We present subannual observations (2009–2014) of a major West Antarctic glacier (Pine Island Glacier) and the neighboring ocean. Ongoing glacier retreat and accelerated ice flow were likely triggered ...a few decades ago by increased ocean‐induced thinning, which may have initiated marine ice sheet instability. Following a subsequent 60% drop in ocean heat content from early 2012 to late 2013, ice flow slowed, but by < 4%, with flow recovering as the ocean warmed to prior temperatures. During this cold‐ocean period, the evolving glacier‐bed/ice shelf system was also in a geometry favorable to stabilization. However, despite a minor, temporary decrease in ice discharge, the basin‐wide thinning signal did not change. Thus, as predicted by theory, once marine ice sheet instability is underway, a single transient high‐amplitude ocean cooling has only a relatively minor effect on ice flow. The long‐term effects of ocean temperature variability on ice flow, however, are not yet known.
Key Points
Pine Island Glacier speed is correlated with ocean temperature
Grounded ice speed slowed by only ~1% despite ~60% drop in ocean heat content
Ice speed recovered after the cold‐ocean anomaly ended
The Greenland Ice Sheet (GIS) contains the equivalent of 7.4 metres of global sea-level rise. Its stability in our warming climate is therefore a pressing concern. However, the sparse proxy evidence ...of the palaeo-stability of the GIS means that its history is controversial (compare refs 2 and 3 to ref. 4). Here we show that Greenland was deglaciated for extended periods during the Pleistocene epoch (from 2.6 million years ago to 11,700 years ago), based on new measurements of cosmic-ray-produced beryllium and aluminium isotopes (
Be and
Al) in a bedrock core from beneath an ice core near the GIS summit. Models indicate that when this bedrock site is ice-free, any remaining ice is concentrated in the eastern Greenland highlands and the GIS is reduced to less than ten per cent of its current volume. Our results narrow the spectrum of possible GIS histories: the longest period of stability of the present ice sheet that is consistent with the measurements is 1.1 million years, assuming that this was preceded by more than 280,000 years of ice-free conditions. Other scenarios, in which Greenland was ice-free during any or all Pleistocene interglacials, may be more realistic. Our observations are incompatible with most existing model simulations that present a continuously existing Pleistocene GIS. Future simulations of the GIS should take into account that Greenland was nearly ice-free for extended periods under Pleistocene climate forcing.
North Atlantic variability in general, and the North Atlantic Oscillation (NAO) in particular, is a long‐studied, very important but still not well‐understood problem in climatology. The recent trend ...to a higher wintertime NAO index was accompanied by an additional increase in the Azores High not coupled to changes in the Icelandic Low, as shown by a self‐organizing maps (SOMs) analysis of monthly mean DJF mean sea level pressure data from 1957 to 2002. SOMs are a nonlinear tool to optimally extract a user‐specified number of patterns or icons from an input data set and to uniquely relate any input data field to an icon, allowing analyses of occurrence frequencies and transitions complementary to principal component analysis (PCA). SOMs analysis of ERA‐40 data finds a North Atlantic “monopole” roughly colocated with the mean position of the Azores High, as well as the well‐known NAO dipole involving the Icelandic Low and the subtropical high. Little trend is shown in December, but the Azores High increased along with the NAO in January and February over the study interval, with implications for storminess in northwestern Europe. In short, our SOM‐based analyses of winter MSLP have both confirmed prior knowledge and expanded it through the relative ease of use and power with nonlinear systems of the SOM‐based approach to climatological analysis.
Glacier motion responds dynamically to changing meltwater inputs, but the multi-decadal response of basal sliding to climate remains poorly constrained due to its sensitivity across multiple ...timescales. Observational records of glacier motion provide critical benchmarks to decode processes influencing glacier dynamics, but multi-decadal records that precede satellite observation and modern warming are rare. Here we present a record of motion in the ablation zone of Saskatchewan Glacier that spans seven decades. We combine in situ and remote-sensing observations to inform a first-order glacier flow model used to estimate the relative contributions of sliding and internal deformation on dynamics. We find a significant increase in basal sliding rates between melt-seasons in the 1950s and those in the 1990s and 2010s and explore three process-based explanations for this anomalous behavior: (i) the glacier surface steepened over seven decades, maintaining flow-driving stresses despite sustained thinning; (ii) the formation of a proglacial lake after 1955 may support elevated basal water pressures; and (iii) subglacial topography may cause dynamic responses specific to Saskatchewan Glacier. Although further constraints are necessary to ascertain which processes are of greatest importance for Saskatchewan Glacier's dynamic evolution, this record provides a benchmark for studies of multi-decadal glacier dynamics.
Modeling Ice-Sheet Flow Alley, Richard B.; Joughin, Ian
Science (American Association for the Advancement of Science),
05/2012, Letnik:
336, Številka:
6081
Journal Article
Recenzirano
The great Greenland and Antarctic ice sheets are the "wild cards" in projections of sea-level change (1). Early models of the coupled ocean-atmosphere system treated the ice sheets as static white ...mountains. Observations since then have shown that ice sheets can change quickly (2): In some places, the tides strongly modulate coastal ice flow; in others, warming-induced ice-shelf loss has caused the flow speed of the subsequently unbuttressed inland ice to increase almost 10-fold within a few weeks (3, 4). A new generation of full-stress ice-sheet models incorporates the physics needed to reproduce such processes (see the figure) (5-7). Including full stresses does improve ice-flow simulations (8). Well-validated, robust projections of ice-sheet behavior under climate change nevertheless remain a challenge, as they will require an ensemble of model ice sheets coupled to the rest of the climate system.
