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Large-ensemble perturbed-parameter forward ice-flow modeling can provide useful insights to uncertainties in inversions for basal drag or other ice-flow parameters. Inversion and data assimilation provide estimates of poorly known parameters that are essential for accurate prognostic modeling. Because ice flow depends on many such parameters with their associated uncertainties, which may interact in nonlinear ways, full uncertainty assessment for parameter estimates is challenging. With rising computational power, it is increasingly practicable to explore co-dependencies and sensitivities. Here, we use a well-characterized higher-order flowline model configured for a well-lubricated ("shelfy") ice stream to run large ensembles, perturbing the magnitude and spatial pattern of basal drag, basal topography, and input flux. We find for steady state that ice-stream velocity and thickness along the entire domain are especially correlated to drag at the downstream end, but with greater local correlation during transients. The modeled effects of basal topographic perturbations on velocity and ice thickness are primarily local. Perturbations of input ice fluxes interact with the others in interesting ways. These insights point to the value of inversions informed by concentrated observations during forced transients such as lake-drainage events, accumulation-rate fluctuations or ice-shelf losses, and to the care needed when interpreting local results of some inversions for basal-drag parameters. • Perturbed-parameter ensembles reveal ice-flow-model uncertainties and interactions. • Basal-friction perturbations at the downstream end especially affect ice-stream flow. • During transients, basal-friction perturbations influence local ice-stream flow more. • Basal-topography perturbations mainly affect ice-stream thickness locally. • Large uncertainties in input flux can obscure other ice-flow controls. [ABSTRACT FROM AUTHOR]

Abstract: Climate is continually changing on numerous time scales, driven by a range of factors. In general, longer-lived changes are somewhat larger, but much slower to occur, than shorter-lived changes. Processes linked with continental drift have affected atmospheric circulation, oceanic currents, and the composition of the atmosphere over tens of millions of years. A global cooling trend over the last 60 million years has altered conditions near sea level in the Arctic from ice-free year-round to completely ice covered. Variations in arctic insolation over tens of thousands of years in response to orbital forcing have caused regular cycles of warming and cooling that were roughly half the size of the continental-drift-linked changes. This “glacial-interglacial” cycling was amplified by the reduced greenhouse gases in colder times and by greater surface albedo from more-extensive ice cover. Glacial-interglacial cycling was punctuated by abrupt millennial oscillations, which near the North Atlantic were roughly half as large as the glacial-interglacial cycles, but which were much smaller Arctic-wide and beyond. The current interglaciation, the Holocene, has been influenced by brief cooling events from single volcanic eruptions, slower but longer lasting changes from random fluctuations in the frequency of volcanic eruptions, from weak solar variability, and perhaps by other classes of events. Human-forced climate changes appear similar in size and duration to the fastest natural changes of the past, but future changes may have no natural analog. [Copyright &y& Elsevier]

Abstract: Arctic sea-ice extent and volume are declining rapidly. Several studies project that the Arctic Ocean may become seasonally ice-free by the year 2040 or even earlier. Putting this into perspective requires information on the history of Arctic sea-ice conditions through the geologic past. This information can be provided by proxy records from the Arctic Ocean floor and from the surrounding coasts. Although existing records are far from complete, they indicate that sea ice became a feature of the Arctic by 47Ma, following a pronounced decline in atmospheric pCO2 after the Paleocene–Eocene Thermal Optimum, and consistently covered at least part of the Arctic Ocean for no less than the last 13–14 million years. Ice was apparently most widespread during the last 2–3 million years, in accordance with Earth''s overall cooler climate. Nevertheless, episodes of considerably reduced sea ice or even seasonally ice-free conditions occurred during warmer periods linked to orbital variations. The last low-ice event related to orbital forcing (high insolation) was in the early Holocene, after which the northern high latitudes cooled overall, with some superimposed shorter-term (multidecadal to millennial-scale) and lower-magnitude variability. The current reduction in Arctic ice cover started in the late 19th century, consistent with the rapidly warming climate, and became very pronounced over the last three decades. This ice loss appears to be unmatched over at least the last few thousand years and unexplainable by any of the known natural variabilities. [Copyright &y& Elsevier]

Abstract: Paleoclimatic records show that the Greenland Ice Sheet consistently has lost mass in response to warming, and grown in response to cooling. Such changes have occurred even at times of slow or zero sea-level change, so changing sea level cannot have been the cause of at least some of the ice-sheet changes. In contrast, there are no documented major ice-sheet changes that occurred independent of temperature changes. Moreover, snowfall has increased when the climate warmed, but the ice sheet lost mass nonetheless; increased accumulation in the ice sheet''s center has not been sufficient to counteract increased melting and flow near the edges. Most documented forcings and ice-sheet responses spanned periods of several thousand years, but limited data also show rapid response to rapid forcings. In particular, regions near the ice margin have responded within decades. However, major changes of central regions of the ice sheet are thought to require centuries to millennia. The paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur. The evidence suggests nearly total ice-sheet loss may result from warming of more than a few degrees above mean 20th century values, but this threshold is poorly defined (perhaps as little as 2°C or more than 7°C). Paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming, or changes may have been induced by factors other than temperature, without having been recorded. [Copyright &y& Elsevier]

