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Ian Joughin

Senior Principal Engineer

Affiliate Professor, Earth and Space Sciences






Ian Joughin continues his pioneering research into the use of differential SAR interferometry for the estimation of surface motion and topography of ice sheets. He combines the remote sensing with field work and modeling to solve ice dynamics problems. Solving the problems helps him understand the mass balance of the Greenland and Antarctic Ice Sheets in response to climate change.

In addition to polar research, he also contributed to the development of algorithms that were used to mosaic data for the near-global map of topography from the Shuttle Radar Topography Mission (SRTM).

Department Affiliation

Polar Science Center


B.S. Electrical Engineering, University of Vermont, 1986

M.S. Electrical Engineering, University of Vermont, 1990

Ph.D. Electrical Engineering, University of Washington, 1995


2000-present and while at APL-UW

Increased ice flow in Western Palmer Land linked to ocean melting

Hogg, A.E., and 11 others including I. Joughin, "Increased ice flow in Western Palmer Land linked to ocean melting," Geophys. Res. Lett., 44, 4159-4167, doi:10.1002/2016GL072110, 2017.

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16 May 2017

A decrease in the mass and volume of Western Palmer Land has raised the prospect that ice speed has increased in this marine-based sector of Antarctica. To assess this possibility, we measure ice velocity over 25 years using satellite imagery and an optimized modeling approach. More than 30 unnamed outlet glaciers drain the 800 km coastline of Western Palmer Land at speeds ranging from 0.5 to 2.5 m/d, interspersed with near-stagnant ice. Between 1992 and 2015, most of the outlet glaciers sped up by 0.2 to 0.3 m/d, leading to a 13% increase in ice flow and a 15 km3/yr increase in ice discharge across the sector as a whole. Speedup is greatest where glaciers are grounded more than 300 m below sea level, consistent with a loss of buttressing caused by ice shelf thinning in a region of shoaling warm circumpolar water.

Drainage of southeast Greenland firn aquifer water through crevasses to the bed

Poinar, K., I. Joughin, D. Lilien, L. Drucker, L. Kehre, and S. Nowicki, "Drainage of southeast Greenland firn aquifer water through crevasses to the bed," Front. Earth Sci., 5, doi:10.3389/feart.2017.00005, 2017.

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7 Feb 2017

A firn aquifer in the Helheim Glacier catchment of Southeast Greenland lies directly upstream of a crevasse field. Previous measurements show that a 3.5-km long segment of the aquifer lost a large volume of water (26,000–65,000 m2 in cross section) between spring 2012 and spring 2013, compared to annual meltwater accumulation of 6000–15,000 m2. The water is thought to have entered the crevasses, but whether the water reached the bed or refroze within the ice sheet is unknown. We used a thermo-visco-elastic model for crevasse propagation to calculate the depths and volumes of these water-filled crevasses. We compared our model output to data from the Airborne Topographic Mapper (ATM), which reveals the near-surface geometry of specific crevasses, and WorldView images, which capture the surface expressions of crevasses across our 1.5-km study area. We found a best fit with a shear modulus between 0.2 and 1.5 GPa within our study area. We show that surface meltwater can drive crevasses to the top surface of the firn aquifer (~20 m depth), whereupon it receives water at rates corresponding to the water flux through the aquifer. Our model shows that crevasses receiving firn-aquifer water hydrofracture through to the bed, ~1000 m below, in 10–40 days. Englacial refreezing of firn-aquifer water raises the average local ice temperature by ~4°C over a ten-year period, which enhances deformational ice motion by ~50 m year–1, compared to the observed surface velocity of ~200 m year–1. The effect of the basal water on the sliding velocity remains unknown. Were the firn aquifer not present to concentrate surface meltwater into crevasses, we find that no surface melt would reach the bed; instead, it would refreeze annually in crevasses at depths <500 m. The crevasse field downstream of the firn aquifer likely allows a large fraction of the aquifer water in our study area to reach the bed. Thus, future studies should consider the aquifer and crevasses as part of a common system. This system may uniquely affect ice-sheet dynamics by routing a large volume of water to the bed outside of the typical runoff period.

Englacial latent-heat transfer has limited influence on seaward ice flux in western Greenland

Poinar, K., I. Joughin, J.T.M. Lenaerts, and M.R. Van Den Broeke, "Englacial latent-heat transfer has limited influence on seaward ice flux in western Greenland," 63, doi:10.1017/jog.2016.103, 2017.

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1 Feb 2017

Surface meltwater can refreeze within firn layers and crevasses to warm ice through latent-heat transfer on decadal to millennial timescales. Earlier work posited that the consequent softening of the ice might accelerate ice flow, potentially increasing ice-sheet mass loss. Here, we calculate the effect of meltwater refreezing on ice temperature and softness in the Pakitsoq (near Swiss Camp) and Jakobshavn Isbrae regions of western Greenland using a numeric model and existing borehole measurements.

We show that in the Jakobshavn catchment, meltwater percolation within the firn warms the ice at depth by 3–5°C. By contrast, meltwater refreezing in crevasses (cryo-hydrologic warming) at depths of ~300 m warms the ice in Pakitsoq by up to 10°C, but this causes minimal increase in ice motion (<10 m a-1). Pakitsoq is representative of western Greenland's land-terminating ice, where the slow movement of ice through a wide ablation zone provides ideal conditions for cryo-hydrologic warming to occur. We find that only ~37% of the western Greenland ice flux, however, travels through such areas. Overall, our findings suggest that cryo-hydrologic warming will likely have only a limited effect on the dynamic evolution of the Greenland ice sheet.

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Greenland Ice Sheet flow response to runoff variability

Stevens, L.A., M.D. Behn, S.B. Das, I. Joughin, B.P.Y Noël, M.R. van den Broeke, and T. Herring, "Greenland Ice Sheet flow response to runoff variability," Geophys. Res. Lett., 43, 11,295-11,303, doi:10.1002/2016GL070414, 2016.

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16 Nov 2016

We use observations of ice sheet surface motion from a Global Positioning System network operating from 2006 to 2014 around North Lake in west Greenland to investigate the dynamical response of the Greenland Ice Sheet's ablation area to interannual variability in surface melting. We find no statistically significant relationship between runoff season characteristics and ice flow velocities within a given year or season. Over the 7 year time series, annual velocities at North Lake decrease at an average rate of –0.9 ± 1.1 m yr-2, consistent with the negative trend in annual velocities observed in neighboring regions over recent decades. We find that net runoff integrated over several preceding years has a negative correlation with annual velocities, similar to findings from the two other available decadal records of ice velocity in western Greenland. However, we argue that this correlation is not necessarily evidence for a direct hydrologic mechanism acting on the timescale of multiple years but could be a statistical construct. Finally, we stress that neither the decadal slowdown trend nor the negative correlation between velocity and integrated runoff is predicted by current ice-sheet models, underscoring that these models do not yet capture all the relevant feedbacks between runoff and ice dynamics needed to predict long-term trends in ice sheet flow.

Sensitivity of Pine Island Glacier to observed ocean forcing

Christianson, K., and 19 others, including P. Dutrieux and I.R. Joughin, "Sensitivity of Pine Island Glacier to observed ocean forcing," Geophys. Res. Lett., 43, 10,817-10,825, doi:10.1002/2016GL070500, 2016.

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28 Oct 2016

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.

Antarctic firm compaction rates from repeat-track airborne radar data: I: Methods

Medley, B., S.R.M. Ligtenberg, I. Joughin, M.R. van den Broeke, S. Gogineni, and S. Nowicki, "Antarctic firm compaction rates from repeat-track airborne radar data: I: Methods," Ann. Glaciol., 56, 155-166, doi:10.3189/2015AoG70A203, 2015.

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1 Oct 2015

While measurements of ice-sheet surface elevation change are increasingly used to assess mass change, the processes that control the elevation fluctuations not related to ice-flow dynamics (e.g. firn compaction and accumulation) remain difficult to measure. Here we use radar data from the Thwaites Glacier (West Antarctica) catchment to measure the rate of thickness change between horizons of constant age over different time intervals: 2009–10, 2010–11 and 2009–11. The average compaction rate to ~25 m depth is 0.33 m a-1, with largest compaction rates near the surface. Our measurements indicate that the accumulation rate controls much of the spatio-temporal variations in the compaction rate while the role of temperature is unclear due to a lack of measurements. Based on a semi-empirical, steady-state densification model, we find that surveying older firn horizons minimizes the potential bias resulting from the variable depth of the constant age horizon. Our results suggest that the spatiotemporal variations in the firn compaction rate are an important consideration when converting surface elevation change to ice mass change. Compaction rates varied by up to 0.12 m a-1 over distances <6 km and were on average >20% larger during the 2010–11 interval than during 2009–10.

