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THIS POST IS A CRITICAL REVIEW OF THE CLIMATE CHANGE RESEARCH PAPER {History, mass loss, structure, and dynamic behavior of the Antarctic Ice Sheet, Robin E. Bell1,*, Helene Seroussi2 See all authors and affiliations Science 20 Mar 2020: Vol. 367, Issue 6484, pp. 1321-1325 DOI: 10.1126/science.aaz5489} ABSTRACT  (abbreviated) {Satellite measurements show that several regions of Antarctica where ice is exposed to warm ocean waters are losing mass, flowing faster, and retreating.  Antarctica’s contribution to sea level rise since 1992 is now 8 millimeters. A continuation of warming ocean waters and increased surface meltwater will trigger faster ice flow, and sea level rise will accelerate.

ABOUT ROBIN ELIZABETH BELL

The lead author of the paper is Professor Robin Bell, Earth Science Professor at Columbia University. Her research emphasis has been Antarctica ice melt and sea level rise with a focus on ice shelves [LINK] . She says that she studies “evidence for changing ice” and that her wake-up call came when the Larsen B ice shelf collapsed in 2002 because “The Antarctic Peninsula is where global temperatures have risen the most, more than 7°F  in 50 years“. The focus of her work is catastrophic sea level rise by way of AGW driven ice melt in Antarctica. This unique career may have been the result of her emotional reaction to the spectacle of the Larsen B ice shelf collapse. She has written about this event profusely and described the event as an AGW horror and a product of fossil fuel emissions. 

CLAIM=WHAT THE PAPER SAYS. RESPONSE=CRITICAL EVALUATION OF THE CLAIM

CLAIM:  GRACE data show mass loss in West Antarctica, focused in the Amundsen and Bellingshausen Sea sectors, and mass gain in some regions of East Antarctica and along Kamb Ice Stream.  RESPONSE:  This well known pattern of ice melt in Antarctica is described in a related post [LINK] where the difference is interpreted in terms of rocky support for East Antarctic Ice versus oceanic bottoms of West Antarctic glaciers and in terms of geological activity in West Antarctica with the West Antarctic Rift system and the Marie Byrd Mantle Plume cited as the basis for that assessment.

CLAIM: Lowering of the surface elevation has been measured over the 25-year altimetry period and it is pronounced in the Amundsen and Bellingshausen Sea sectors of West Antarctica and Wilkes Land in East Antarctica. Pine Island, Thwaites, and Smith-Pope-Kohler Glaciers experienced the greatest elevation drop over this period, with changes of up to 9 m/year. Mass gain in East Antarctica with rising elevation is attributed to increased snowfall.  RESPONSE:  The pattern of surface ice loss is consistent with its geological interpretation described in the related post cited above [LINK] and inconsistent with a uniform atmospheric cause thought to be driven by the rising greenhouse effect of fossil fuel emissions such that the ice loss in Antarctica and its sea level effects can be attenuated with climate action in the form of reducing fossil fuel emissions.

CLAIM: Velocity measurements show that changes in ice sheet velocity are striking in the Peninsula, where a substantial acceleration of glaciers feeding the Larsen ice shelves was observed after their collapse, as well as in the Amundsen Sea sector. In this region, Pine Island Glacier’s velocity doubled from the 1990s to the 2010s while its grounding line retreated by more than 30 km. The velocity observations are used to calculate the flux of ice discharge into the ocean.  RESPONSE: The Antarctic Peninsula and the Pine Island Glacier are both subject to intense geological activity and geothermal heat. That the acceleration is found to be localized to these geothermal hot spots does not imply a uniform atmospheric cause by way of fossil fuel emissions that can be attenuated by reducing fossil fuel emissions.