Abstract: Arctic amplification, the observation that surface air temperature changes in the Arctic exceed those of the Northern Hemisphere as a whole, is a pervasive feature of climate models, and has recently emerged in observational data relative to the warming trend of the past century. The magnitude of Arctic amplification is an important, but poorly constrained variable necessary to estimate global average temperature change over the next century. Here we evaluate the mechanisms responsible for Arctic amplification on Quaternary timescales, and review evidence from four intervals in the past 3 Ma for which sufficient paleoclimate data and model simulations are available to estimate the magnitude of Arctic amplification under climate states both warmer and colder than present. Despite differences in forcings and feedbacks for these reconstructions compared to today, the Arctic temperature change consistently exceeds the Northern Hemisphere average by a factor of 3–4, suggesting that Arctic warming will continue to greatly exceed the global average over the coming century, with concomitant reductions in terrestrial ice masses and, consequently, an increasing rate of sea level rise. [Copyright &y& Elsevier]

Abstract: A simple shallow-ice flowline glacier model coupled to a model of sediment transport and deposition is used to simulate formation and preservation of moraines. The number, positions, and volume of moraines formed all are sensitive to the climate history assumed. We drive the model with the GISP2 central-Greenland temperature record, and with reduced-millennial-amplitude versions of that record, to test the hypothesis that the Younger Dryas and other millennial oscillations were primarily wintertime events and thus had less influence on glacier behavior than did the Last Glacial Maximum with its strong summertime as well as wintertime signal. We find that forcing the model by GISP2, with dampened strength of the millennial-scale signal, provides modeled moraine configurations that reflect observed moraine records in the Northern Hemisphere. [Copyright &y& Elsevier]

Abstract: A case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the North Atlantic region during the last glaciation and into the Holocene. The primary evidence comes from mismatches between mean annual temperatures from Greenland ice cores in comparison with snowline changes in East Greenland, northern Europe, and North America. The most likely explanation is a shutdown (or reduction in strength) of the conveyor. This allows the spread of winter sea ice across the North Atlantic, thus causing the northern region to experience much colder winters. Because they mimic the Greenland temperature rather than the snowline signal, changes in the Atlantic Intertropical Convergence Zone and the Asian monsoon may also share a winter linkage with Greenland. Thus the paleoclimate record is consistent with the notion that a huge continental sector of the Northern Hemisphere, stretching from Greenland to Asia, was close to an extreme winter threshold during much of the last glaciation. Winter climate crossed this threshold repeatedly, with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics. [Copyright &y& Elsevier]

Abstract: A prominent, abrupt climate event about 8200 years ago brought generally cold and dry conditions to broad northern-hemisphere regions especially in wintertime, in response to a very large outburst flood that freshened the North Atlantic. Changes were much larger than typical climate variability before and after the event, with anomalies up to many degrees contributing to major displacement of vegetative patterns. This “8k” event provides a clear case of cause and effect in the paleoclimatic realm, and so offers an excellent opportunity for model testing. The response to North Atlantic freshening has the same general anomaly pattern as observed for older events associated with abrupt climate changes following North Atlantic freshening, and so greatly strengthens the case that those older events also reflect North Atlantic changes. The North Atlantic involvement in the 8k event helps in estimating limits on climate anomalies that might result in the future if warming-caused ice-melt and hydrologic-cycle intensification at high latitudes lead to major changes in North Atlantic circulation. Few model experiments have directly addressed the 8k event, and most studies of proxy records across this event lack the time resolution to fully characterize the anomalies, so much work remains to be done. [Copyright &y& Elsevier]

The Greenland ice sheet is likely to make a faster contribution to sea-level rise in a warming world than previously believed, based on numerical modelling using a parameterization of recent results showing surface-meltwater lubrication of ice flow. Zwally et al. (Science 297(557) (2002) 218) documented correlation between increased ice velocity and increased surface melt (as parameterized by positive degree days). They argued that surface water is piped directly to the bed with little delay, causing increased basal-water pressures and basal-sliding velocities, an effect not included in recent Greenland ice-sheet models known to the authors.Using the Pennsylvania State University/University of Chicago thermomechanical flowline model, numerous simulations were conducted to test a wide range of parameter space linking surface melt with a new sliding law based on the Zwally et al. data under three different global warming scenarios (2×CO2, 4×CO2, and 8×CO2). Comparisons to reconstructions generated with a traditional sliding parameterization illustrate an enhanced sensitivity of the ice sheet to surface warming resulting in higher ablation rates, additional thinning and retreat of the margin, and a reduction in ice volume leading to an increased contribution to global sea-level rise. [Copyright &y& Elsevier]