Greenland supraglacial lake drainages triggered by hydrologically induced basal slip

Stevens, L.A., M.D. Behn, J.J. McGuire, S.B. Das, I. Joughin, T. Herring, D.E. Shean, and M.A. King, "Greenland supraglacial lake drainages triggered by hydrologically induced basal slip," Nature, 522, 73-76, doi:10.1038/nature14480, 2015.

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4 Jun 2015

Water-driven fracture propagation beneath supraglacial lakes rapidly transports large volumes of surface meltwater to the base of the Greenland Ice Sheet. These drainage events drive transient ice-sheet acceleration and establish conduits for additional surface-to-bed meltwater transport for the remainder of the melt season. Although it is well established that cracks must remain water-filled to propagate to the bed, the precise mechanisms that initiate hydro-fracture events beneath lakes are unknown. Here we show that, for a lake on the western Greenland Ice Sheet, drainage events are preceded by a 6–12 hour period of ice-sheet uplift and/or enhanced basal slip. Our observations from a dense Global Positioning System (GPS) network allow us to determine the distribution of meltwater at the ice-sheet bed before, during, and after three rapid drainages in 2011–2013, each of which generates tensile stresses that promote hydro-fracture beneath the lake. We hypothesize that these precursors are associated with the introduction of meltwater to the bed through neighbouring moulin systems (vertical conduits connecting the surface and base of the ice sheet). Our results imply that as lakes form in less crevassed, interior regions of the ice sheet, where water at the bed is currently less pervasive, the creation of new surface-to-bed conduits caused by lake-draining hydro-fractures may be limited.

Seismicity on the western Greenland Ice Sheet: Surface fracture in the vicinity of active moulins

Carmichael, J.D., I. Joughin, M.D. Behn, S. Das, M.A. King, L. Stevens, and D. Lizarralde, "Seismicity on the western Greenland Ice Sheet: Surface fracture in the vicinity of active moulins," J. Geophys. Res., 120, 1082-1106, doi:10.1002/2014JF003398, 2015.

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1 Jun 2015

We analyzed geophone and GPS measurements collected within the ablation zone of the western Greenland Ice Sheet during a ~35 day period of the 2011 melt season to study changes in ice deformation before, during, and after a supraglacial lake drainage event. During rapid lake drainage, ice flow speeds increased to ~400% of winter values, and icequake activity peaked. At times >7 days after drainage, this seismicity developed variability over both diurnal and longer periods (~10 days), while coincident ice speeds fell to ~150% of winter values and showed nightly peaks in spatial variability. Approximately 95% of all detected seismicity in the lake basin and its immediate vicinity was triggered by fracture propagation within near-surface ice (<330 m deep) that generated Rayleigh waves. Icequakes occurring before and during drainage frequently were collocated with the down flow (west) end of the primary hydrofracture through which the lake drained but shifted farther west and outside the lake basin after the drainage. We interpret these results to reveal vertical hydrofracture opening and local uplift during the drainage, followed by enhanced seismicity and ice flow on the downstream side of the lake basin. This region collocates with interferometric synthetic aperture radar-measured speedup in previous years and could reflect the migration path of the meltwater supplied to the bed by the lake. The diurnal seismic signal can be associated with nightly reductions in surface melt input that increase effective basal pressure and traction, thereby promoting elevated strain in the surficial ice.

Seasonal to multiyear variability of glacier surface velocity, terminus position, and sea ice/ice melange in northwest Greenland

Moon, T., I, Joughin, and B. Smith, "Seasonal to multiyear variability of glacier surface velocity, terminus position, and sea ice/ice melange in northwest Greenland," J. Geophys. Res., 120, 818-833, doi:10.1002/2015JF003494, 2015.

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13 May 2015

Glacier ice discharge, which depends on ice velocity and terminus fluctuations, is a primary component of Greenland Ice Sheet mass loss. Some research suggests that ice melange influences terminus calving, in turn affecting glacier velocity. The details and broad spatiotemporal consistency of these relationships, however, is undetermined. Focusing on 16 northwestern Greenland glaciers during 2009 through summer 2014, we examined seasonal surface velocity changes, glacier terminus position, and sea ice and ice melange conditions. For a longer-term analysis, we also produced extended records of four glaciers from 1999 to 2014. There is a strong correspondence between seasonal near-terminus sea ice/melange conditions and terminus change, with rigid ice melange conditions associated with advance and open water associated with retreat. Extended sea ice-free periods and reduced rigid melange are also linked with anomalously large terminus retreat. In all but one case, sustained multiyear retreat of greater than 1 km during both the 15-year and 6-year records was accompanied by interannual velocity increases. Seasonal velocity patterns, however, correspond more strongly with runoff changes than terminus behavior. Projections of continued warming and longer sea ice-free periods around Greenland indicate that notable retreat over wide areas may continue. This sustained retreat likely will contribute to multiyear speedup. Longer melt seasons and earlier breakup of melange may also alter the timing of seasonal ice flow variability.

Limits to future expansion of surface-melt-enhanced ice flow into the interior of western Greenland

Poinar, K., I. Joughin, S.B. Das, M.D. Behn, J.T.M. Lanaerts, and M.R. van den Broeke, "Limits to future expansion of surface-melt-enhanced ice flow into the interior of western Greenland," Geophys. Res. Let., 42, 1800-1807, doi:10.1002/2015GL063192, 2015.

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28 Mar 2015

Moulins are important conduits for surface meltwater to reach the bed of the Greenland Ice Sheet. It has been proposed that in a warming climate, newly formed moulins associated with the inland migration of supraglacial lakes could introduce surface melt to new regions of the bed, introducing or enhancing sliding there. By examining surface strain rates, we found that the upper limit to where crevasses, and therefore moulins, are likely to form is ~1600 m. This is also roughly the elevation above which lakes do not drain completely. Thus, meltwater above this elevation will largely flow tens of kilometers through surface streams into existing moulins downstream. Furthermore, results from a thermal ice sheet model indicate that the ~1600 m crevassing limit is well below the wet-frozen basal transition (~2000 m). Together, these data sets suggest that new supraglacial lakes will have a limited effect on the inland expansion of melt-induced seasonal acceleration.

Seasonal and interannual variations in ice melange and its impact on terminus stability, Jakobshavn Isbrae, Greenland

Cassotto, R., M. Fahnestock, J.M. Amundson, M. Truffer, and I. Joughin, "Seasonal and interannual variations in ice melange and its impact on terminus stability, Jakobshavn Isbrae, Greenland," J. Glaciol., 61, 76-88, doi:10.3189/2015JoG13J235, 2015.

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1 Feb 2015

We used satellite-derived surface temperatures and time-lapse photography to infer temporal variations in the proglacial ice melange at Jakobshavn Isbræ, a large and rapidly retreating outlet glacier in Greenland. Freezing of the melange-covered fjord surface during winter is indicated by a decrease in fjord surface temperatures and is associated with (1) a decrease in ice melange mobility and (2) a drastic reduction in iceberg production. Vigorous calving resumes in spring, typically abruptly, following the steady up-fjord retreat of the sea-ice/ice-melange margin. An analysis of pixel displacement from time-lapse imagery demonstrates that melange motion increases prior to calving and subsequently decreases following several events. We find that secular changes in ice melange extent, character and persistence can influence iceberg calving, and therefore glacier dynamics over daily-to-monthly timescales, which, if sustained, will influence the mass balance of an ice sheet.

Marine ice sheet collapse potentially underway for the Thwaites Glacier Basin, West Antarctica

Joughin, I., B.E. Smith, and B. Medley, "Marine ice sheet collapse potentially underway for the Thwaites Glacier Basin, West Antarctica," Science, 344, 735-738, doi: 10.1126/science.1249055, 2014

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16 May 2014

Resting atop a deep marine basin, the West Antarctic Ice Sheet has long been considered prone to instability. Using a numerical model, we investigate the sensitivity of Thwaites Glacier to ocean melt and whether unstable retreat is already underway. Our model reproduces observed losses when forced with ocean melt comparable to estimates. Simulated losses are moderate (<0.25 mm per year sea level) over the 21st Century, but generally increase thereafter. Except possibly for the lowest-melt scenario, the simulations indicate early-stage collapse has begun. Less certain is the timescale, with onset of rapid (> 1 mm per year of sea-level rise) collapse for the different simulations within the range of two to nine centuries.

