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THIS POST IS A CRITICAL REVIEW OF ALARMING AGW DRIVEN GREENLAND ICE SHEET MELT REPORTED IN FORBES [LINK] AND CLIMATE HOME NEWS [LINK] WHERE THE CITED SOURCE IS AN IMBIE RESERCH PAPER [LINK] IMBIE STANDS FOR ICE SHEET MASS BALANCE INTER COMPARISON EXERCISE

PART-1: CLAIMS MADE IN THESE SOURCES

(1)  THE FORBES SCIENCE ARTICLE “Greenland And Antarctica Are Melting Six Times Faster Than In The 1990s” [LINK] :  Throughout the 1990s, Greenland and Antarctica together lost 81 billion tons of ice per year. But this month, a comprehensive assessment of the changing ice sheets published in the journal Nature, found that in the 2010s, the rate of ice loss has risen by a factor six. This means that the two ice sheets are now losing 475 billion tons of ice per year. The IPCC Fifth Assessment Report predicted a rise in global sea levels of 28 inches by 2100. But this new study shows that ice losses from both Antarctica and Greenland are rising faster than expected, tracking with the IPCC’s worst-case scenario. The Ice Sheet Mass Balance Intercomparison Exercise team, an international team of 89 polar scientists from 50 organizations, conducted the study. They combined 26 surveys to calculate changes in the mass of the Greenland and Antarctic ice sheets between 1992 and 2018, using data from 11 satellite missions, including measurements of the ice sheets’ changing volume, flow and gravity. The ice loss coincides with several years of intense surface melting in Greenland, including last summer’s Arctic heatwave, which means that 2019 is also likely to set a new record for polar ice sheet loss. Almost all of the ice lost from Antarctica and half of that lost from Greenland has been triggered by oceans melting their outlet glaciers, which causes them to speed up. The remainder of Greenland’s ice losses are due rising air temperature, which has melted the ice sheet at its surface.

(2)  THE CLIMATE HOME ARTICLE: [LINK]  Greenland ice loss much faster than expected. New results combine data from multiple satellite missions for an up-to-date assessment of changes across the ice-sheet. Between 1992 and 2017, Greenland lost 3.8 trillion tonnes of ice about seven times faster than expected. This corresponds to a 10.6 mm contribution to global sea-level rise. The Greenland ice sheet is losing mass seven times faster than in the 1990s, In a paper published today in Nature, an international team of 89 polar scientists, working in collaboration with ESA and NASA, have produced the most complete picture of Greenland ice loss to date. Over the study period, the rate of ice loss was found to have increased seven-fold from 33 billion tonnes per year 1990s to 254 billion tonnes per year in the last decade. The IMBIE teach combined data from 11 satellites including ESA’s ERS-1, ERS-2, Envisat and Cryosat missions, as well as the Copernicus Sentinel-1 and Sentinel-2 missions to monitor changes in the ice sheet’s volume, flow and gravity. Using observational data spanning three decades, the team has produced Greenland’s mass balance. This study condenses the available data and provides a consensus view regarding Greenland’s ice loss enabling more accurate projections of future sea rise to be made  allowing coastal areas to prepare, and highlighting the urgent need for the international community to curtail greenhouse gas emissions. The IPCC had predicted a 60cm rise in global sea levels by 2100, putting 260 million people at risk of annual coastal flooding. The faster-than-expected rate reported by the IMBIE team shows that ice loss is following the IPCC’s high-end climate warming scenario, which predicts sea level will rise by an additional 7cm.  For every 1cm rise in sea level, another six million people are exposed to coastal flooding. Greenland ice melt will cause 100 million people to be flooded each year by the end of the century. These changes will devastate coastal communities.” Climate models show that over half of the losses were because of increased surface meltwater runoff, driven by warming air temperatures. The remaining losses were the result of increased glacier flow triggered by rising ocean temperatures. Ice loss peaked at 335 gigatons/yr in 2011 dropping to an average of 238 gigatons/yr 2012- 2018 but still seven times higher than observed in the 1990s. The variable nature of the ice losses from Greenland over the last three decades (is a consequence of the wide range of physical processes affecting different sectors of the ice sheet and reflects the value of monitoring year-to-year fluctuations when attempting to close the global sea level budget. Greenhouse gas emissions are still going up, not down. We are leaving future generations to be confronted with increasingly severe impacts of climate change, such as rising sea levels. We need to redouble efforts to meet the internationally agreed goal to limit global warming to 1.5°C over pre-industrial levels.

