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WEST ANTARCTIC GLACIERS ARE TOAST

Posted on: March 9, 2020

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THIS POST IS A REVIEW OF A 2014 NASA VIDEO [LINK] ABOUT GLACIERS IN WEST ANTARCTICA. IT IS PRESENTED IN THREE PARTS. PART-1 IS A TRANSCRIPT OF THE VIDEO. PART-2 IS ITS CRITICAL EVALUATION, AND PART-3 IS A RELEVANT BIBLIOGRAPHY.   

RELATED POSTS: [LINK] [LINK] [LINK] [LINK] [LINK][LINK] 

 

 

 

PART-1: TRANSCRIPT OF THE VIDEO LECTURE 

(1)  Over the years, temperatures around the world have ratcheted upward and climate change researchers have kept a weary eye perhaps on one place more than any other – the West Antarctic Ice Sheet (WAIS) – and particularly, the fastest melting part of it – the glaciers that flow into the Amundsen Sea. In that region, six glaciers hang in precarious balance, partially supported by land and partially floating in waters just off-shore.

 

(2)  There is enough water frozen in the ice sheets that feed these icy giants to raise global sea levels by 4 feet if they were to melt. That’s troubling because the glaciers ARE MELTING. Moreover, a new study finds that their decline appears to be IRREVERSIBLE. We have passed the point of no return according to glaciologist Eric Rignot who is working jointly with NASA’s Jet Propulsion Laboratory (JPL) and the University of California Irvine. He and his research team used 19 years of NASA satellite radar data to map the fast melting glaciers. Their paper, published in Geophysical Research Letters, finds that this section of West Antarctica is undergoing a “Marine Ice Sheet Instability that will significantly contribute to sea level rise in the centuries ahead.

 

(3)  A key concept in this study is the grounding line. It is the dividing line between land and water underneath the glacier. Because, virtually all the melt occurs where the glacier’s underside touches the ocean, pinpointing the grounding line is crucial for estimating melt rates. The problem is that grounding lines are buried under thousands of feet of glacial ice. It is challenging for a human observer to figure out where they are. There is nothing obvious that sticks out on the surface to indicate that “this is where the glacier goes afloat”.

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(4)  To find the hidden grounding line, they use radar images of the glaciers made by the European Space Agency’s first remote sensing satellite from 1992-2011. Glaciers flex in response to tides. By analyzing these flexing motions, they were able to trace the grounding line. This led to a key discovery. In all the glaciers they studied, grounding lines were rapidly retreating away from the sea. In this sector, we are seeing retreat rates that we don’t see anywhere else on earth.

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(5)   The Smith Glacier’s line moves the fastest, retreating 22 miles upstream. The other glaciers retreated from 6 to 19 miles. When glaciers melt and lose weight, they float off the land where they used to sit. Water gets under the grounding line and pushes the grounding line inland. This in turn reduces friction between the glacier and its bed. The glacier speeds up, stretches out, and thins which drives the grounding line to retreat even farther inland. This is a positive feedback loop that leads to out of control melting.

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(6)  The only natural factor that can slow or stop this process is a pinning point in the bedrock –  a bump or projection that snares the glacier from underneath and keeps it from sliding towards the sea. A map of these features derived from an aerial survey of the region revealed that the glaciers had already floated off many of their small pinning points. In short, there is no turning back – and at current melt rates, these glaciers will be history within a few hundred years.

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PART-2: CRITICAL COMMENTARY

 The chart below displays the tropospheric temperature trends for each calendar month measured above the Antarctic Ocean. None of these trends  is statistically significant and most months show cooling. These data do not indicate that AGW climate change is warming the waters of the Antarctic Ocean and causing it to melt ice shelves and glaciers. In this regard, the warmer temperature of the Circumpolar Deep water Circulation does not have an AGW climate change interpretation. This warmth is more likely to be geothermal heat particularly since the warmth is found only in deep water in an area  known to be geologically active. The relevant geological features of this region are described in a related post [LINK] . Geothermal heat, particularly from hydrothermal vents, is recognized in this context in several of the papers cited in the bibliography below. The AGW climate change interpretation of glacial ice shelf melt phenomena in the West Antarctic ice sheet is rejected on this basis. 