Further speedup of Jakobshavn Isbrae

Joughin, I., B.E. Smith, D.E. Shean, and D. Floricioiu, "Further speedup of Jakobshavn Isbrae," The Cryosphere, 8, 209-214, doi:doi:10.5194/tc-8-209-2014, 2014.

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3 Feb 2014

We have extended the record of flow speed on Jakobshavn Isbrae through the summer of 2013. These new data reveal large seasonal speedups, 30 to 50% greater than previous summers. At a point a few kilometres inland from the terminus, the mean annual speed for 2012 is nearly three times as great as that in the mid-1990s, while the peak summer speeds are more than a factor of four greater. These speeds were achieved as the glacier terminus appears to have retreated to the bottom of an over-deepened basin with a depth of ~ 1300 m below sea level. The terminus is likely to reach the deepest section of the trough within a few decades, after which it could rapidly retreat to the shallower regions ~ 50 km farther upstream, potentially by the end of this century.

Tropical Pacific influence on the source and transport of marine aerosols to West Antarctica

Criscitiello, A.S., S.B. Das, K.B. Karnauskas, M.J. Evans, K.E. Frey, I. Joughin, E.J. Steig, J.R. McConnell, and B. Medley, "Tropical Pacific influence on the source and transport of marine aerosols to West Antarctica," J. Clim., 27, 1343-1363, doi:10.1175/JCLI-D-13-00148.1, 2014.

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1 Feb 2014

The climate of West Antarctica is strongly influenced by remote forcing from the tropical Pacific. For example, recent surface warming over West Antarctica reflects atmospheric circulation changes over the Amundsen Sea, driven by an atmospheric Rossby wave response to tropical sea surface temperature (SST) anomalies. Here, it is demonstrated that tropical Pacific SST anomalies also influence the source and transport of marine-derived aerosols to the West Antarctic Ice Sheet. Using records from four firn cores collected along the Amundsen coast of West Antarctica, the relationship between sea ice-modulated chemical species and large-scale atmospheric variability in the tropical Pacific from 1979 to 2010 is investigated. Significant correlations are found between marine biogenic aerosols and sea salts, and SST and sea level pressure in the tropical Pacific. In particular, La Nina-like conditions generate an atmospheric Rossby wave response that influences atmospheric circulation over Pine Island Bay. Seasonal regression of atmospheric fields on methanesulfonic acid (MSA) reveals a reduction in onshore wind velocities in summer at Pine Island Bay, consistent with enhanced katabatic flow, polynya opening, and coastal dimethyl sulfide production. Seasonal regression of atmospheric fields on chloride (Cl-) reveals an intensification in onshore wind velocities in winter, consistent with sea salt transport from offshore source regions. Both the source and transport of marine aerosols to West Antarctica are found to be modulated by similar atmospheric dynamics in response to remote forcing. Finally, the regional ice-core array suggests that there is both a temporally and a spatially varying response to remote tropical forcing.

Pine Island glacier ice shelf melt distributed at kilometre scales

Dutrieux, P., D.G. Vaughan, H.F.J. Corr, A. Jenkins, P.R. Holland, I. Joughin, and A.H. Fleming, "Pine Island glacier ice shelf melt distributed at kilometre scales," Cryosphere, 7, 1543-1555, doi:10.5194/tc-7-1543-2013, 2013.

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26 Sep 2013

By thinning and accelerating, West Antarctic ice streams are contributing about 10% of the observed global sea level rise. Much of this ice loss is from Pine Island Glacier, which has thinned since at least 1992, driven by changes in ocean heat transport beneath its ice shelf and retreat of the grounding line. Details of the processes driving this change, however, remain largely elusive, hampering our ability to predict the future behaviour of this and similar systems. Here, a Lagrangian methodology is developed to measure oceanic melting of such rapidly advecting ice. High-resolution satellite and airborne observations of ice surface velocity and elevation are used to quantify patterns of basal melt under the Pine Island Glacier ice shelf and the associated adjustments to ice flow. At the broad scale, melt rates of up to 100 m yr-1 occur near the grounding line, reducing to 30 m yr-1 just 20 km downstream. Between 2008 and 2011, basal melting was largely compensated by ice advection, allowing us to estimate an average loss of ice to the ocean of 87 km3 yr-1, in close agreement with 2009 oceanographically constrained estimates. At smaller scales, a network of basal channels typically 500 m to 3 km wide is sculpted by concentrated melt, with kilometre-scale anomalies reaching 50% of the broad-scale basal melt. Basal melting enlarges the channels close to the grounding line, but farther downstream melting tends to diminish them. Kilometre-scale variations in melt are a key component of the complex ice–ocean interaction beneath the ice shelf, implying that greater understanding of their effect, or very high resolution models, are required to predict the sea-level contribution of the region.

Future sea-level rise from Greenland's main outlet glaciers in a warming climate

Nick, F.M., et al., including I. Joughin, "Future sea-level rise from Greenland's main outlet glaciers in a warming climate," Nature, 497, 235-238, doi:10.1038/nature12068, 2013.

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8 May 2013

Over the past decade, ice loss from the Greenland Ice Sheet increased as a result of both increased surface melting and ice discharge to the ocean. The latter is controlled by the acceleration of ice flow and subsequent thinning of fast-flowing marine-terminating outlet glaciers. Quantifying the future dynamic contribution of such glaciers to sea-level rise (SLR) remains a major challenge because outlet glacier dynamics are poorly understood. Here we present a glacier flow model that includes a fully dynamic treatment of marine termini. We use this model to simulate behaviour of four major marine-terminating outlet glaciers, which collectively drain about 22 per cent of the Greenland Ice Sheet. Using atmospheric and oceanic forcing from a mid-range future warming scenario that predicts warming by 2.8 degrees Celsius by 2100, we project a contribution of 19 to 30 millimetres to SLR from these glaciers by 2200. This contribution is largely (80 per cent) dynamic in origin and is caused by several episodic retreats past overdeepenings in outlet glacier troughs. After initial increases, however, dynamic losses from these four outlets remain relatively constant and contribute to SLR individually at rates of about 0.01 to 0.06 millimetres per year. These rates correspond to ice fluxes that are less than twice those of the late 1990s, well below previous upper bounds. For a more extreme future warming scenario (warming by 4.5 degrees Celsius by 2100), the projected losses increase by more than 50 per cent, producing a cumulative SLR of 29 to 49 millimetres by 2200.

Recurring dynamically induced thinning during 1985 to 2010 on Upernavik Isstrøm, West Greenland

Khan, S.A., et al., including I. Joughin, "Recurring dynamically induced thinning during 1985 to 2010 on Upernavik Isstrøm, West Greenland," J. Geophys. Res., 118, 111-121, doi:10.1029/2012JF002481, 2013.

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1 Mar 2013

Many glaciers along the southeast and northwest coasts of Greenland have accelerated, increasing the ice sheet's contribution to global sea-level rise. In this article, we map elevation changes on Upernavik Isstrøm (UI), West Greenland, during 2003 to 2009 using high-resolution ice, cloud and land elevation satellite laser altimeter data supplemented with altimeter surveys from NASA's Airborne Topographic Mapper during 2002 to 2010. To assess thinning prior to 2002, we analyze aerial photographs from 1985. We document at least two distinct periods of dynamically induced ice loss during 1985 to 2010 characterized by a rapid retreat of the calving front, increased ice speed, and lowering of the ice surface. The first period occurred before 1991, whereas the latter occurred during 2005 to 2009. Analyses of air and sea-surface temperature suggest a combination of relatively warm air and ocean water as a potential trigger for the dynamically induced ice loss. We estimate a total catchment-wide ice-mass loss of UI caused by the two events of 72.3 ± 15.8 Gt during 1985 to 2010, whereas the total melt-induced ice-mass loss during this same period is 19.8 ± 2.8 Gt. Thus, 79% of the total ice-mass loss of the UI catchment was caused by ice dynamics, indicating the importance of including dynamically induced ice loss in the total mass change budget of the Greenland ice sheet.

Ice sheet record of recent sea ice behavior and polynya variability in the Amundsen Sea, West Antarctica

Criscitiello, A.S., S.B. Das, M.J. Evans, K.E. Frey, H. Conway, I. Joughin, B. Medley, and E.J. Steig, "Ice sheet record of recent sea ice behavior and polynya variability in the Amundsen Sea, West Antarctica," J. Geophys. Res., 118, 118-130, doi:10.1029/2012JC008077, 2013.