Re-analysis of existing data sets shows that they are fully consistent with northern-insolation control of Ice-Age cycles, and do not provide strong support for alternate models invoking tropical or southern controls. The coldest time of the last 100 ka in central Greenland was nearly synchronous with the minimum in summer insolation about 24 ka BP (thousand years before 1950). Approximately one-third of the total Termination-I warming since then was achieved over the almost 10 ka before the abrupt warming into the Bolling, which some workers have interpreted as the start of the warming. Southern warming as recorded in the Byrd Station, Antarctica ice-isotopic record lagged northern warming. CO2 rise was roughly synchronous with southern warming, methane rise may have been delayed a little as was sea-level rise, and the change in the isotopic composition of atmospheric oxygen was delayed significantly. The data for the extrema just before Termination II are consistent with occurrence of the same phasing as for Termination I, but with somewhat different progression of the two terminations once initiated, perhaps because of differences in the forcing. [Copyright &y& Elsevier]

• Timing and rate of glacier retreat is dictated by basal rheology and topography. • Retreat rate across mixed-rheology beds depends on spacing between basal bumps. • Ice-ocean interactions around isolated topographic highs impact stability. The spatially variable basal flow law suggested by geophysical data from Thwaites Glacier, West Antarctica produces modeled ice-flow response to warming that differs notably from the response of commonly assumed spatially uniform flow laws, with implications for future sea-level rise. Unlike many ice-sheet outlets, Thwaites flows across rather than along prominent topography, with hard stoss faces where form drag is understood to give a low-stress-exponent basal flow law and soft lee-side tills likely to give nearly-plastic behavior. Applying the PSU 2-D higher-order flowline model to a range of idealized Thwaites-like topographies, we test the impact of nearly plastic, nearly viscous, and spatially variable basal rheology on forced grounding-line evolution. In agreement with previous findings, for spatially uniform basal rheology, the timing and rate of grounding-line retreat depend strongly on the bed exponent, with plastic rheology promoting longer stability at the current grounding line but faster retreat once destabilized. Additionally, we find that retreat over mixed-rheology beds is sensitive to the wavelength of the basal topography. For outlet-glacier flow across long-wavelength bumps (≳10 km), behavior falls between that for uniform viscous and plastic beds, yet closer to plastic. However, over shorter-wavelength topography, at times, grounding-line retreat is slower than for either uniform-rheology end-member. Accurately accounting for variable basal conditions in ice-sheet models thus is critical for improving projections of both the timing and magnitude of retreat. [ABSTRACT FROM AUTHOR]

Abstract Seismic measurements on Thwaites Glacier show a spatially variable bed character, with implications for ice-sheet stability. The West Antarctic Ice Sheet is losing mass rapidly through outlet glaciers and ice streams in the Amundsen Sea Embayment, including Thwaites Glacier, where limited observations and modeling suggest that ice-flow rates depend on bed properties. Here we characterize bed properties of Thwaites Glacier based on amplitude analysis of reflection-seismic data from a ∼40-km-long profile collected in the approximate flow direction and two ∼10-km-long profiles transverse to flow. The upstream portion of the seismic profile reveals a ∼12-km long sedimentary basin with ice-flow-aligned bedforms capped by a continuous till layer that is likely soft and deforming (porosity ∼0.4–0.45), with several locations where water has pooled at the bed. Downstream of the sedimentary basin, the bed rises by ∼400 m over ∼25 km into subglacial highlands. Our seismic survey of these subglacial highlands reveals strong spatial variations in bed character across rugged topography (∼200 m amplitude at ∼2- to 5-km wavelength) resembling crag-and-tails. Till on the stoss sides (facing upglacier) of topographic highs is more consolidated (porosity ∼0.3–0.35 or lower), whereas the lee sides (facing downglacier) and flat regions exhibit porosity similar to the till of the upstream sedimentary basin. Modeling studies could use the observed correlation between bed character and bed aspect and slope to extend our observations to other parts of Thwaites Glacier, resulting in more-realistic models of future grounding-line retreat. Our findings highlight the need for more geophysical constraints on bed properties for important outlets in Antarctica and Greenland. Highlights • P -wave reflectivities show bed character varies with subglacial morphology. • Relatively flat, ∼12-km-long basin is "soft" bedded, capped by dilatant till. • "Hard" beds on stoss, "soft" beds on lee sides of topography in subglacial highlands. • Localized water layers are scattered beneath upper Thwaites Glacier. [ABSTRACT FROM AUTHOR]