(3)  THE CITED RESEARCH PAPER: [LINK] Article: Published: 10 December 2019
Mass balance of the Greenland Ice Sheet from 1992 to 2018: The IMBIE Team
Nature volume 579, pages233–239(2020): Abstract: The Greenland Ice Sheet has been a major contributor to global sea-level rise in recent decades and it is expected to continue to be so. Although increases in glacier flow and surface melting have been driven by oceanic and atmospheric warming, the magnitude and trajectory of the ice sheet’s mass imbalance remain uncertain. Here we compare and combine 26 individual satellite measurements of changes in the ice sheet’s volume, flow and gravitational potential to produce a reconciled estimate of its mass balance. The ice sheet was close to a state of balance in the 1990s, but annual losses have risen since then, peaking at 345 ± 66 billion tonnes per year in 2011. In all, Greenland lost 3,902 ± 342 billion tonnes of ice between 1992 and 2018, causing the mean sea level to rise by 10.8 ± 0.9 mm. Using three regional climate models, we show that the reduced surface mass balance has driven 1,964 ± 565 billion tonnes (50.3 per cent) of the ice loss owing to increased meltwater runoff. The remaining 1,938 ± 541 billion tonnes (49.7 per cent) of ice loss was due to increased glacier dynamical imbalance, which rose from 46 ± 37 billion tonnes per year in the 1990s to 87 ± 25 billion tonnes per year since then. The total rate of ice loss slowed to 222 ± 30 billion tonnes per year between 2013 and 2017, on average, as atmospheric circulation favored cooler conditions and ocean temperatures fell at the terminus of Jakobshavn Isbræ. Cumulative ice losses from Greenland as a whole have been close to the rates predicted by the IPCC for their high-end climate warming scenario which forecast an additional 70 to 130 mm of global sea-level rise by 2100 compared with their central estimate.

PART-2: CRITICAL COMMENTARY

(1)  THE ESSENTIAL FINDING OF THE CITED PAPER: The essential finding of the IMBIE team reported in this paper is that in the 27-year study period 1992 to 2018, Greenland lost 3,902 gigatonnes (GT) of ice.  The corresponding annual rate is 144.52 GT/year. If this average rate sustains, the whole of the Greenland Ice Sheet (GIS) will be gone in 18,177.6 years raising global mean sea level (GMSL) by 7,360 mm 18,177.6 years from now at a rate of  0.405 mm/year. The corresponding sea level rise forecast for the year 2100 is 33.6 mm, well short of the IPCC & climate model forecast of 70 to 130 mm [LINK] . It is unlikely that the balance can be provided by Antarctica and other sources. These changes are implicitly attributed to AGW climate change and the fossil fuel emissions of the industrial economy with the implication that they can be moderated with climate action in the form of reducing emissions and eliminating the use of fossil fuels by moving the world’s energy infrastructure to renewables. The ice melt forecast at a millennial time scale will likely be interrupted by the next glaciation as we are now 12,000 years into the Holocene interglacial. The last interglacial, the Eemian, had lasted 15,000 years. It is therefore unlikely that the whole of the GIS will be melted by the Holocene. It is noted that the GIS had survived the Eemian interglacial considered by Paleo climatologists to have been a more violent ice-melt event than the Holocene [LINK]. Yet another consideration is that the paleo climate history of temperature variations in the last 12,000 years of the Holocene shows violent cycles of warming and cooling at millennial and centennial time scales [LINK] and therefore these trends cannot be extrapolated over millennia. 

(2)  SEARCHING FOR CALAMITY: It appears that the finding of a melt rate of 144.5 GT/year with corresponding GMSL rise of 0.405 mm/year was a disappointment to the IMBIE team who might have been looking in the data for something that would provide more alarming evidence of a “climate breakdown” or “climate emergency” or crisis that would serve as the rationale for costly climate action. It is likely for this reason that it became necessary for these scientists to review the data for a more creative presentation that could serve as an emergency with an urgent need for climate action. A look through the 27-year study period 1992-2018 revealed that if the mass balance time series is truncated at 2013, a higher annual average melt rate is found in the shorter 22-year time series 1992-2013. In this early portion of the data, the average annual melt rate is 254 GT/yr. At this rate, the whole of the GIS will be gone in 10,342 years raising sea levels by 0.712 mm/yr or 62.7mm by the year 2100. The corresponding forecast for the year 2100 is a sea level rise of 62.7mm, much closer to the IPCC and climate model forecast of 70 to 130 mm [LINK] with a reasonable possibility that the balance can be provided by Antarctica and other sources. 