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1 Jan 2013

Our understanding of past sea-ice variability is limited by the short length of satellite and instrumental records. Proxy records can extend these observations but require further development and validation. We compare methanesulfonic acid (MSA) and chloride (Cl) concentrations from a new firn core from coastal West Antarctica with satellite-derived observations of regional sea-ice concentration (SIC) in the Amundsen Sea (AS) to evaluate spatial and temporal correlations from 2002–2010. The high accumulation rate (~39 g x cm-2 yr-1) provides monthly resolved records of MSA and Cl, allowing detailed investigation of how regional SIC is recorded in the ice-sheet stratigraphy. Over the period 2002–2010 we find that the ice-sheet chemistry is significantly correlated with SIC variability within the AS and Pine Island Bay polynyas. Based on this result, we evaluate the use of ice-core chemistry as a proxy for interannual polynya variability in this region, one of the largest and most persistent polynya areas in Antarctica. MSA concentrations correlate strongly with summer SIC within the polynya regions, consistent with MSA at this site being derived from marine biological productivity during the spring and summer. Cl concentrations correlate strongly with winter SIC within the polynyas as well as some regions outside the polynyas, consistent with Cl at this site originating primarily from winter sea-ice formation. Spatial correlations were generally insignificant outside of the polynya areas, with some notable exceptions. Ice-core glaciochemical records from this dynamic region thus may provide a proxy for reconstructing AS and Pine Island Bay polynya variability prior to the satellite era.

A reconciled estimate of ice-sheet mass balance

Shepherd, A., and 46 others, including I. Joughin and B. Smith, "A reconciled estimate of ice-sheet mass balance," Science, 338, 1183-1189, doi:10.1126/science.1228102, 2012.

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30 Nov 2012

We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth's polar ice sheets. We find that there is good agreement between different satellite methods—especially in Greenland and West Antarctica—and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by –142 ± 49, +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes year-1, respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year-1 to the rate of global sea-level rise.

Ice-sheet response to oceanic forcing

Joughin, I., R.B. Alley, and D.M. Holland, "Ice-sheet response to oceanic forcing," Science, 338, 1172-1176, doi:10.1126/science.1226481, 2012.

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30 Nov 2012

The ice sheets of Greenland and Antarctica are losing ice at accelerating rates, much of which is a response to oceanic forcing, especially of the floating ice shelves. Recent observations establish a clear correspondence between the increased delivery of oceanic heat to the ice-sheet margin and increased ice loss. In Antarctica, most of these processes are reasonably well understood but have not been rigorously quantified. In Greenland, an understanding of the processes by which warmer ocean temperatures drive the observed retreat remains elusive. Experiments designed to identify the relevant processes are confounded by the logistical difficulties of instrumenting ice-choked fjords with actively calving glaciers. For both ice sheets, multiple challenges remain before the fully coupled ice-ocean-atmosphere models needed for rigorous sea-level projection are available.

Seasonal to decadal scale variations in the surface velocity of Jakobshavn Isbrae, Greenland: Observation and model-based analysis

Joughin, I., B.E. Smith, I.M. Howat, D. Floriciolu, R.B. Alley, M. Truffer, and M. Fahnestock, "Seasonal to decadal scale variations in the surface velocity of Jakobshavn Isbrae, Greenland: Observation and model-based analysis," J. Geophys. Res., 117, doi:10.1029/2011JF002110, 2012.

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25 May 2012

Using new data, we build upon the nearly two-decade long record of observations from Jakobshavn Isbrae to investigate the processes driving its dynamic evolution. While winter flow speed has not increased substantially over the last three winters, there remains a strong seasonal variation in flow speed that coincides with a cycle of summer thinning and winter thickening. We relate changes in glacier speed to geometry through variations in basal traction and horizontal stresses, using ice-flow models constrained by satellite and airborne observations. These results suggest that the bed provides little flow resistance along the main trough within about 20 km of the terminus. While the loss of buttressing from the retreat of grounded and floating ice likely contributed to the initial speedup, other processes are of comparable significance at seasonal to decadal time scales. From analysis of the models, we hypothesize that thinning-induced change in basal effective pressure is the dominant process influencing near-terminus behavior, while diffusive processes drive the upstream response. The apparent need for the terminus to thin to near flotation before it can calve may limit the rate at which retreat occurs. Our analysis of the processes controlling the speed suggests little potential for further large acceleration. Thinning and elevated speeds may continue at rates similar to present, however, putting the glacier on course to retreat to the head of its deep trough in about a century, at which point it likely would stabilize with a thinner terminus.

21st-century evolution of Greenland outlet glacier velocities

Moon, T., I. Joughin, B. Smith, and I. Howat, "21st-century evolution of Greenland outlet glacier velocities," Science, 336, 576-578, doi:10.1126/science.1219985, 2012.

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4 May 2012

Earlier observations on several of Greenland%u2019s outlet glaciers, starting near the turn of the 21st century, indicated rapid (annual-scale) and large (>100%) increases in glacier velocity. Combining data from several satellites, we produce a decade-long (2000 to 2010) record documenting the ongoing velocity evolution of nearly all (200 ) of Greenland%u2019s major outlet glaciers, revealing complex spatial and temporal patterns. Changes on fast-flow marine-terminating glaciers contrast with steady velocities on ice-shelf%u2013terminating glaciers and slow speeds on land-terminating glaciers. Regionally, glaciers in the northwest accelerated steadily, with more variability in the southeast and relatively steady flow elsewhere. Intraregional variability shows a complex response to regional and local forcing. Observed acceleration indicates that sea level rise from Greenland may fall well below proposed upper bounds.

Modeling ice-sheet flow

Alley, R.B., and I. Joughin, "Modeling ice-sheet flow," Science, 336, 551-552, doi:10.1126/science.1220530, 2012.

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4 May 2012

The great Greenland and Antarctic ice sheets are the "wild cards" in projections of sea-level change. 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: 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. A new generation of full-stress ice-sheet models incorporates the physics needed to reproduce such processes. Including full stresses does improve ice-flow simulations. 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.

Constraining ice mass loss from Jakobshavn Isbrae (Greenland) using InSAR-measured crustal uplift

Liu, L., J. Wahr, I. Howat, S.A. Khan, I. Joughin, and M. Furuya, "Constraining ice mass loss from Jakobshavn Isbrae (Greenland) using InSAR-measured crustal uplift," Geophys. J. Int., 188, 994-1006, doi:10.1111/j.1365-246X.2011.05317.x, 2012.

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1 Mar 2012

Jakobshavn Isbrae in west Greenland has been undergoing dramatic thinning since 1997. Applying the interferometric synthetic aperture radar (InSAR) technique to Radarsat-1 SAR data, we measure crustal uplift near Jakobshavn Isbrae caused by recent ice mass loss. The crustal uplift is predominantly at long spatial wavelengths (larger than 10 km), and thus is difficult to separate from InSAR orbit errors. We reduce the effects of orbit errors by removing long-wavelength deformation signals using conventional InSAR baseline fitting methods. We find good agreement between the remaining short-scale InSAR-estimated deformation rates during 2004–2008 and the corresponding short-scale components of a deformation model that is based on changes in ice elevation measured by NASA's Airborne Topographic Mapper (ATM). We are also able to use the InSAR-measured deformation to invert for the spatial pattern of ice thinning. Overall, our results suggest that despite the inherent difficulties of working with a signal that has significant large-scale components, InSAR-measured crustal deformation can be used to study the ice mass loss of a rapidly thinning glacier and its surrounding catchment, providing both a constraint on any existing model of ice mass loss and a data source that can be used to invert for ice mass loss. These new applications of InSAR can help to better understand a glacier's rapid response to a warming climate.

Oceanic controls on the mass balance of Wilkins Ice Shelf, Antarctica

Padman, L., D.P. Costa, M.S. Dinniman, H.A. Fricker, M.E. Goebel, L. A. Huckstadt, A. Humbert, I. Joughin, J.T.M. Lenaerts, S.R.M. Ligtenberg, T. Scambos, M.R. van den Broeke, "Oceanic controls on the mass balance of Wilkins Ice Shelf, Antarctica," J. Geophys. Res., 117, doi:10.1029/2011JC007301, 2012.