CLAIM:  Between 1950 and 2000, the average air temperature in the Peninsula increased by 4°C. During this warming period, the Larsen A and B ice shelves collapsed in 1995 and 2002, respectively. The glaciers feeding the Larsen B Ice Shelf sped up after the loss of the backward stress or buttressing. Before the Larsen B collapse, the ice shelf surface was covered by lakes, indicating that warming air temperatures and surface meltwater can destabilize ice shelves, leading to faster flow of Antarctic ice into the global oceans and highlighting the protecting role of ice shelves.  RESPONSE: Monthly mean mid troposphere temperatures from October 1994 to April 2002 for South Polar land and ocean surfaces are shown in the chart below. No significant warming is seen that would explain meltwater lakes on the top of the ice shelves either in 1995 or in 2002. Therefore, the localized warming trend found only on the Antarctic Peninsula in the period 1950-2000 does not have an AGW climate change interpretation such that it can be attributed to fossil fuel emissions and that it could have been prevented with climate action in the form of reducing emissions. This tiny corner of Antarctica is known to be extremely geologically active by way of the West Antarctic Rift system below it and is also known for the prevalence of katabatic (foehn) winds that can create melt ponds on ice shelves as seen in the bibliography below . The more likely cause of this localized warming event and ice shelf collapse is therefore a localized warming phenomenon such as geothermal heat and katabatic winds both of which are found in the Antarctic Peninsula and neither of which has an AGW climate change interpretation.

GEOTHERMAL HEAT CURIE DEPTH IN ANTARCTICA 

CLAIM: These remote sensing observations allow scientists to observe ice sheet changes and decipher the causes of such changes. Both the ocean surrounding Antarctica and the atmosphere, especially in the Peninsula region, have warmed over the 25-year observational record of ice change. Antarctica is losing most of its mass through increased ice flow of the outlet glaciers and ice streams. This contrasts with the Greenland Ice Sheet, where half of the loss is due to faster ice flow and half is due to increased melting of the ice sheet surface (35). Surface melt is not yet a major contributor to ice loss in Antarctica, and global climate models suggest that an increase in snowfall in East Antarctica could partially offset the dynamic mass loss (36). Although these changes have been ongoing for the past three decades, more rapid and dramatic mass loss cannot be excluded. The marine portions of the ice sheet with subglacial topography that deepens inland and glaciers with thick marine terminating fronts are prone to instabilities (37, 38).  RESPONSE: Remote sensing observations do allow scientists to observe ice sheet changes but they do not contain information that the cause of those changes is global warming particularly so when these changes are highly localized anomalies in a tiny corner of Antarctica known for high geothermal heat flux as shown in the image above. These observations tell us only that Antarctica is losing ice. They do not tell us that the ice loss is driven by fossil fuel emissions and that they can be attenuated by reducing fossil fuel emissions.

CLAIM:  Although the surface waters surrounding Antarctica are cold, the underlying waters of the Circumpolar Deep Water are warmer and can influence the ice sheet when they reach the ice shelves and grounding lines, where the ice becomes afloat. The concentration of changes in West Antarctica points to the dominant role the warming ocean plays in recently observed change (39, 40).  RESPONSE: With regard to the anomalous warmth of the Circumpolar Deep Water Circulation (CDWC), it is noted that in a highly geologically active area such as West Antarctica and the Antarctic Ocean in general, a role for geothermal heat cannot be ignored. Recent research papers on the CDWC have pointed to significant geothermal heat sources on the sea floor that offer a more ready explanation for the warmth of the CDWC as can be seen in the papers cited in the bibliography below. The geological activity In West Antarctica and the Antarctic Ocean are described in a related post [LINK] where we see the important role of the West Antarctic Rift system and the Marie Byrd Mantle Plume in the interpretation ocean temperature and ice melt in this region. The relative warmth of the CDWC is not evidence of a “warming ocean” in terms of AGW climate change. The CDWC is warmer than surface water but that warmth does not contain a long term warming trend. 