(3)  CALAMITY FOUND: The large difference in mean annual melt rate between the 27-year time series (144 GT/yr) and the 22-year time series (254 GT/yr) likely provided the motivation for these scientists to rationalize the use of the shorter 22-year time series of melt data instead of the full span of the available data in the longer 27-year time series of melt data. A rationale was soon found. The scientists determined that the last 5-years of the full span of the data time series contains climate anomalies that must be removed from the data for a purely AGW climate change interpretation of the data. These anomalies are described as (1)Atmospheric circulation favored cooler conditions and ocean temperatures fell at the terminus of Jakobshavn Isbræ during the last 5 years of the sample period and (2)The melt rate is a closer fit to climate model and to IPCC forecasts when the last 5 years are removed from the data time series. Based on these considerations, the IMBIE scientists determined that the shorter time series 1992-2013 must be used to evaluate the impact of AGW climate change on the Greenland Ice Sheet / Accordingly, they determined that the Greenland Ice Sheet is being melted by AGW climate change at a rate of 254 GT/year and that this rate being consistent with the IPCC high emission scenario provides empirical evidence for the urgency of climate action to prevent sea level rise holocaust in the form of tidal floods in low lying regions of the world. 

(4) ERRORS IN THE CALAMITY LOGIC OF IMBIE SCIENTISTS: ERROR#1: FORECAST STATISTICS: For making forecasts, we need as long a time series as possible as the basis for the forecast. In fact, the greater the variance in the data, the longer the data time series needs to be for the forecast being made to the year 2100. The high variance argument does not support the use of a shorter data time series for making the forecast to the year 2100. ERROR#2: CIRCULAR REASONING: A fundamental principle in statistics is that the data used to construct a hypothesis may not be used to test that hypothesis because that involves circular reasoning of the worst form best described as the TEXAS SHARPSHOOTER FALLACY [LINK] The circular reasoning used here are: (1)The shorter time series yields a higher average melt rate that better fits the theory and therefore it must be the better data set with which to test the theory. and (2)The shorter time series is a better fit to the IPCC forecast and therefore the lower melt rate in the longer time series must have an explanation in terms of climate and temperature anomalies and if we look for them we can surely find some climate and temperature anomalies to blame that on.  ERROR#3: CIRCULAR REASONING. REDEFINING CALAMITY TO FIT THE DATA: In response to their failure to provide evidence of catastrophic sea level rise we had been told to fear, climate science has restructured sea level rise fear in terms of the few centimeters of SLR that they have evidence for, so that even low levels of sea level rise can be described as a climate change catastrophe. It is claimed that for every 10mm of sea level rise, 6 million people in low lying coastal areas are put at risk and that therefore we must cut fossil fuel emissions to save these people. This argument is weakened into irrelevance by two considerations. First, the cited study [LINK]  uses eustatic mean sea level against DEM satellite data on coastal land elevation. These data contain a very high level of uncertainty that can create catastrophe out of nothing. Besides if there are only 6 million people for every 10mm rise in sea level, it would be much easier for the rest of the 7,800 million people of the world to take care of the unfortunate coastal lowland dwellers that too give up fossil fuels to reduce their high tide floods. 

SUMMARY

The fear of AGW driven melt of the Greenland Ice Sheet and the resultant sea level rise presented in the IMBIE study cited above is not credible because the study suffers from methodological and statistical weaknesses in the form of circular reasoning.  The idea, derived from uncertain satellite land elevation data that we should fear 10mm of sea level rise is yet another case of circular reasoning that says in effect that if we can’t find the sea level rise we fear we must fear the sea level rise we can find.

A further issue is that the large and incoherent variability of year to year ice melt may have a geothermal heat flux explanation as Greenland sits on a geologically active area as seen in a related post [LINK] . Rather than seek out parts of the time series with lower variance, the authors should study and attempt to understand the apparently random variability in year to year ice melt. A bibliography on geothermal heat under the Greenland ice sheet is provided below.

A second bibliography below implies that the failure to find catastrophic ice melt in Greenland this late in the Holocene may have to do with the finding that most of the interglacial effects on the GrIS have already occurred early in the Holocene. See also [LINK] .