Ice-penetrating radar and kinematic GPS observations from Subglacial Lake Whillans (SLW), West Antarctica, reveal a shallow lake that is confined by steep basal topographic features. Radar imaging of ...SLW, although indicating wet basal conditions, is consistent with a water column depth of only ~6m or less during the near low-stand state at the time of the survey. Kinematic GPS profiles reveal that SLW is generally defined by a ~15m surface depression centered at S 84.237° W 153.614°. This point coincides with the area of lowest hydropotential in the lake basin and also the largest surface elevation range in ICESat data. Therefore this location appears to be an opportune site for subglacial access drilling of this active subglacial lake. A distinct basal topographic ridge on the grid south side of the basin is coincident with a strong contrast in relative basal reflectivity (~6dB), which we interpret as the lake boundary. Mapped hydropotential (calculated assuming hydrostatic equilibrium) shows that water enters the lake from the upstream direction and drains downstream. We hypothesize that a lake-level rise of ~5m plus flexural effects is sufficient to overtop a drainage divide. Thus SLW acts as a temporary storage basin for water beneath Whillans Ice Stream.
► Radar and GPS surveys geophysically characterize a dynamic subglacial lake. ► Subglacial lakes store and release transient water flow impacting ice dynamics. ► A depression in hydropotential coincides with the greatest surface elevation change. ► Ice flexural effects may play a key role in lake drainage. ► Water flux during filling (or draining) is modest (∼10m3/s).
Sedimentation filling space beneath ice shelves helps to stabilize ice sheets against grounding-line retreat in response to a rise in relative sea level of at least several meters. Recent Antarctic ...changes thus cannot be attributed to sea-level rise, strengthening earlier interpretations that warming has driven ice-sheet mass loss. Large sea-level rise, such as the almost equal to100-meter rise at the end of the last ice age, may overwhelm the stabilizing feedback from sedimentation, but smaller sea-level changes are unlikely to have synchronized the behavior of ice sheets in the past.
Interpretation of ice core trace gas records depends on an accurate understanding of the processes that smooth the atmospheric signal in the firn. Much work has been done to understand the processes ...affecting air transport in the open pores of the firn, but a paucity of data from air trapped in bubbles in the firn‐ice transition region has limited the ability to constrain the effect of bubble closure processes. Here we present high‐resolution measurements of firn density, methane concentrations, nitrogen isotopes, and total air content that show layering in the firn‐ice transition region at the West Antarctic Ice Sheet (WAIS) Divide ice core site. Using the notion that bubble trapping is a stochastic process, we derive a new parameterization for closed porosity that incorporates the effects of layering in a steady state firn modeling approach. We include the process of bubble trapping into an open‐porosity firn air transport model and obtain a good fit to the firn core data. We find that layering broadens the depth range over which bubbles are trapped, widens the modeled gas age distribution of air in closed bubbles, reduces the mean gas age of air in closed bubbles, and introduces stratigraphic irregularities in the gas age scale that have a peak‐to‐peak variability of ~10 years at WAIS Divide. For a more complete understanding of gas occlusion and its impact on ice core records, we suggest that this experiment be repeated at sites climatically different from WAIS Divide, for example, on the East Antarctic plateau.
Key Points
Gas occlusion and firn layering are observed in high‐resolution measurements
We present an improved porosity parameterization that accounts for layering
Understanding layering is important for interpretation of ice core gas records
Understanding the processes that affect streaming ice flow and the mass balance of glaciers and ice sheets requires sound knowledge of their subglacial environments. Previous studies have shown that ...an extensive deformable subglacial sediment layer favors fast ice-stream flow. However, areas of high basal drag, termed sticky spots, are of particular interest because they inhibit the fast flow of the overriding ice. The stick-slip behavior of Whillans Ice Stream (WIS) is perhaps the most conspicuous manifestation of a subglacial sticky spot. We present new ice-thickness and seismic-reflection measurements collected over the main sticky spot in the ice plain of WIS, allowing us to elucidate its role in the behavior of the ice stream. Ice-thickness and surface-elevation data show that the sticky spot occupies a subglacial topographic high. Water flow in response to the hydrological potential gradient will be routed around the sticky spot if effective pressures are similar on the sticky spot and elsewhere. The seismic experiment imaged a laterally continuous basal layer approximately 6 m thick, having compressional wave velocities of greater than 1800 m s−1 and density greater than 1800 kg m−3, indicative of a till layer that is stiffer than corresponding till beneath well-lubricated parts of the ice stream. This layer likely continues to deform under the higher shear stress of the sticky spot, and some water may be pumped up onto the sticky spot during motion events.
•A laterally continuous basal till layer of approximately 6 m thickness is imaged.•Stiff till beneath the sticky spot on Whillans Ice Stream (WIS) influences its flow.•High bed elevation and low hydropotential are the likely cause of the sticky spot.•Effective pressure at the sticky spot is higher than the streaming part of the WIS.
We used satellite images to examine the calving behavior of Helheim and Kangerdlugssuaq Glaciers, Greenland, from 2001 to 2006, a period in which they retreated and sped up. These data show that many ...large iceberg‐calving episodes coincided with teleseismically detected glacial earthquakes, suggesting that calving‐related processes are the source of the seismicity. For each of several events for which we have observations, the ice front calved back to a large, pre‐existing rift. These rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high, which agrees well with earlier theoretical predictions. In addition to the recent retreat in a period of higher temperatures, analysis of several images shows that Helheim retreated in the 20th Century during a warmer period and then re‐advanced during a subsequent cooler period. This apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water. The cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in Greenland can advance rapidly. Greenland's larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance.
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