RELEVANT BIBLIOGRAPHY

1. Bouzette, Ariane, and Roland Souchez. “Katabatic wind influence on meltwater supply to fuel glacier–substrate interactions at the grounding line, Terra Nova Bay, East Antarctica.” Annals of glaciology 28 (1999): 272-276. The co-isotopic composition, both in δDand in δ 18O, of interbedded debris-rich and clear ice layers, thought to have been formed at the grounding line of Hells Gate Ice Shelf, indicates freezing by a double-diffusion effect between continental meltwater and sea water within a subglacial sediment. A source of meltwater is required to sustain the process, since the temperature of the bed is below the freshwater melting point. The most likely source is a surficially frozen meltwater lake. Rock outcrops, kept mostly snow-free by the action of katabatic winds, absorb solar radiation so local production of liquid water becomes possible in an area with year-round subfreezing air-temperature conditions. The meltwater accumulated in a marginal lake can eventually reach the subglacial substratum near a pinning point where the ice is relatively thin and fractured.
2. G. L. Lyon &W. F. Giggenbach, Geothermal activity in Victoria Land, Antarctica. Pages 511-521 | Received 23 Oct 1973, Published online: 07 Jan 2012. Fumarolic ice towers and areas of steaming ground are the only surface manifestations of geothermal activity near the summits of Mounts Melbourne and Erebus, Victoria Land, Antarctica. The distribution, nature, and limited range of the geothermal features reflect the lack of liquid water in the normal environment of these volcanoes, where modes of heat transport are confined to conduction and convection of air and water vapour. Under such conditions, only localised occurrence of liquid water is considered possible.
3. Fountain, Andrew G., et al. “Evolution of cryoconite holes and their contribution to meltwater runoff from glaciers in the McMurdo Dry Valleys, Antarctica.” Journal of Glaciology 50.168 (2004): 35-45.  Cryoconite holes are water-filled holes in the surface of a glacier caused by enhanced ice melt around trapped sediment. Measurements on the ablation zones of four glaciers in Taylor Valley, Antarctica, show that cryoconite holes cover about 4–6% of the ice surface. They typically vary in diameter from 5 to 145 cm, with depths ranging from 4 to 56 cm. In some cases, huge holes form with 5 m depths and 30 m diameters. Unlike cryoconite holes elsewhere, these have ice lids up to 36 cm thick and melt from within each spring. About one-half of the holes are connected to the near-surface hydrologic system and the remainder are isolated. The duration of isolation, estimated from the chloride accumulation in hole waters, commonly shows ages of several years, with one hole of 10 years. The cryoconite holes in the McMurdo Dry Valleys create a near-surface hydrologic system tens of cm below the ice surface. The glacier surface itself is generally frozen and dry. Comparison of water levels between holes a few meters apart shows independent cycles of water storage and release. Most likely, local freeze–thaw effects control water passage and therefore temporary storage. Rough calculations indicate that the holes generate at least 13% of the observed runoff on the one glacier measured. This hydrologic system represents the transition between a melting ice cover with supraglacial streams and one entirely frozen and absent of water.
4. Banwell, Alison F., Douglas R. MacAyeal, and Olga V. Sergienko. “Breakup of the Larsen B Ice Shelf triggered by chain reaction drainage of supraglacial lakes.” Geophysical Research Letters 40.22 (2013): 5872-5876.  The explosive disintegration of the Larsen B Ice Shelf poses two unresolved questions: What process (1) set a horizontal fracture spacing sufficiently small to predispose the subsequent ice shelf fragments to capsize and (2) synchronized the widespread drainage of >2750 supraglacial meltwater lakes observed in the days prior to break up? We answer both questions through analysis of the ice shelf’s elastic flexure response to the supraglacial lakes on the ice shelf prior to break up. By expanding the previously articulated role of lakes beyond mere water reservoirs supporting hydrofracture, we show that lake‐induced flexural stresses produce a fracture network with appropriate horizontal spacing to induce capsize‐driven breakup. The analysis of flexural stresses suggests that drainage of a single lake can cause neighboring lakes to drain, which, in turn, causes farther removed lakes to drain. Such self‐stimulating behavior can account for the sudden, widespread appearance of a fracture system capable of driving explosive break up.