BIBLIOGRAPHY: THE ROLE OF GEOTHERMAL HEAT IN GREENLAND

The studies below show significant impact of basal geothermal heat flux on ice melt

1. Fahnestock, Mark, et al. “High geothermal heat flow, basal melt, and the origin of rapid ice flow in central Greenland.” Science 294.5550 (2001): 2338-2342.  Age-depth relations from internal layering reveal a large region of rapid basal melting in Greenland. Melt is localized at the onset of rapid ice flow in the large ice stream that drains north off the summit dome and other areas in the northeast quadrant of the ice sheet. Locally, high melt rates indicate geothermal fluxes 15 to 30 times continental background. The southern limit of melt coincides with magnetic anomalies and topography that suggest a volcanic origin.
2. Greve, Ralf. “Relation of measured basal temperatures and the spatial distribution of the geothermal heat flux for the Greenland ice sheet.” Annals of Glaciology 42 (2005): 424-432. The thermomechanical, three-dimensional ice-sheet model SICOPOLIS is applied to the Greenland ice sheet. Simulations over two glacial–interglacial cycles are carried out, driven by a climatic forcing interpolated between present conditions and Last Glacial Maximum anomalies. Based on the global heat-flow representation by Pollack and others (1993), we attempt to constrain the spatial pattern of the geothermal heat flux by comparing simulation results to direct measurements of basal temperatures at the GRIP, NorthGRIP, Camp Century and Dye 3 ice-core locations. The obtained heat-flux map shows an increasing trend from west to east, a high-heat-flux anomaly around NorthGRIP with values up to 135 mWm–2 and a low-heat-flux anomaly around Dye 3 with values down to 20 mW m–2. Validation is provided by the generally good fit between observed and measured ice thicknesses. Residual discrepancies are most likely due to deficiencies of the input precipitation rate and further variability of the geothermal heat flux not captured here.
3. Greve, Ralf, and Kolumban Hutter. “Polythermal three-dimensional modelling of the Greenland ice sheet with varied geothermal heat flux.” Annals of Glaciology 21 (1995): 8-12.  Computations over 50 000 years into steady state with Greve’s polythermal ice-sheet model and its numerical code are performed for the Greenland ice sheet with today’s climatological input (surface temperature and accumulation function) and three values of the geothermal heat flux: (42, 54.6, 29.4) mW m−2. It is shown that through the thermo-mechanical coupling the geometry as well as the thermal regime, in particular that close to the bed, respond surprisingly strongly to the basal thermal heat input. The most sensitive variable is the basal temperature field, but the maximum height of the summit also varies by more than ±100m. Furthermore, some intercomparison of the model outputs with the real ice sheet is carried out, showing that the model provides reasonable results for the ice-sheet geometry as well as for the englacial temperatures.
4. van der Veen, Cornelis J., et al. “Subglacial topography and geothermal heat flux: Potential interactions with drainage of the Greenland ice sheet.” Geophysical research letters 34.12 (2007).  Many of the outlet glaciers in Greenland overlie deep and narrow trenches cut into the bedrock. It is well known that pronounced topography intensifies the geothermal heat flux in deep valleys and attenuates this flux on mountains. Here we investigate the magnitude of this effect for two subglacial trenches in Greenland. Heat flux variations are estimated for idealized geometries using solutions for plane slopes derived by Lachenbruch (1968). It is found that for channels such as the one under Jakobshavn Isbræ, topographic effects may increase the local geothermal heat flux by as much as 100%.
5. Dahl-Jensen, Dorthe, et al. “Past temperatures directly from the Greenland ice sheet.” Science 282.5387 (1998): 268-271.  A Monte Carlo inverse method has been used on the temperature profiles measured down through the Greenland Ice Core Project (GRIP) borehole, at the summit of the Greenland Ice Sheet, and the Dye 3 borehole 865 kilometers farther south. The result is a 50,000-year-long temperature history at GRIP and a 7000-year history at Dye 3. The Last Glacial Maximum, the Climatic Optimum, the Medieval Warmth, the Little Ice Age, and a warm period at 1930 A.D. are resolved from the GRIP reconstruction with the amplitudes –23 kelvin, +2.5 kelvin, +1 kelvin, –1 kelvin, and +0.5 kelvin, respectively. The Dye 3 temperature is similar to the GRIP history but has an amplitude 1.5 times larger, indicating higher climatic variability there. The calculated terrestrial heat flow density from the GRIP inversion is 51.3 milliwatts per square meter.