 

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CIRCUMPOLAR DEEP WATER CIRCULATION BIBLIOGRAPHY

  1. Thoma, Malte, et al. “Modelling circumpolar deep water intrusions on the Amundsen Sea continental shelf, Antarctica.” Geophysical Research Letters 35.18 (2008). [FULL TEXT]   Results are presented from an isopycnic coordinate model of ocean circulation in the Amundsen Sea, focusing on the delivery of Circumpolar Deep Water (CDW) to the inner continental shelf around Pine Island Bay. The warmest waters to reach this region are channeled through a submarine trough, accessed via bathymetric irregularities along the shelf break. Temporal variability in the influx of CDW is related to regional wind forcing. Easterly winds over the shelf edge change to westerlies when the Amundsen Sea Low migrates west and south in winter/spring. This drives seasonal on‐shelf flow, while inter‐annual changes in the wind forcing lead to inflow variability on a decadal timescale. A modelled period of warming following low CDW influx in the late 1980’s and early 1990’s coincides with a period of observed thinning and acceleration of Pine Island Glacier.
  2. Moffat, C., B. Owens, and R. C. Beardsley. “On the characteristics of Circumpolar Deep Water intrusions to the west Antarctic Peninsula continental shelf.” Journal of Geophysical Research: Oceans 114.C5 (2009).  [FULL TEXT]  Hydrographic and current velocity observations collected from March 2001 to February 2003 on the west Antarctic Peninsula shelf as part of the Southern Ocean Global Ecosystems Dynamics program are used to characterize intrusions of Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW) onto the shelf and Marguerite Bay. UCDW is found on the middle and outer shelf along Marguerite Trough, which connects the shelf break to Marguerite Bay, and at another location farther south. UCDW intrudes in the form of frequent (four per month) and small horizontal scales (≈4 km) warm eddy‐like structures with maximum vertical scales of a few hundred meters. However, no evidence of UCDW intrusions was found in Marguerite Bay. LCDW was found in several deep depressions connected to the shelf break, including Marguerite Trough, forming a tongue of relatively dense water 95 m thick (on average) that reaches into Marguerite Bay through Marguerite Trough. A steady advective‐diffusive balance for the LCDW intrusion is used to make an estimation of the average upwelling rate and diffusivity in the deep layer within Marguerite Trough, which suggest the LCDW layer is renewed approximately every six weeks.
  3. Wåhlin, A. K., et al. “Inflow of warm Circumpolar Deep Water in the central Amundsen shelf.” Journal of Physical Oceanography 40.6 (2010): 1427-1434. [FULL TEXT]   The thinning and acceleration of the West Antarctic Ice Sheet has been attributed to basal melting induced by intrusions of relatively warm salty water across the continental shelf. A hydrographic section including lowered acoustic Doppler current profiler measurements showing such an inflow in the channel leading to the Getz and Dotson Ice Shelves is presented here. The flow rate was 0.3–0.4 Sv (1 Sv ≡ 106 m3 s−1), and the subsurface heat loss was estimated to be 1.2–1.6 TW. Assuming that the inflow persists throughout the year, it corresponds to an ice melt of 110–130 km3 yr−1, which exceeds recent estimates of the net ice glacier ice volume loss in the Amundsen Sea. The results also show a 100–150-m-thick intermediate water mass consisting of Circumpolar Deep Water that has been modified (cooled and freshened) by subsurface melting of ice shelves and/or icebergs. This water mass has not previously been reported in the region, possibly because of the paucity of historical data.
  4. Dinniman, Michael S., John M. Klinck, and Walker O. Smith Jr. “A model study of Circumpolar Deep Water on the West Antarctic Peninsula and Ross Sea continental shelves.” Deep Sea Research Part II: Topical Studies in Oceanography 58.13-16 (2011): 1508-1523.  [FULL TEXT]   Transport of relatively warm, nutrient-rich Circumpolar Deep Water (CDW) onto continental shelves around Antarctica has important effects on physical and biological processes. However, the characteristics of the CDW along the shelf break, as well as what happens to it once it has been advected onto the continental shelf, differ spatially. In the present study high resolution (4–5 km) regional models of the Ross Sea and the West Antarctic Peninsula coastal ocean are used to compare differences in CDW transport. The models compared very well with observations from both regions. Examining the fluxes not only of heat, but also of a simulated “dye” representing CDW, shows that in both cases CDW crosses the shelf break in specific locations primarily determined by the bathymetry, but eventually floods much of the shelf. The frequency of intrusions in Marguerite Trough was ca. 2–3 per month, similar to recent mooring observations. A significant correlation between the along shelf break wind stress and the cross shelf break dye flux through Marguerite Trough was observed, suggesting that intrusions are at least partially related to short duration wind events. The primary difference between the CDW intrusions on the Ross and west Antarctic Peninsula shelves is that there is more vigorous mixing of the CDW with the surface waters in the Ross Sea, especially in the west where High Salinity Shelf Water is created. The models show that the CDW moving across the Antarctic Peninsula continental shelf towards the base of the ice shelves not only is warmer initially and travels a shorter distance than that advected towards the base of the Ross Ice Shelf, but it is also subjected to less vertical mixing with surface waters, which conserves the heat available to be advected under the ice shelves. This difference in vertical mixing also likely leads to differences in the supply of nutrients from the CDW into the upper water column, and thus modulates the impacts on surface biogeochemical processes.
  5. Steig, Eric J., et al. “Tropical forcing of Circumpolar Deep Water inflow and outlet glacier thinning in the Amundsen Sea Embayment, West Antarctica.” Annals of Glaciology 53.60 (2012): 19-28. [FULL TEXT]   Outlet glaciers draining the Antarctic ice sheet into the Amundsen Sea Embayment (ASE) have accelerated in recent decades, most likely as a result of increased melting of their ice-shelf termini by warm Circumpolar Deep Water (CDW). An ocean model forced with climate reanalysis data shows that, beginning in the early 1990s, an increase in westerly wind stress near the continental shelf edge drove an increase in CDW inflow onto the shelf. The change in local wind stress occurred predominantly in fall and early winter, associated with anomalous high sea-level pressure (SLP) to the north of the ASE and an increase in sea surface temperature (SST) in the central tropical Pacific. The SLP change is associated with geopotential height anomalies in the middle and upper troposphere, characteristic of a stationary Rossby wave response to tropical SST forcing, rather than with changes in the zonally symmetric circulation. Tropical Pacific warming similar to that of the 1990s occurred in the 1940s, and thus is a candidate for initiating the current period of ASE glacier retreat.
  6. 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.
  7. Stewart, Andrew L., and Andrew F. Thompson. “Eddy‐mediated transport of warm Circumpolar Deep Water across the Antarctic shelf break.” Geophysical Research Letters 42.2 (2015): 432-440. [FULL TEXT]  The Antarctic Slope Front (ASF) modulates ventilation of the abyssal ocean via the export of dense Antarctic Bottom Water (AABW) and constrains shoreward transport of warm Circumpolar Deep Water (CDW) toward marine‐terminating glaciers. Along certain stretches of the continental shelf, particularly where AABW is exported, density surfaces connect the shelf waters to the mid-depth Circumpolar Deep Water offshore, offering a pathway for mesoscale eddies to transport CDW directly onto the continental shelf. Using an eddy‐resolving process model of the ASF, the authors show that mesoscale eddies can supply a dynamically significant transport of heat and mass across the continental shelf break. The shoreward transport of surface waters is purely wind driven, while the shoreward CDW transport is entirely due to mesoscale eddy transfer. The CDW flux is sensitive to all aspects of the model’s surface forcing and geometry, suggesting that shoreward eddy heat transport may be localized to favorable sections of the continental slope.
  8. 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.
  9. 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.

  10. 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.  

 

 

 

 

 

 

 

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