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20 Jan 2012

Several Antarctic Peninsula (AP) ice shelves have lost significant fractions of their volume over the past decades, coincident with rapid regional climate change. Wilkins Ice Shelf (WIS), on the western side of the AP, is the most recent, experiencing a sequence of large calving events in 2008 and 2009. We analyze the mass balance for WIS for the period 1992–2008 and find that the averaged rate of ice-shelf thinning was ~0.8 m a-1, driven by a mean basal melt rate of <wb>=1.3±0.4 m a-1. Interannual variability was large, associated with changes in both surface mass accumulation and <wb>. Basal melt rate declined significantly near 2000 from 1.8±0.4 m a-1 for 1992–2000 to ~0.75±0.55 m a-1 for 2001–2008, the latter value corresponding to approximately steady state ice-shelf mass.

Observations of ocean temperature T obtained during 2007–2009 by instrumented seals reveal a cold, deep halo of Winter Water (WW; T ~ –1.6°C) surrounding WIS. The base of the WW in the halo is ~170 m, approximately the mean ice draft for WIS. We hypothesize that the transition in <wb> in 2000 was caused by a small perturbation (~10–20 m) in the relative depths of the ice base and the bottom of the WW layer in the halo. We conclude that basal melting of thin ice shelves like WIS is very sensitive to upper-ocean and coastal processes that act on shorter time and space scales than those affecting basal melting of thicker West Antarctic ice shelves.

Warming of waters in an East Greenland fjord prior to glacier retreat: Mechanisms and connection to large-scale atmospheric conditions

Christoffersen, P., R.I. Mugford, K.J. Heywood, I. Joughin, J.A. Dowdeswell, J.P.M. Syvitski, A. Luckman, and T.J. Benham, "Warming of waters in an East Greenland fjord prior to glacier retreat: Mechanisms and connection to large-scale atmospheric conditions," Cryosphere Discuss., 5, 1335-1364, doi:10.5194/tcd-5-1335-2011, 2011.

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9 Sep 2011

Hydrographic data acquired in Kangerlugssuaq Fjord and adjacent seas in 1993 and 2004 are used together with ocean reanalysis to elucidate water mass change and ice-ocean-atmosphere interactions in East Greenland. The hydrographic data show substantial warming of fjord waters between 1993 and 2004 and warm subsurface conditions coincide with the rapid retreat of Kangerlugssuaq Glacier in 2004-2005. The ocean reanalysis shows that the warm properties of fjord waters in 2004 are related to a major peak in oceanic shoreward heat flux into a cross-shelf trough on the outer continental shelf. The heat flux into this trough varies according to seasonal exchanges with the atmosphere as well as from deep seasonal intrusions of subtropical waters. Both mechanisms contribute to high (low) shoreward heat flux when winds from the northeast are weak (strong). The combined effect of surface heating and inflow of subtropical waters is seen in the hydrographic data, which were collected after periods when along-shore coastal winds from the north were strong (1993) and weak (2004). We show that coastal winds vary according to the pressure gradient defined by a semi-permanent atmospheric pressure system over Greenland and a persistent atmospheric low situated near Iceland. The magnitude of this pressure gradient is controlled by longitudinal variability in the position of the Icelandic Low.

Seasonal speedup of a Greenland marine-terminating outlet glacier forced by surface melt-induced changes in subglacial hydrology

Sole, A.J., D.W.F. Mair, P.W. Nienow, I.D. Bartholomew, M.A. King, M.J. Burke, and I. Joughin, "Seasonal speedup of a Greenland marine-terminating outlet glacier forced by surface melt-induced changes in subglacial hydrology," J. Geophys. Res., 116, doi: 10.1029/2010JF001948, 2011.

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23 Aug 2011

We present subdaily ice flow measurements at four GPS sites between 36 and 72 km from the margin of a marine-terminating Greenland outlet glacier spanning the 2009 melt season. Our data show that >35 km from the margin, seasonal and shorter-time scale ice flow variations are controlled by surface melt-induced changes in subglacial hydrology. Following the onset of melting at each site, ice motion increased above background for up to 2 months with resultant up-glacier migration of both the onset and peak of acceleration. Later in our survey, ice flow at all sites decreased to below background. Multiple 1 to 15 day speedups increased ice motion by up to 40% above background. These events were typically accompanied by uplift and coincided with enhanced surface melt or lake drainage. Our results indicate that the subglacial drainage system evolved through the season with efficient drainage extending to at least 48 km inland during the melt season. While we can explain our observations with reference to evolution of the glacier drainage system, the net effect of the summer speed variations on annual motion is small (~1%). This, in part, is because the speedups are compensated for by slowdowns beneath background associated with the establishment of an efficient subglacial drainage system. In addition, the speedups are less pronounced in comparison to land-terminating systems. Our results reveal similarities between the inland ice flow response of Greenland marine- and land-terminating outlet glaciers.

Stability of the West Antarctic ice sheet in a warming world

Joughin, I. and R.B. Alley, "Stability of the West Antarctic ice sheet in a warming world," Nature Geosci., 4, 506-513, doi: 10.1038/ngeo1194, .2011.

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24 Jul 2011

Ice sheets are expected to shrink in size as the world warms, which in turn will raise sea level. The West Antarctic ice sheet is of particular concern, because it was probably much smaller at times during the past million years when temperatures were comparable to levels that might be reached or exceeded within the next few centuries. Much of the grounded ice in West Antarctica lies on a bed that deepens inland and extends well below sea level. Oceanic and atmospheric warming threaten to reduce or eliminate the floating ice shelves that buttress the ice sheet at present. Loss of the ice shelves would accelerate the flow of non-floating ice near the coast. Because of the slope of the sea bed, the consequent thinning could ultimately float much of the ice sheet's interior. In this scenario, global sea level would rise by more than three metres, at an unknown rate. Simplified analyses suggest that much of the ice sheet will survive beyond this century. We do not know how likely or inevitable eventual collapse of the West Antarctic ice sheet is at this stage, but the possibility cannot be discarded. For confident projections of the fate of the ice sheet and the rate of any collapse, further work including the development of well-validated physical models will be required.

Changes in the dynamics of marine terminating outlet glaciers in west Greenland (2000-2009)

McFadden, E.M., I.M. Howat, I. Joughin, B.E. Smith, and Y. Ahn, "Changes in the dynamics of marine terminating outlet glaciers in west Greenland (2000-2009)," J. Geophys. Res., 116, doi:10.1029/2010F001757, 2011.

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23 Jun 2011

Recent changes in the dynamics of Greenland's marine terminating outlet glaciers indicate a rapid and complex response to external forcing. Despite observed ice front retreat and recent geophysical evidence for accelerated mass loss along Greenland's northwestern margin, it is unclear whether west Greenland glaciers have undergone the synchronous speed-up and subsequent slow-down as observed in southeastern glaciers earlier in the decade. To investigate changes in west Greenland outlet glacier dynamics and the potential controls behind their behavior, we derive time series of front position, surface elevation, and surface slope for 59 marine terminating outlet glaciers and surface speeds for select glaciers in west Greenland from 2000 to 2009. Using these data, we look for relationships between retreat, thinning, acceleration, and geometric parameters to determine the first-order controls on glacier behavior. Our data indicate that changes in front positions and surface elevations were asynchronous on annual time scales, though nearly all glaciers retreated and thinned over the decade. We found no direct relationship between retreat, acceleration, and external forcing applicable to the entire region. In regard to geometry, we found that, following retreat, (1) glaciers with grounded termini experienced more pronounced changes in dynamics than those with floating termini and (2) thinning rates declined more quickly for glaciers with steeper slopes. Overall, glacier geometry should influence outlet glacier dynamics via stress redistribution following perturbations at the front, but our data indicate that the relative importance of geometry as a control of glacier behavior is highly variable throughout west Greenland.

The propsed DESDynI mission - From science to implementation

Rosen, P.A., H. Eisen, Y. Shen, S. Hensley, S. Shaffer, L. Veilleux, R. Dubayah, K.J. Ranson, A. Dress, J.B., Blair, S. Luthcke, B.H. Hager, and I. Joughin, "The propsed DESDynI mission - From science to implementation," Radar Conference (RADAR), 23-27 May, Kansas City, MO, 1129-1131, doi:10.1109/RADAR.2011.5960710 (IEEE, 2011).