6. Petrunin, A. G., et al. “Heat flux variations beneath central Greenland’s ice due to anomalously thin lithosphere.” Nature Geoscience 6.9 (2013): 746-750.  At the Earth’s surface, heat fluxes from the interior¹ are generally insignificant compared with those from the Sun and atmosphere², except in areas permanently blanketed by ice. Modelling studies show that geothermal heat flux influences the internal thermal structure of ice sheets and the distribution of basal melt water³, and it should be taken into account in planning deep ice drilling campaigns and climate reconstructions⁴. Here we use a coupled ice–lithosphere model driven by climate and show that the oldest and thickest part of the Greenland Ice Sheet is strongly influenced by heat flow from the deep Earth. We find that the geothermal heat flux in central Greenland increases from west to east due to thinning of the lithosphere, which is only about 25–66% as thick as is typical for terrains of early Proterozoic age⁵. Complex interactions between geothermal heat flow and glaciation-induced thermal perturbations in the upper crust over glacial cycles lead to strong regional variations in basal ice conditions, with areas of rapid basal melting adjoining areas of extremely cold basal ice. Our findings demonstrate the role that the structure of the solid Earth plays in the dynamics of surface processes.
7. Brinkerhoff, Douglas J., et al. “Sensitivity of the frozen/melted basal boundary to perturbations of basal traction and geothermal heat flux: Isunnguata Sermia, western Greenland.” Annals of Glaciology 52.59 (2011): 43-50.  A full-stress, thermomechanically coupled, numerical model is used to explore the interaction between basal thermal conditions and motion of a terrestrially terminating section of the west Greenland ice sheet. The model domain is a two-dimensional flowline profile extending from the ice divide to the margin. We use data-assimilation techniques based on the adjoint model in order to optimize the basal traction field, minimizing the difference between modeled and observed surface velocities. We monitor the sensitivity of the frozen/melted boundary (FMB) to changes in prescribed geothermal heat flux and sliding speed by applying perturbations to each of these parameters. The FMB shows sensitivity to the prescribed geothermal heat flux below an upper threshold where a maximum portion of the bed is already melted. The position of the FMB is insensitive to perturbations applied to the basal traction field. This insensitivity is due to the short distances over which longitudinal stresses act in an ice sheet.
8. Tarasov, Lev, and W. Richard Peltier. “Greenland glacial history, borehole constraints, and Eemian extent.” Journal of Geophysical Research: Solid Earth 108.B3 (2003).  We examine the extent to which observations from the Greenland ice sheet combined with three‐dimensional dynamical ice sheet models and semi‐Lagrangian tracer methods can be used to constrain inferences of the Eemian evolution of the ice sheet, of the extent and frequency of summit migration during the 100 kyr ice age cycle, and of the deep geothermal flux of heat from the Earth into the base of the ice sheet. Relative sea level, present‐day surface geometry, basal temperature, and age and temperature profiles from the Greenland Ice Project (GRIP) are imposed as constraints to tune ice sheet model and climate forcing parameters. Despite the paucity of observations, model‐based inferences suggest a significant northeast gradient in geothermal heat flux. Our analyses also suggest that during the glacial cycle, the contemporaneous summit only occupied the present‐day location during interglacial periods. On the basis of the development and use of a high‐resolution semi‐Lagrangian tracer analysis methodology for δ¹⁸O, we rule out isotropic flow disturbances due to summit migration as a possible source of the high Eemian variability of the GRIP δ¹⁸O record. Finally, in contrast with results obtained in some recent attempts to infer the extent to which Greenland may have contributed to the anomalous highstand of Eemian sea level, we find that conservative bounds for this contribution are 2–5.2 m, with a more likely range of 2.7–4.5 m.