Abstract: The Younger Dryas event is by far the best studied of the millennial-scale cold snaps of glacial time. Yet its origin remains a subject of debate. The long-held scenario that the Younger Dryas was a one-time outlier triggered by a flood of water stored in proglacial Lake Agassiz has fallen from favor due to lack of a clear geomorphic signature at the correct time and place on the landscape. The recent suggestion that the Younger Dryas was triggered by the impact of a comet has not gained traction. Instead, evidence from Chinese stalagmites suggests that, rather than being a freak occurrence, the Younger Dryas is an integral part of the deglacial sequence of events that produced the last termination on a global scale. [Copyright &y& Elsevier]

Abstract: Thirty-eight new cosmogenic (10Be) exposure ages from the Scoresby Sund region of east Greenland indicate that prominent moraine sets deposited by mountain glaciers date from 780 to 310yr, approximately during the Little Ice Age, from 11 660 to 10 630yr, at the end of the Younger Dryas cold interval or during Preboreal time, and from 13 010 to 11 630yr, during lateglacial time. Equilibrium line altitudes (ELAs) interpreted from lateglacial to Early Holocene moraines indicate summertime cooling between ∼3.9 and 6.6°C relative to today''s value, much less than the extreme Younger Dryas cooling registered by Greenland ice cores (mean-annual temperatures of ∼15°C colder than today''s value). This apparent discrepancy between paleotemperature records supports the contention that Younger Dryas cooling was primarily a wintertime phenomenon. 10Be ages of lateglacial and Holocene moraines show that mountain glaciers during the Little Ice Age were more extensive than at any other time since the Early Holocene Epoch. In addition, 10Be ages of lateglacial moraines show extensive reworking of boulders with cosmogenic nuclides inherited from prior periods of exposure, consistent with our geomorphic observations and cosmogenic-exposure dating studies in other Arctic regions. [Copyright &y& Elsevier]

Abstract: Glaciohydraulic supercooling has been observed at many modern glaciers and has been shown to generate thick zones of debris-laden basal ice which, when exposed to melting, can produce debris-flow and melt-out till deposits, as well as hummocky terrain. Glaciohydraulic supercooling probably occurred in former continental ice sheets, especially where ice flowed out of deep basins, or up steep slopes. Evidence to support glaciohydraulic supercooling in former ice sheets, however, is limited but may include sedimentary sequences of melt-out till and debris-flow deposits and hummocky terrain along the distal rim of overdeepenings and crests of adverse bed slopes. [Copyright &y& Elsevier]

Abstract: Using atmospheric methane and the isotopic composition of O2 as correlation tools, we place the record of ice from the Siple Dome (West Antarctica) ice core on a precise common chronology with the GISP2 (Greenland) ice core for the period from 9 to 57ka. The onset of major millennial warming events in Siple Dome preceded major abrupt warmings in Greenland, and the pattern of millennial change at Siple Dome was broadly similar, though not identical, to that previously observed for the Byrd ice core (also in West Antarctica). The addition of Siple Dome to the database of well-dated Antarctic paleoclimate records supports the case for a coherent regional pattern of millennial-scale climate change in Antarctica during much of the last ice age and glacial–interglacial transition. [Copyright &y& Elsevier]

Thwaites Glacier (TG), West Antarctica, is losing mass in response to oceanic forcing. Future evolution could lead to deglaciation of the marine basins of the West Antarctic Ice Sheet, depending on ongoing and future climate forcings, but also on basal topography/bathymetry, basal properties, and physical processes operating within the grounding zone. Hence, it is important to know the distribution of bed types of TG's interior and grounding zone, and to incorporate them accurately in models in order to improve estimates of retreat rates and stability. Here we estimate properties of the bed by determining its acoustic impedance from amplitude analysis of reflection seismic data. We report on the results from two lines – a longitudinal (L-Line) and a transverse (N-Line) – on a central flowline of TG ∼100 km inland from the grounding zone. The data show considerable spatial variability in bed forms and properties, similar to results from a comparable survey farther inland. Notably, we find the same pattern here of hard (presumed bedrock) material on the stoss side of bumps and soft (presumed till) on the lee side. Physical understanding indicates the basal flow law describing motion over different regions of TG's bed likely varies from nearly-viscous over the bedrock regions to nearly-plastic over till regions, providing guidance for modeling. • The "hard" stoss and "soft" lee bed pattern persists downstream of prior observations. • Localized regions of water occur in the lees of bumps and the downstream end of lower Thwaites Glacier. • "Stoss-side-moats" in regions with high topographic relief are "hard" and may be locations of increased scour. [ABSTRACT FROM AUTHOR]