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23 May 2011

The proposed DESDynI mission is being planned by NASA to study earth change in three distinct disciplines ecosystems, solid earth, and cryospheric sciences. DESDynI would provide unique and unprecedented capabilities to the science community, with an imaging L-band radar proposed to include new modes and observational techniques, and a first-of a-kind multi-beam lidar for measuring canopy height metrics at fine spatial resolution. Under current planning scenarios, DESDynI could be ready to launch in 2017. In this paper, we describe the science objectives, how these lead to the measurements that achieve these objectives, and how these requirements lead to a mission design. The properties of the radar are then described, including a number of new radar modes and capabilities such as "SweepSAR" scan-on-receive techniques and split-spectrum acquisitions in single and multi pol configurations.

Seasonal speedup of the Greenland Ice Sheet linked to routing of surface water

Palmer, S., A. Shepherd, P. Nienow, and I. Joughin, "Seasonal speedup of the Greenland Ice Sheet linked to routing of surface water," Earth Planet. Sci. Lett., 302, 423-428, doi:10.1016/j.epsl.2010.12.037, 2011.

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1 Feb 2011

We use interferometric synthetic aperture radar observations recorded in a land-terminating sector of western Greenland to characterise the ice sheet surface hydrology and to quantify spatial variations in the seasonality of ice sheet flow. Our data reveal a non-uniform pattern of late-summer ice speedup that, in places, extends over 100 km inland. We show that the degree of late-summer speedup is positively correlated with modelled runoff within the 10 glacier catchments of our survey, and that the pattern of late-summer speedup follows that of water routed at the ice sheet surface. In late-summer, ice within the largest catchment flows on average 48% faster than during winter, whereas changes in smaller catchments are less pronounced. Our observations show that the routing of seasonal runoff at the ice sheet surface plays an important role in shaping the magnitude and extent of seasonal ice sheet speedup.

GPS measurements of crustal uplift near Jakobshavn Isbrae due to glacial ice mass loss

Khan, S.A., L. Liu, J. Wahr, I. Howat, I. Joughin, T. van Dam, and K. Fleming, "GPS measurements of crustal uplift near Jakobshavn Isbrae due to glacial ice mass loss," J. Geophys. Res., 115, doi:10.1029/2010JB007490, 2010.

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16 Sep 2010

We analyze 2006–2009 data from four continuous Global Positioning System (GPS) receivers located between 5 and 150 km from the glacier Jakobshavn Isbrae, West Greenland. The GPS stations were established on bedrock to determine the vertical crustal motion due to the unloading of ice from Jakobshavn Isbrae. All stations experienced uplift, but the uplift rate at Kangia North, only 5 km from the glacier front, was about 10 mm yr-1 larger than the rate at Ilulissat, located only ~45 km further away. This suggests that most of the uplift is due to the unloading of the Earth's surface as Jakobshavn thins and loses mass.

Our estimate of Jakobshavn's contribution to uplift rates at Kangia North and Ilulissat are 14.6 plus/minus 1.7 mm yr-1 and 4.9 plus/minus 1.1 mm yr-1, respectively. The observed rates are consistent with a glacier thinning model based on repeat altimeter surveys from NASA's Airborne Topographic Mapper (ATM), which shows that Jakobshavn lost mass at an average rate of 22 plus/minus 2 km3 yr-1 between 2006 and 2009. At Kangia North and Ilulissat, the predicted uplift rates computed using thinning estimates from the ATM laser altimetry are 12.1 plus/minus 0.9 mm yr-1 and 3.2 x 0.3 mm yr-1, respectively. The observed rates are slightly larger than the predicted rates. The fact that the GPS uplift rates are much larger closer to Jakobshavn than further away, and are consistent with rates inferred using the ATM-based glacier thinning model, shows that GPS measurements of crustal motion are a potentially useful method for assessing ice-mass change models.

Greenland flow variability from ice-sheet-wide velocity mapping

Joughin, I., B.E. Smith, I.M. Howat, T. Scambos, and T. Moon, "Greenland flow variability from ice-sheet-wide velocity mapping," J. Glaciol., 56, 415-430, doi:10.3189/002214310792447734, 2010.

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1 Aug 2010

Using RADARSAT synthetic aperture radar data, we have mapped the flow velocity over much of the Greenland ice sheet for the winters of 2000/01 and 2005/06. These maps provide a detailed view of the ice-sheet flow, including that of the hundreds of glaciers draining the interior. The focused patterns of flow at the coast suggest a strong influence of bedrock topography. Differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes. The overall pattern is one of speed-up accompanied by terminus retreat, but there are also several instances of surge behavior and a few cases of glacier slowdown. Comprehensive mappings such as these, at regular intervals, provide an important new observational capability for understanding ice-sheet variability.

Synthesizing multiple remote-sensing techniques for subglacial hydrologic mapping: Application to a lake system beneath MacAyeal Ice Stream, West Antarctica

Fricker, H.A., T. Scambos, S. Carter, C. Davis, T. Haran, and I. Joughin, "Synthesizing multiple remote-sensing techniques for subglacial hydrologic mapping: Application to a lake system beneath MacAyeal Ice Stream, West Antarctica," J. Glaciol., 56, 187-199, doi:10.3189/002214310791968557, 2010.

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1 Jun 2010

We present an analysis of the active hydrologic system of MacAyeal Ice Stream (MacIS), West Antarctica, from a synthesis of multiple remote-sensing techniques: satellite laser altimetry; satellite image differencing; and hydrologic potential mapping (using a satellite-derived DEM and a bedrock DEM from airborne radio-echo sounding). Combining these techniques augments the information provided by each one individually, and allows us to develop a protocol for studying subglacial hydrologic systems in a holistic manner.

Our study reveals five large active subglacial lakes under MacIS, the largest of which undergoes volume changes of at least 1.0 km3. We discuss the hydrologic properties of this system and present evidence for links between the lakes. At least three of the lakes are co-located with sticky spots, i.e., regions of high local basal shear stress. We also find evidence for surface elevation changes due to ice-dynamic effects (not just water movement) caused by changes in basal resistance. Lastly, we show that satellite radar altimetry is of limited use for monitoring lake activity on fast-flowing ice streams with surfaces that undulate on ~10 km length scales.

An inventory of active subglacial lakes in Antarctica detected by ICESat (2003-2008)

Smith, B.E., H.A. Fricker, I.R. Joughin, and S. Tulaczyk, "An inventory of active subglacial lakes in Antarctica detected by ICESat (2003-2008)," J. Glaciol., 55, 573-595, doi:10.3189/002214309789470879, 2009.

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1 Sep 2009

Through the detection of surface deformation in response to water movement, recent satellite studies have demonstrated the existence of subglacial lakes in Antarctica that fill and drain on timescales of months to years. These studies, however, were confined to specific regions of the ice sheet. Here we present the first comprehensive study of these 'active' lakes for the Antarctic ice sheet north of 86°S, based on 4.5 years (2003-08) of NASA's Ice, Cloud and land Elevation Satellite (ICESat) laser altimeter data. Our analysis has detected 124 lakes that were active during this period, and we estimate volume changes for each lake. The ICESat-detected lakes are prevalent in coastal Antarctica, and are present under most of the largest ice-stream catchments. Lakes sometimes appear to transfer water from one to another, but also often exchange water with distributed sources undetectable by ICESat, suggesting that the lakes may provide water to or withdraw water from the hydrologic systems that lubricate glacier flow. Thus, these reservoirs may contribute pulses of water to produce rapid temporal changes in glacier speeds, but also may withdraw water at other times to slow flow.

Constraints on the lake volume required for hydro-fracture through ice sheets

Krawczynski, M.J., M.D. Behn, S.B. Das, and I. Joughin, "Constraints on the lake volume required for hydro-fracture through ice sheets," Geophys. Res. Lett., 36, 10.1029/2008GL036765, 2009.

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16 May 2009

Water-filled cracks are an effective mechanism to drive hydro-fractures through thick ice sheets. Crack geometry is therefore critical in assessing whether a supraglacial lake contains a sufficient volume of water to keep a crack water-filled until it reaches the bed. In this study, we investigate fracture propagation using a linear elastic fracture mechanics model to calculate the dimensions of water-filled cracks beneath supraglacial lakes. We find that the cross-sectional area of water-filled cracks increases non-linearly with ice sheet thickness. Using these results, we place volumetric constraints on the amount of water necessary to drive cracks through ~1 km of sub-freezing ice. For ice sheet regions under little tension, lakes larger than 0.25–0.80 km in diameter contain sufficient water to rapidly drive hydro-fractures through 1–1.5 km of subfreezing ice. This represents ~98% of the meltwater volume held in supraglacial lakes in the central western margin of the Greenland Ice Sheet.