BIBLIOGRAPHY : A HISTORY OF THE GREENLAND ICE SHEET

The bibliography shows that the significant changes to the GrIS expected from AGW climate change occurred early in the Holocene but not since then. 

1. Funder, Svend, et al. “The Greenland Ice Sheet during the past 300,000 years: A review.” Developments in Quaternary Sciences. Vol. 15. Elsevier, 2011. 699-713.   The Greenland ice sheet‘s response to climate change is a major issue in the climate debate. This report reviews existing evidence on how the ice sheet margins reacted to climate change during the past 300,000 years—how it responded to the warm climate of the last interglacial and expanded on to the shelf during the last ice age. Compared to the other large ice sheets in the northern hemisphere, the Greenland ice sheet showed remarkable resilience to temperature change—a good omen for the future.  [FULL TEXT]. 
2. Ó Cofaigh, C., et al. “An extensive and dynamic ice sheet on the West Greenland shelf during the last glacial cycle.” Geology 41.2 (2013): 219-222.  Considerable uncertainty surrounds the extent and timing of the advance and retreat of the Greenland Ice Sheet (GIS) on the continental shelf bordering Baffin Bay during the last glacial cycle. Here we use marine geophysical and geological data to show that fast-flowing ice sheet outlets, including the ancestral Jakobshavn Isbræ, expanded several hundred kilometers to the shelf edge during the last glaciation ca. 20 ka. Retreat of these outlets was asynchronous. Initial retreat from the shelf edge was underway by 14,880 calibrated (cal) yr B.P. in Uummannaq trough. Radiocarbon dates from the adjacent Disko trough and adjoining trough-mouth fan imply later deglaciation of Jakobshavn Isbræ, and, significantly, an extensive readvance and rapid retreat of this outlet during the Younger Dryas stadial (YD). This is notable because it is the first evidence of a major advance of the GIS during the YD on the West Greenland shelf, although the short duration suggests that it may have been out of phase with YD temperatures. [FULL TEXT]
3. Jennings, Anne E., et al. “Paleoenvironments during Younger Dryas‐E arly Holocene retreat of the Greenland Ice Sheet from outer Disko Trough, central west Greenland.” Journal of Quaternary Science 29.1 (2014): 27-40.  Paleo-environments during the late Younger Dryas through early Holocene retreat of the Greenland Ice Sheet from the outer shelf in the Disko Trough system of central West Greenland were investigated via lithofacies, foraminifera, dinocysts and sediment provenance analyses in radiocarbon‐dated sediment cores from the upper slope (JR175‐VC35) and outer shelf (JR175‐VC20 and HU2008029‐070CC). Core data show that the ice margin retreated rapidly from the outer shelf by calving, beginning by 12.2k cal a BP under cold paleoceanographic conditions with up to 11 months of sea‐ice. Ice retreat into Disko Bugt was well underway by 10.9k cal a BP. Enhanced ice‐sheet ablation in Disko Bugt and elsewhere along the West Greenland coast is inferred from cold glacial marine conditions associated with high sedimentation rates between 10.9 and 9.5k cal a BP on the outer shelf. Glacial marine conditions are recorded on the outer shelf until 7.8k cal a BP. Detrital carbonate‐bearing sediments rich in >2‐mm clasts deposited between 11.6 and 10.6 k cal a BP indicate that icebergs calved from northern Baffin Bay ice margins were melting and releasing sediments along West Greenland while the Greenland Ice Sheet margin was retreating into Disko Bugt. [FULL TEXT]
4. Lecavalier, Benoit S., et al. “A model of Greenland ice sheet deglaciation constrained by observations of relative sea level and ice extent.” Quaternary Science Reviews 102 (2014): 54-84.  An ice sheet model was constrained to reconstruct the evolution of the Greenland Ice Sheet (GrIS) from the Last Glacial Maximum (LGM) to present to improve our understanding of its response to climate change. The study involved applying a glaciological model in series with a glacial isostatic adjustment and relative sea-level (RSL) model. The model reconstruction builds upon the work of Simpson et al. (2009) through four main extensions: (1) a larger constraint database consisting of RSL and ice extent data; model improvements to the (2) climate and (3) sea-level forcing components; (4) accounting for uncertainties in non-Greenland ice. The research was conducted primarily to address data-model misfits and to quantify inherent model uncertainties with the Earth structure and non-Greenland ice. Our new model (termed Huy3) fits the majority of observations and is characterised by a number of defining features. During the LGM, the ice sheet had an excess of 4.7 m ice-equivalent sea-level (IESL), which reached a maximum volume of 5.1 m IESL at 16.5 cal ka BP. Modelled retreat of ice from the continental shelf progressed at different rates and timings in different sectors. Southwest and Southeast Greenland began to retreat from the continental shelf by ∼16 to 14 cal ka BP, thus responding in part to the Bølling-Allerød warm event (c. 14.5 cal ka BP); subsequently ice at the southern tip of Greenland readvanced during the Younger