5. Luckman, Adrian, et al. “Surface melt and ponding on Larsen C Ice Shelf and the impact of föhn winds.” Antarctic Science 26.6 (2014): 625-635.  A common precursor to ice shelf disintegration, most notably that of Larsen B Ice Shelf, is unusually intense or prolonged surface melt and the presence of surface standing water. However, there has been little research into detailed patterns of melt on ice shelves or the nature of summer melt ponds. We investigated surface melt on Larsen C Ice Shelf at high resolution using Envisat advanced synthetic aperture radar (ASAR) data and explored melt ponds in a range of satellite images. The improved spatial resolution of SAR over alternative approaches revealed anomalously long melt duration in western inlets. Meteorological modelling explained this pattern by föhn winds which were common in this region. Melt ponds are difficult to detect using optical imagery because cloud-free conditions are rare in this region and ponds quickly freeze over, but can be monitored using SAR in all weather conditions. Melt ponds up to tens of kilometres in length were common in Cabinet Inlet, where melt duration was most prolonged. The pattern of melt explains the previously observed distribution of ice shelf densification, which in parts had reached levels that preceded the collapse of Larsen B Ice Shelf, suggesting a potential role for föhn winds in promoting unstable conditions on ice shelves. [FULL TEXT] 
6. Fisher, Andrew T., et al. “High geothermal heat flux measured below the West Antarctic Ice Sheet.” Science advances 1.6 (2015): e1500093.  The geothermal heat flux is a critical thermal boundary condition that influences the melting, flow, and mass balance of ice sheets, but measurements of this parameter are difficult to make in ice-covered regions. We report the first direct measurement of geothermal heat flux into the base of the West Antarctic Ice Sheet (WAIS), below Subglacial Lake Whillans, determined from the thermal gradient and the thermal conductivity of sediment under the lake. The heat flux at this site is 285 ± 80 mW/m², significantly higher than the continental and regional averages estimated for this site using regional geophysical and glaciological models. Independent temperature measurements in the ice indicate an upward heat flux through the WAIS of 105 ± 13 mW/m². The difference between these heat flux values could contribute to basal melting and/or be advected from Subglacial Lake Whillans by flowing water. The high geothermal heat flux may help to explain why ice streams and subglacial lakes are so abundant and dynamic in this region.
7. Hubbard, Bryn, et al. “Massive subsurface ice formed by refreezing of ice-shelf melt ponds.” Nature communications 7.1 (2016): 1-6.  Surface melt ponds form intermittently on several Antarctic ice shelves. Although implicated in ice-shelf break up, the consequences of such ponding for ice formation and ice-shelf structure have not been evaluated. Here we report the discovery of a massive subsurface ice layer, at least 16 km across, several kilometres long and tens of metres deep, located in an area of intense melting and intermittent ponding on Larsen C Ice Shelf, Antarctica. We combine borehole optical televiewer logging and radar measurements with remote sensing and firn modelling to investigate the layer, found to be ∼10 °C warmer and ∼170 kg m⁻³ denser than anticipated in the absence of ponding and hitherto used in models of ice-shelf fracture and flow. Surface ponding and ice layers such as the one we report are likely to form on a wider range of Antarctic ice shelves in response to climatic warming in forthcoming decades.
8. Lenaerts, J. T. M., et al. “Meltwater produced by wind–albedo interaction stored in an East Antarctic ice shelf.” Nature climate change 7.1 (2017): 58-62.  Surface melt and subsequent firn air depletion can ultimately lead to disintegration of Antarctic ice shelves^1,2 causing grounded glaciers to accelerate³ and sea level to rise. In the Antarctic Peninsula, foehn winds enhance melting near the grounding line⁴, which in the recent past has led to the disintegration of the most northerly ice shelves^5,6. Here, we provide observational and model evidence that this process also occurs over an East Antarctic ice shelf, where meltwater-induced firn air depletion is found in the grounding zone. Unlike the Antarctic Peninsula, where foehn events originate from episodic interaction of the circumpolar westerlies with the topography, in coastal East Antarctica high temperatures are caused by persistent katabatic winds originating from the ice sheet’s interior. Katabatic winds warm and mix the air as it flows downward and cause widespread snow erosion, explaining >3 K higher near-surface temperatures in summer and surface melt doubling in the grounding zone compared with its surroundings. Additionally, these winds expose blue ice and firn with lower surface albedo, further enhancing melt. The in situ observation of supraglacial flow and englacial storage of meltwater suggests that ice-shelf grounding zones in East Antarctica, like their Antarctic Peninsula counterparts, are vulnerable to hydrofracturing. [FULL TEXT]