Basal conditions for Pine Island and Thwaites glaciers, West Antarctica, determined using satellite and airborne data

Joughin, I., S. Tulaczyk, J.L. Bamber, D. Blankenship, J.W. Holt, T. Scambos, and D.G. Vaughan, "Basal conditions for Pine Island and Thwaites glaciers, West Antarctica, determined using satellite and airborne data," J. Glaciol., 55, 245-257, doi:10.3189/002214309788608705, 2009.

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1 Apr 2009

We use models constrained by remotely sensed data from Pine Island and Thwaites Glaciers, West Antarctica, to infer basal properties that are difficult to observe directly. The results indicate strong basal melting in areas upstream of the grounding lines of both glaciers, where the ice flow is fast and the basal shear stress is large. Farther inland, we find that both glaciers have 'mixed' bed conditions, with extensive areas of both bedrock and weak till. In particular, there are weak areas along much of Pine Island Glacier's main trunk that could prove unstable if it retreats past the band of strong bed just above its current grounding line.

In agreement with earlier studies, our forward ice-stream model shows a strong sensitivity to small perturbations in the grounding line position. These results also reveal a large sensitivity to the assumed bed (sliding or deforming) model, with non-linear sliding laws producing substantially greater dynamic response than earlier simulations that assume a linear-viscous till rheology. Finally, comparison indicates that our results using a plastic bed are compatible with the limited observational constraints and theoretical work that suggests an upper bound exists on maximum basal shear stress.

Greenland ice sheet motion coupled with daily melting in late summer

Shepherd, A., A. Hubbard, M. King, M. McMillan, and I. Joughin, "Greenland ice sheet motion coupled with daily melting in late summer," Geophys. Res. Lett., 36, 10.1029/2008GL035758, 2009.

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7 Jan 2009

We use ground-based and satellite observations to detect large diurnal and longer-period variations in the flow of the Greenland Ice Sheet (GrIS) during late summer that are strongly coupled with changes in its surface hydrology. The diurnal signals are associated with periodic changes in surface melting, and the longer-period signals are associated with the episodic drainage of supra-glacial lakes. Ice velocity doubles around 2 hours after peak daily melting and returns approximately to wintertime levels around 12 hours afterwards, demonstrating an intimate link between the surface and basal hydrology. During late summer, the ice sheet accelerates by 35% per positive degree-day of melting. The observed link between surface melting and enhanced flow is typical of Alpine glaciers, which may provide an appropriate analogue for the evolution of the GrIS in a warming climate.

A simple law for ice-shelf calving

Alley, R.B., H.J. Horgan, I. Joughin, K.M Cuffey, T.K. Dupont, B.R. Parizek, S. Anandakrishnan, J. Bassis, "A simple law for ice-shelf calving," Science, 322, 1344, doi:10.1126/science.1162543, 2008.

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28 Nov 2008

A major problem for ice-sheet models is that no physically based law for the calving process has been established. Comparison across a diverse set of ice shelves demonstrates that iceberg calving increases with the along-flow spreading rate of a shelf. This relation suggests that frictional buttressing loss, which increases spreading, also leads to shelf retreat, a process known to accelerate ice-sheet flow and contribute to sea-level rise.

Continued evolution of Jakobshavn Isbrae following its rapid speedup

Joughin, I., I. Howat, M. Fahnestock, B. Smith, W. Krabill, R. Alley, H. Stern, and M. Truffer, "Continued evolution of Jakobshavn Isbrae following its rapid speedup," J. Geophys. Res., 113, doi:10.1029/2008JF001023, 2008.

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28 Oct 2008

Several new data sets reveal that thinning and speedup of Jakobshavn Isbrae continue, following its recent rapid increase in speed as its floating ice tongue disintegrated. The present speedup rate of –5% a-1 over much of the fast-moving region appears to be a diffusive response to the initial much larger speedup near the front. There is strong seasonality in speed over much of the fast-flowing main trunk that shows a good inverse correlation with the seasonally varying length of a short (typically ~6 km) floating ice tongue. This modulation of speed with ice front position supports the hypothesis that the major speedup was caused by loss of the larger floating ice tongue from 1998 to 2003. Analysis of image time series suggests that the transient winter ice tongue is formed when sea ice bonds glacier ice in the fjord to produce a nearly rigid mass that almost entirely suppresses calving. Major calving only resumes in late winter when much of this ice clears from the fjord. The collapse of the ice tongue in the late 1990s followed almost immediately after a sharp decline in winter sea-ice concentration in Disko Bay. This decline may have extended the length of the calving season for several consecutive years, leading to the ice tongue's collapse.

Rates of southeast Greenland ice volume loss from combined ICESat and ASTER observations

Howat, I.M., B.E. Smith, I. Joughin, and T.A. Scambos, "Rates of southeast Greenland ice volume loss from combined ICESat and ASTER observations," Geophys. Res. Lett., 35, doi:10.1029/2008GL034496, 2008.

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9 Sep 2008

Repeat satellite laser altimetry is critical for observing the rapidly changing mass balance of the Greenland Ice Sheet. However, sparse sampling and high surface slopes over rapidly thinning, coastal outlet glaciers may result in underestimation of mass loss. Here we supplement ICESat-derived surface elevation changes with differenced ASTER digital elevation models of outlet glaciers in southeastern Greenland, the region with the largest concentrated change in outlet glacier mass loss.

We estimate a 2002–2005 regional volume-loss rate of 108 km3/yr. Our results are consistent with drainage-scale GRACE and mass-budget estimates when differences in observation periods are taken into account. The two largest glaciers, Kangerdlugssuaq and Helheim, account for only 28% of the mass loss, illustrating the combined importance of smaller glaciers and the need for complete observational coverage. Additionally, we find that rapid, concentrated thinning within the outlets represents a small contribution to the total volume change compared to dispersed inland thinning.

Changes in ice front position on Greenland's outlet glaciers from 1992 to 2007

Moon, T., and I. Joughin, "Changes in ice front position on Greenland's outlet glaciers from 1992 to 2007," J. Geophys. Res., 113, doi:10.1029/2007JF000927, 2008.

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7 Jun 2008

Previous studies in Greenland show that retreat of tidewater glaciers may be linked to recent increases in ice loss, raising Greenland's contribution to sea level rise. We examined ice front changes of 203 tidewater glaciers, land-terminating glaciers, and glaciers terminating with ice shelves to understand Greenland glacier behavior over three periods: 1992–2000, 2000–2006, and 2006–2007. We observed synchronous, ice sheet-wide increases in tidewater retreat during 2000–2006 relative to 1992–2000, coinciding with a 1.1°C increase in mean summer temperature at coastal weather stations. Rates of retreat for the southeast and east slowed during 2006–2007 when temperatures were slightly cooler than the 2000–2006 average. Our work suggests that regional Greenland tidewater retreat responds strongly to climate change, with higher temperatures corresponding to increasing retreat, and helps confirm a link between ice thickness, velocity, and ice front position.

Fracture propagation to the base of the Greenland ice sheet during supraglacial lake drainage

Das, S.B., I. Joughin, M.D. Behn, I.M. Howat, M.A. King, D. Lizarralde, and M.P. Bhatia, "Fracture propagation to the base of the Greenland ice sheet during supraglacial lake drainage," Science, 320, 778-781, 2008.

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9 May 2008

Surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change. The process whereby such a pathway is created through thick, cold ice has not, however, been previously observed. We describe the rapid (<2 hours) drainage of a large supraglacial lake down 980 meters through to the bed of the Greenland Ice Sheet initiated by water-driven fracture propagation evolving into moulin flow. Drainage coincided with increased seismicity, transient acceleration, ice-sheet uplift, and horizontal displacement. Subsidence and deceleration occurred over the subsequent 24 hours. The short-lived dynamic response suggests that an efficient drainage system dispersed the meltwater subglacially. The integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup.

Seasonal speedup along the western flank of the Greenland ice sheet

Joughin, I., S.B. Das, M.A. King, B.E. Smith, I.M. Howat, and T. Moon, "Seasonal speedup along the western flank of the Greenland ice sheet," Science, 320, 791-883, 2008.