Dryas cold event. In northern Greenland the ice retreated rapidly from the continental shelf upon the climatic recovery out of the Younger Dryas to present-day conditions. Upon entering the Holocene (11.7 cal ka BP), the ice sheet soon became land-based. During the Holocene Thermal Maximum (HTM; 9-5 cal ka BP), air temperatures across Greenland were marginally higher than those at present and the GrIS margin retreated inland of its present-day southwest position by 40–60 km at 4 cal ka BP which produced a deficit volume of 0.16 m IESL relative to present. In response to the HTM warmth, our optimal model reconstruction lost mass at a maximum centennial rate of c. 103.4 Gt/yr. Our results suggest that remaining data-model discrepancies are affiliated with missing physics and sub-grid processes of the glaciological model, uncertainties in the climate forcing, lateral Earth structure, and non-Greenland ice (particularly the North American component). Finally, applying the Huy3 Greenland reconstruction with our optimal Earth model we generate present-day uplift rates across Greenland due to past changes in the ocean and ice loads with explicit error bars due to uncertainties in the Earth structure. Present-day uplift rates due to past changes are spatially variable and range from 3.5 to −7 mm/a (including Earth model uncertainty). [FULL TEXT]
5. Larsen, Nicolaj K., et al. “Rapid early Holocene ice retreat in West Greenland.” Quaternary Science Reviews 92 (2014): 310-323.  The possible demise of the Greenland ice sheet and its effect on global sea level rank among the most serious climate threats to society. To improve our knowledge about the future behaviour of the ice margin, we studied the ice sheet’s response to early Holocene warming in West Greenland using 47 cosmogenic ¹⁰Be exposure ages, 26 optically-stimulated luminescence ages as well as 15 new and 28 previously published radiocarbon ages. Paired bedrock and boulder ages show that the entire area was covered by warm-based ice during the Last Glacial Maximum (LGM), although glacial erosion was insufficient to completely remove the upper rock surface containing ¹⁰Be inherited from a previous period of exposure in bedrock samples above an elevation of 800 m. Our compilation of ¹⁰Be and ¹⁴C ages demonstrates that the ice sheet retreated from the outer-coast to the present ice margin between c. 11.4 and 10.4 cal. ka BP in the Godthåbsfjord system and between 10.7 ± 0.6 and 10.1 ± 0.4 ka ago in Buksefjord, whereas the coast at Sermilik became ice free at c. 10.5 cal. ka BP. We find no significant changes in the retreat rates between the deep Godthåbsfjord system and the Buksefjord-Sermilik region, which is characterized by only a few narrow and shallow fjords. However, deglaciation was initiated c. 700–900 years earlier in the Godthåbsfjord system indicating that the deep fjords probably triggered land-based deglaciation by dynamic ice loss leading to an overall rapid early Holocene ice retreat and drawdown of the ice sheet in West Greenland. These results demonstrate that even if there was a topographic control on the onset of deglaciation, fast ice retreat is not restricted to deep fjord systems but may occur independently of the topographic setting. [FULL TEXT]
6. Young, Nicolás E., et al. “Age of the Fjord Stade moraines in the Disko Bugt region, western Greenland, and the 9.3 and 8.2 ka cooling events.” Quaternary Science Reviews 60 (2013): 76-90.  Retreat of the western Greenland Ice Sheet during the early Holocene was interrupted by deposition of the Fjord Stade moraine system. The Fjord Stade moraine system spans several hundred kilometers of western Greenland’s ice-free fringe and represents an important period in the western Greenland Ice Sheet’s deglaciation history, but the origin and timing of moraine deposition remain uncertain. Here, we combine new and previously published ¹⁰Be and ¹⁴C ages from Disko Bugt, western Greenland to constrain the timing of Fjord Stade moraine deposition at two locations ∼60 km apart. At Jakobshavn Isfjord, the northern of two study sites, we show that Jakobshavn Isbræ advanced to deposit moraines ca 9.2 and 8.2–8.0 ka. In southeastern Disko Bugt, the ice sheet deposited moraines ca 9.4–9.0 and 8.5–8.1 ka. Our ice-margin chronology indicates that the Greenland Ice Sheet in two distant regions responded in unison to early Holocene abrupt cooling 9.3 and 8.2 ka, as recorded in central Greenland ice cores. Although the timing of Fjord Stade moraine deposition was synchronous in Jakobshavn Isfjord and southeastern Disko Bugt, within uncertainties, we suggest that Jakobshavn Isbræ advanced while the southeastern Disko Bugt ice margin experienced stillstands during the 9.3 and 8.2 ka events based on regional geomorphology and the distribution of ¹⁰Be ages at each location. The contrasting style of ice-margin response was likely regulated by site-specific ice-flow characteristics. Jakobshavn Isbræ’s high ice flux results in an amplified ice-margin response to a climate perturbation, both warming and cooling, whereas the comparatively low-flux sector of the ice sheet in southeastern Disko Bugt experiences a more subdued response to climate perturbations. Our chronology indicates that the western Greenland Ice Sheet advanced and retreated in concert with early Holocene temperature variations, and the 9.3 and 8.2 ka events, although brief, were of sufficient duration to elicit a significant response of the western Greenland Ice Sheet. [FULL TEXT]