9. Bell, Robin E., et al. “Antarctic ice shelf potentially stabilized by export of meltwater in surface river.” Nature 544.7650 (2017): 344-348.  Meltwater stored in ponds¹ and crevasses can weaken and fracture ice shelves, triggering their rapid disintegration². This ice-shelf collapse results in an increased flux of ice from adjacent glaciers³ and ice streams, thereby raising sea level globally⁴. However, surface rivers forming on ice shelves could potentially export stored meltwater and prevent its destructive effects. Here we present evidence for persistent active drainage networks—interconnected streams, ponds and rivers—on the Nansen Ice Shelf in Antarctica that export a large fraction of the ice shelf’s meltwater into the ocean. We find that active drainage has exported water off the ice surface through waterfalls and dolines for more than a century. The surface river terminates in a 130-metre-wide waterfall that can export the entire annual surface melt over the course of seven days. During warmer melt seasons, these drainage networks adapt to changing environmental conditions by remaining active for longer and exporting more water. Similar networks are present on the ice shelf in front of Petermann Glacier, Greenland, but other systems, such as on the Larsen C and Amery Ice Shelves, retain surface water at present. The underlying reasons for export versus retention remain unclear. Nonetheless our results suggest that, in a future warming climate, surface rivers could export melt off the large ice shelves surrounding Antarctica—contrary to present Antarctic ice-sheet models¹, which assume that meltwater is stored on the ice surface where it triggers ice-shelf disintegration.
10. Kingslake, Jonathan, et al. “Widespread movement of meltwater onto and across Antarctic ice shelves.” Nature 544.7650 (2017): 349-352.  Surface meltwater drains across ice sheets, forming melt ponds that can trigger ice-shelf collapse^1,2, acceleration of grounded ice flow and increased sea-level rise^3,4,5. Numerical models of the Antarctic Ice Sheet that incorporate meltwater’s impact on ice shelves, but ignore the movement of water across the ice surface, predict a metre of global sea-level rise this century⁵ in response to atmospheric warming⁶. To understand the impact of water moving across the ice surface a broad quantification of surface meltwater and its drainage is needed. Yet, despite extensive research in Greenland^7,8,9,10 and observations of individual drainage systems in Antarctica^{10,11,12,13,14,15,16,17}, we have little understanding of Antarctic-wide surface hydrology or how it will evolve. Here we show widespread drainage of meltwater across the surface of the ice sheet through surface streams and ponds (hereafter ‘surface drainage’) as far south as 85° S and as high as 1,300 metres above sea level. Our findings are based on satellite imagery from 1973 onwards and aerial photography from 1947 onwards. Surface drainage has persisted for decades, transporting water up to 120 kilometres from grounded ice onto and across ice shelves, feeding vast melt ponds up to 80 kilometres long. Large-scale surface drainage could deliver water to areas of ice shelves vulnerable to collapse, as melt rates increase this century. While Antarctic surface melt ponds are relatively well documented on some ice shelves, we have discovered that ponds often form part of widespread, large-scale surface drainage systems. In a warming climate, enhanced surface drainage could accelerate future ice-mass loss from Antarctic, potentially via positive feedbacks between the extent of exposed rock, melting and thinning of the ice sheet.
11. Mashayek, A., et al. “The role of the geothermal heat flux in driving the abyssal ocean circulation.” Geophysical Research Letters 40.12 (2013): 3144-3149. [FULL TEXT]  The results presented in this paper demonstrate that the geothermal heat flux (GHF) from the solid Earth into the ocean plays a non‐negligible role in determining both abyssal stratification and circulation strength. Based upon an ocean data set, we show that the map of upward heat flux at the ocean floor is consistent (within a factor of 2) with the ocean floor age‐dependent map of GHF. The observed buoyancy flux above the ocean floor is consistent with previous suggestions that the GHF acts to erode the abyssal stratification and thereby enhances the strength of the abyssal circulation. Idealized numerical simulations are performed using a zonally averaged single‐basin model which enables us to address the GHF impact as a function of the depth dependence of diapycnal diffusivity. We show that ignoring this vertical variation leads to an under‐prediction of the influence of the GHF on the abyssal circulation. Independent of the diffusivity profile, introduction of the GHF in the model leads to steepening of the Southern Ocean isopycnals and to strengthening of the eddy‐induced circulation and the Antarctic bottom water cell. The enhanced circulation ventilates the GHF derived heating to shallow depths, primarily in the Southern Ocean.
