, surface glaciological observations and regional atmospheric models to study the surface albedo, accumulation and the number of days bare ice was exposed over the GrIS in 2010. Their results indicated that the high near-surface temperatures over the GrIS led to a strongly negative surface mass balance—defined as the difference between accumulation and ablation of ice and snow—which was further intensified by the decrease in albedo and the increase in the number of days bare ice was exposed in the GrIS. Thus, the authors concluded that these anomalous conditions led to a longer melting season and contributed to the strongly negative surface mass balance of the GrIS in 2010.
Tedesco, M., Fettweis, X., van den Broeke, M. R., van de Wal, R. S. W., Smeets, C. J. P. P.,  van  de Berg, W. J., Serreze, M. C., Box, J. E. 2011. The role of albedo and accumulation in the  2010 melting record in Greenland. doi:10.1088/1748-9326/6/1/014005

In large areas of the ablation zone in the south of the GrIS, the melting season had started 50 days earlier than the average melting season (measured from 1979 to 2009) and had ended exceptionally late in 2010. While the increase in surface melting can be positively correlated with the increase in near-surface temperatures, recent studies have shown that the melting of the GrIS also depends on the accumulation, radiation, and refreezing and sublimation conditions. The surface mass balance is also strongly correlated with albedo because when melting increases, the grain size of the snow increases and which consequently, decreases the albedo. In this study, the authors used moderate-resolution imaging spectroradiometer (MODIS) albedo product to study anomalies in albedo; they also used data from automatic weather stations and regional surface and energy models to study the surface mass balance anomalies in 2010. They found the largest negative albedo anomalies occurred  in August along the south west coast of the ice sheet; they hypothesized that the reduced amount of snowfall, enhanced melting and increased number of bare ice exposure days could have led to the 2010 albedo anomalies. While the early melt season was triggered by the large increase in near-surface temperatures, the reduced accumulation and albedo were more likely to be responsible for the premature bare ice exposure. Thus, the authors inferred that the anomalously warm conditions reduced the accumulation and albedo, which led to the strongly negative surface mass balance of the GrIS in 2010.

Efficient Subglacial Drainage Systems Reduce Velocity and Duration of Ice Flow at Warm Temperatures

Recent studies on the mass balance of the Greenland Ice Sheet (GrIS) have shown that the thickening of the sheet’s interior is offset by its mass loss near coastal regions due to basal lubrication (a process by which the surface meltwater penetrates to the base of the ice sheet and enhances basal sliding). To study this mechanism further, Sundal et al. (2011) observed the spatial and temporal variations in ice flow of six glaciers in the GrIS over a period of five years. They found that there was a significant increase in the speed of the ice flow in the summer in comparison to that in the winter. Peak rates of ice flow are known to be positively correlated with an increased degree of melting; however, this trend is not universally observed, as ice speeds slow down significantly in warm summers. The authors hypothesize that a more efficient subglacial drainage system contributes to the reduction in ice velocities in warm summers.—Sachi Singh
Sundal, A. V., Shepherd, A., Nienow, P., Hanna, E., Palmer, S., Huybrechts, P. 2011. Melt-induced speed-up of Greenland ice sheet offset by efficient subglacial drainage. doi:10.1038/nature09740.

The combined effect of increased surface melting and ice sheet flow is said to hasten the mass loss of the GrIS; however, recent data models on the mass loss of the GrIS have not been able to incorporate the effects of surface melting induced acceleration into their predictions because little is known about the hydraulic forces associated with the melting. To explore these hydraulic forces, the authors studied the satellite observations of ice flow recorded in the southwest corner of the GrIS to examine the development of the ice flow in years of markedly different melting. They found that the average winter speed of glaciers (122 m/year) was significantly lower than the speed in the summer (138 m/year); they also observed an increased variance of glacier speeds over the summer. Thus, the authors conclude that the seasonal variations in melting drives the seasonal variations in ice flow. The variations in the timing, extent and quantity of surface run-off, and variations in the routing of water at the base of the the ice-sheet could all contribute to the seasonal ice flow cycles. The authors investigated further to find that the ice flow in the late summers was three times shorter and significantly slower than that in the early summer. While some scientists believe that greater melting induces greater ice velocity, the authors believe that even though the peak rate of flow increases with high melting, an efficient subglacial drainage system leads to a reduction in the speed as well as the duration of the flow. Since basal lubrication alone cannot explain this phenomenon, the authors claim that the glacial drainage adjusts to accommodate an increase in ice flow; abundant meltwater could trigger a change from an inefficient cavity system to an efficient channelized system of drainage, which could lead to a reduction in subglacial water pressure and ice speeds. These patterns have been observed in the High Arctic and Alaskan valley glaciers.
Since the rates of surface melting of the GrIS are said to double over the 21st century, it would be useful to gain a deeper understanding of the mechanism that drives these changes in subglacial drainage patterns. 

Multi-Model Analyses Reveals the Regional Contribution of Mountain Glaciers and Ice Caps to Future Sea-level Rise

Scientists believe that mountains glaciers and ice caps have been a major contributor to the rise in global sea-levels over the past decades. In this paper, Radić & Hock (2011) investigate the Intergovernmental Panel for Climate Change’s (IPCC) projections for the global sea-level rise in the twenty first century, and they conclude that these estimates are not wholly accurate as they do not account for the effects of precipitation and the regional factors influencing the rise in sea-levels. The authors projected the changes in volume of all the ice caps and glaciers on Earth in response to twenty first century temperature and precipitation projections from ten global climate change models (GCMs) reported by the IPCC. They conclude that glaciers in Arctic Canada, Alaska and Antarctica would be largest contributors to the rise in global sea-levels in 2100. Thus, there will be a significant reduction in total glacier volume by 2100, and some mountainous regions may even lose up to 75% of their present ice volume. – Sachi Singh
Radić, V., Hock, R. 2011. Regionally differentiated contribution of mountain glaciers and ice caps to future sea-level rise. Nature Geoscience. doi:10.1038/ngeo1052.

The Fourth Assessment Report of the IPCC predicts that the wastage of glaciers and ice caps will lead to a 0.07 to 0.17 m rise in global sea-levels in the twenty first century. Another study found the accelerating rates of mass loss from the glacier mass balance data between 1995 and 2005; the authors of this study used this model to predict a 0.240±0.128m rise in sea levels, assuming this rate of acceleration is constant. In order to resolve these discrepancies, Radić & Hock studied the volume changes of mountain glaciers and ice caps in 19 spatially resolved glacierized regions. To quantify future volume changes, the authors developed a calibrated mass balance model to applied it to all the glaciers available in the World Glacier Inventory (WGI-XF). According to their multi-model means, glaciers around the world will cause a 0.124±0.037m rise in sea-level by 2100. Assuming the GCMs are accurate, the authors predict that there will be a global ice volume loss of 0.124±0.037m SLE (sea level equivalents) by 2100. The volume loss varies considerably from region to region; the smallest loss was predicted to be in Greenland and High Mountain Asia, and the largest in the European Alps and New Zealand. However, these places are not significant contributors to the future rise in sea-level. The glaciers in Arctic Canada, Alaska and Antarctica are estimated to be the largest contributers to the rise in sea-levels. While there are some uncertainties associated with the rise initial setup of the model, this study reveals  the main regional contributers to sea-level rise and pinpoint the areas that are most vulnerable to glacier waste. Thus, if warming continues as expected, glaciers will be a large contributer to sea-level rise around the world.