7. Roberts, David H., et al. “New constraints on Greenland ice sheet dynamics during the last glacial cycle: evidence from the Uummannaq ice stream system.” Journal of Geophysical Research: Earth Surface 118.2 (2013): 519-541.  This paper presents the first assessment of the Uummannaq ice stream system (UISS) in West Greenland. The UISS drained ~6% of the Greenland ice sheet (GrIS) at the Last Glacial Maximum (LGM). The onset of the UISS is a function of a convergent network of fjords which feed a geologically controlled trough system running offshore to the shelf break. Mapping, cosmogenic radiogenic nuclide (CRN) dating, and model output reveal that glacially scoured surfaces up to 1266 m above sea level (asl) in fjord‐head areas were produced by warm‐based ice moving offshore during the LGM, with the elevation of warm‐based ice dropping westwards to ~700 m asl as the ice stream trunk zone developed. Marginal plateaux with allochthonous blockfields suggest that warm‐based ice produced till and erratics up to ~1200 m asl, but CRN ages and weathering pits suggest this was pre‐LGM, with only cold‐based ice operating during the LGM. Deglaciation began on the outer shelf at ~14.8 cal. kyrs B.P., with Ubekendt Ejland becoming ice free at ~12.4 ka. The UISS then collapsed with over 100 km of retreat by ~11.4 ka–10.8 cal. kyrs B.P., a rapid and complex response to bathymetric deepening, trough widening, and sea‐level rise coinciding with rapidly increasing air temperatures and solar radiation, but which occurred prior to ocean warming at ~8.4 cal. kyrs B.P. Local fjord constriction temporarily stabilized the unzipped UISS margins at the start of the Holocene before ice retreat inland of the current margin at ~8.7 ka. [FULL TEXT]
8. Knutz, Paul C., et al. “Multiple‐stage deglacial retreat of the southern Greenland Ice Sheet linked with Irminger Current warm water transport.” Paleoceanography 26.3 (2011).  There is limited knowledge pertaining to the history of the Greenland Ice Sheet (GIS) during the last glacial‐interglacial transition as it retreated from the continental margins to an inland position. Here we use multiproxy data, including ice‐rafted debris (IRD); planktonic isotopes; alkenone temperatures; and tephra geochemistry from the northern Labrador Sea, off southwest Greenland, to investigate the deglacial response of the GIS and evaluate its implications for the North Atlantic deglacial development. The results imply that the southern GIS retreated in three successive stages: (1) early deglaciation of the East Greenland margins, by tephra‐rich IRD that embrace Heinrich Event 1; (2) progressive retreat during Allerød culminating in major meltwater releases (δ¹⁸O depletion of 1.2‰) at the Allerød–Younger Dryas transition (12.8–13.0 kyr B.P.); and (3) a final stage of glacial recession during the early Holocene (∼9–11 kyr B.P.). Rather than indicating local temperatures of ambient surface water, the alkenones likely were transported to the core site by the Irminger Current. We attribute the timing of GIS retreat to the incursion of warm intermediate waters along the base of grounded glaciers and below floating ice shelves on the continental margin. The results lend support to the view that GIS meltwater presented a forcing factor for the Younger Dryas cooling. [FULL TEXT]
9. Young, Nicolás E., et al. “Response of Jakobshavn Isbræ, greenland, to Holocene climate change.” Geology 39.2 (2011): 131-134.  Rapid fluctuations in the velocity of Greenland Ice Sheet (GIS) outlet glaciers over the past decade have made it difficult to extrapolate ice-sheet change into the future. This significant short-term variability highlights the need for geologic records of preinstrumental GIS margin fluctuations in order to better predict future GIS response to climate change. Using ¹⁰Be surface exposure ages and radiocarbon-dated lake sediments, we constructed a detailed chronology of ice-margin fluctuations over the past 10 k.y. for Jakobshavn Isbræ, Greenland’s largest outlet glacier. In addition, we present new estimates of corresponding local temperature changes using a continuous record of insect (Chironomidae) remains preserved in lake sediments. We find that following an early Holocene advance just prior to 8 ka, Jakobshavn Isbræ retreated rapidly at a rate of ∼100 m yr⁻¹, likely in response to increasing regional and local temperatures. Ice remained behind its present margin for ∼7 k.y. during a warm period in the middle Holocene with sustained temperatures ∼2 °C warmer than today, then the land-based margin advanced at least 2–4 km between A.D. 1500–1640 and A.D. 1850. The ice margin near Jakobshavn thus underwent large and rapid adjustments in response to relatively modest centennial-scale Holocene temperature changes, which may foreshadow GIS response to future warming.