12. Downes, Stephanie M., et al.  [LINK] “The transient response of southern ocean circulation to geothermal heating in a global climate model.” Journal of Climate 29.16 (2016): 5689-5708.  Model and observational studies have concluded that geothermal heating significantly alters the global overturning circulation and the properties of the widely distributed Antarctic Bottom Water. Here two distinct geothermal heat flux datasets are tested under different experimental designs in a fully coupled model that mimics the control run of a typical Coupled Model Intercomparison Project (CMIP) climate model. The regional analysis herein reveals that bottom temperature and transport changes, due to the inclusion of geothermal heating, are propagated throughout the water column, most prominently in the Southern Ocean, with the background density structure and major circulation pathways acting as drivers of these changes. While geothermal heating enhances Southern Ocean abyssal overturning circulation by 20%–50%, upwelling of warmer deep waters and cooling of upper ocean waters within the Antarctic Circumpolar Current (ACC) region decrease its transport by 3–5 Sv (1 Sv = 10⁶ m³ s⁻¹). The transient responses in regional bottom temperature increases exceed 0.1°C. The large-scale features that are shown to transport anomalies far from their geothermal source all exist in the Southern Ocean. Such features include steeply sloping isopycnals, weak abyssal stratification, voluminous southward flowing deep waters and exported bottom waters, the ACC, and the polar gyres. Recently the Southern Ocean has been identified as a prime region for deep ocean warming; geothermal heating should be included in climate models to ensure accurate representation of these abyssal temperature changes.
13. Barnes, Jowan M., et al. “Idealised modelling of ocean circulation driven by conductive and hydrothermal fluxes at the seabed.” Ocean Modelling 122 (2018): 26-35. [FULL TEXT]   Geothermal heating is increasingly recognised as an important factor affecting ocean circulation, with modelling studies suggesting that this heat source could lead to first-order changes in the formation rate of Antarctic Bottom Water, as well as a significant warming effect in the abyssal ocean. Where it has been represented in numerical models, however, the geothermal heat flux into the ocean is generally treated as an entirely conductive flux, despite an estimated one third of the global geothermal flux being introduced to the ocean via hydrothermal sources. A modelling study is presented which investigates the sensitivity of the geothermally forced circulation to the way heat is supplied to the abyssal ocean. An analytical two-dimensional model of the circulation is described, which demonstrates the effects of a volume flux through the ocean bed. A simulation using the NEMO numerical general circulation model in an idealised domain is then used to partition a heat flux between conductive and hydrothermal sources and explicitly test the sensitivity of the circulation to the formulation of the abyssal heat flux. Our simulations suggest that representing the hydrothermal flux as a mass exchange indeed changes the heat distribution in the abyssal ocean, increasing the advective heat transport from the abyss by up to 35% compared to conductive heat sources. Consequently, we suggest that the inclusion of hydrothermal fluxes can be an important addition to course-resolution ocean models.
14. Downes, Stephanie M., et al. “Hydrothermal heat enhances abyssal mixing in the Antarctic Circumpolar Current.” Geophysical Research Letters 46.2 (2019): 812-821.  [LINK]  Upwelling in the world’s strongest current, the Antarctic Circumpolar Current, is thought to be driven by wind stress, surface buoyancy flux, and mixing generated from the interaction between bottom currents and rough topography. However, the impact of localized injection of heat by hydrothermal vents where the Antarctic Circumpolar Current interacts with mid‐ocean ridges remains poorly understood. Here a circumpolar compilation of helium and physical measurements are used to show that while geothermal heat is transferred to the ocean over a broad area by conduction, heat transfer by convection dominates near hydrothermal vents. Buoyant hydrothermal plumes decrease stratification above the vent source and increase stratification to the south, altering the local vertical diffusivity and diapycnal upwelling within 500 m of the sea floor by an order of magnitude. Both the helium tracer and stratification signals induced by hydrothermal input are advected by the flow and influence properties downstream.