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9 May 2008

It has been widely hypothesized that a warmer climate in Greenland would increase the volume of lubricating surface meltwater reaching the ice-bedrock interface, accelerating ice flow and increasing mass loss. We have assembled a data set that provides a synoptic-scale view, spanning ice-sheet to outlet-glacier flow, with which to evaluate this hypothesis. On the ice sheet, these data reveal summer speedups (50 to 100%) consistent with, but somewhat larger than, earlier observations. The relative speedup of outlet glaciers, however, is far smaller (<15%). Furthermore, the dominant seasonal influence on Jakobshavn Isbrae's flow is the calving front's annual advance and retreat. With other effects producing outlet-glacier speedups an order of magnitude larger, seasonal melt's influence on ice flow is likely confined to those regions dominated by ice-sheet flow.

Ice-front variation and tidewater behavior on Helheim and Kangerdlugssuaq Glaciers, Greenland

Joughin, I., I. Howat, R.B. Alley, G. Ekstrom, M. Fahnestock, T. Moon, M. Nettles, M. Truffer, and V.C. Tsai, "Ice-front variation and tidewater behavior on Helheim and Kangerdlugssuaq Glaciers, Greenland," J. Geophys. Res., 113, doi:10.1029/2007JF000837, 2008.

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26 Jan 2008

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.

Numerical modeling of ocean-ice interactions under Pine Island Bay's ice shelf

Payne, A.J., P.R. Holland, A.P. Shepherd, I.C. Rutt, A. Jenkins, and I. Joughin, "Numerical modeling of ocean-ice interactions under Pine Island Bay's ice shelf," J. Geophys. Res., 112, doi:10.1029/2006JC003733, 2007.

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19 Oct 2007

A two-dimensional numerical model is used to simulate the dynamics of buoyant, meltwater-rich plumes flowing beneath the ice shelf occupying much of Pine Island Bay, West Antarctica. Recent studies have shown that this ice shelf, along with all others fringing the Amundsen Sea, is thinning rapidly. In the model, both the Coriolis effect and subshelf topography are important in controlling plume dynamics and the spatial distribution of ice melt. Melt is concentrated in a narrow zone within ~20 km of the grounding line where steep subshelf slopes and access to warm ambient water allow melt rates to exceed 100 m yr-1. The plume generated by entrainment of ambient water into the meltwater in these areas is guided by the topography of the ice shelf underside and exits the ice shelf at three distinct outflow locations. Melt rates generated along the course of the plume are higher (approximately 2.5x) than rates elsewhere. The model suggests that the observed ice shelf thinning rates could have resulted from a hypothetical instantaneous 0.25°C warming of the ambient water entrained by the plume. A context for this value is provided by the 40-year warming trend documented by Jacobs et al. (2002) for Circumpolar Deep Water in the nearby Ross Sea.

Rapid response of modern day ice sheets to external forcing

Bamber, J.L., R.B. Alley, and I. Joughin, "Rapid response of modern day ice sheets to external forcing," Earth Plant. Sci. Lett., 257, 1-13, doi:10.1016/j.epsl.2007.03.005, 2007.

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15 May 2007

The great ice sheets covering Antarctica and Greenland were, traditionally, believed to take thousands of years to respond to external forcing. Recent observations suggest, however, that major changes in the dynamics of parts of the ice sheets are taking place over timescales of years. These changes were not predicted by numerical models, and the underlying cause(s) remains uncertain. It has been suggested that regional oceanic and/or atmospheric warming are responsible but separating the influence and importance of these two forcings has not been possible. In most cases, the role of atmospheric versus oceanic control remains uncertain. Here, we review the observations of rapid change and discuss the possible mechanisms, in the light of advances in numerical modelling and our understanding of the processes that may be responsible.

Rapid changes in ice discharge from Greenland outlet glaciers

Howat, I.M., I. Joughin, and T.A. Scambos, "Rapid changes in ice discharge from Greenland outlet glaciers," Science, 315, 1559-1561, 2007.

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16 Mar 2007

Using satellite-derived surface elevation and velocity data, we found major short-term variations in recent ice discharge and mass loss at two of Greenland's largest outlet glaciers. Their combined rate of mass loss doubled in less than a year in 2004 and then decreased in 2006 to near the previous rates, likely as a result of fast re-equilibration of calving-front geometry after retreat. Total mass loss is a fraction of concurrent gravity-derived estimates, pointing to an alternative source of loss and the need for high-resolution observations of outlet dynamics and glacier geometry for sea-level rise predictions.

Large subglacial lakes in East Antarctica at the onset of fast-flowing ice streams

Bell, R.E., M. Studinger, C.A. Shuman, M.A. Fahnestock, and I. Joughin, "Large subglacial lakes in East Antarctica at the onset of fast-flowing ice streams," Nature, 445, 907-907, 2007.

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22 Feb 2007

Water plays a crucial role in ice-sheet stability and the onset of ice streams. Subglacial lake water moves between lakes and rapidly drains, causing catastrophic floods. The exact mechanisms by which subglacial lakes influence ice-sheet dynamics are unknown, however, and large subglacial lakes have not been closely associated with rapidly flowing ice streams. Here we use satellite imagery and ice-surface elevations to identify a region of subglacial lakes, similar in total area to Lake Vostok, at the onset region of the Recovery Glacier ice stream in East Antarctica and predicted by ice-sheet models. We define four lakes through extensive, flat, featureless regions of ice surface bounded by upstream troughs and downstream ridges. Using ice velocities determined using interferometric synthetic aperture radar (InSAR), we find the onset of rapid flow (moving at 20 to 30 m yr-1) of the tributaries to the Recovery Glacier ice stream in a 280-km-wide segment at the downslope margins of these four subglacial lakes. We conclude that the subglacial lakes initiate and maintain rapid ice flow through either active modification of the basal thermal regime of the ice sheet by lake accretion or through scouring bedrock channels in periodic drainage events. We suggest that the role of subglacial lakes needs to be considered in ice-sheet mass balance assessments.

East Antarctic ice stream tributary underlain by major sedimentary basin

Bamber, J.L., F. Ferraccioli, I. Joughin, T. Shepherd, D.M. Rippin, M.J. Siegert, and D.G. Vaughan, "East Antarctic ice stream tributary underlain by major sedimentary basin," Geology, 34, 33-36, doi:10.1130/G22160.1, 2006.

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1 Jan 2006

Marine and rift sediments exert a fundamental control on ice stream flow in the West Antarctic Ice Sheet, and hence on its mass balance and stability. In contrast, most ice streams in the much larger East Antarctic Ice Sheet are thought to be relatively stable features resting on till, perhaps underlain by crystalline rock. Any geological controls on East Antarctic Ice Sheet enhanced flow remain largely unknown. We present aerogeophysical evidence indicating that a region of enhanced ice flow in the interior of the East Antarctic Ice Sheet is underlain by subglacial sediments ~3 km thick and that these are influencing the flow regime of the overlying ice. We show that subglacial sediments are important in modulating ice dynamics, not just for the West Antarctic Ice Sheet, but also for its much larger neighbor, and suggest that the sedimentary basin identified here may contain information on the Neogene glacial history of this part of the East Antarctic Ice Sheet.

Rapid retreat and acceleration of Helheim Glacier, east Greenland

Howat, I.M., I. Joughin, S. Tulaczyk, and S. Gogineni, "Rapid retreat and acceleration of Helheim Glacier, east Greenland," Geophys. Res. Lett., 32, 10.1029/2005GL024737, 2005

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22 Nov 2005

A significant amount of the measured coastal thinning of the Greenland ice sheet may be due to recent acceleration of outlet glaciers. Using remote sensing, we measured two major periods of speedup on Helheim Glacier between 2000 and 2005 that increased peak speeds from approximately 8 to 11 km/yr. These speedups coincided with rapid retreats of the calving front, totaling over 7.5 km. The glacier also thinned by over 40 m from 2001 to 2003. Retreat of the ice front appears to decrease resistance to flow and concentrates the gravitational driving force over a smaller area. Farther up-glacier, acceleration may be a delayed response to surface draw-down and steepening of the glacier's main trunk. If the 2005 speedup also produces strong thinning, then much of the glacier's main trunk may un-ground, leading to further retreat.

Continued deceleration of Whillans Ice Stream, West Antarctica

Joughin, I., R.A. Binschadler, M.A. King, D. Voigt, R.B. Alley, S. Anandakrishnan, H. Horgan, L. Peters, P. Winberry, S.B. Das, and G. Catania, "Continued deceleration of Whillans Ice Stream, West Antarctica," Geophys. Res. Lett., 32, doi:10.1029/2005GL024319, 2005.

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17 Nov 2005

Earlier observations indicated that Whillans Ice Stream slowed from 1973 to 1997. We collected new GPS observations of the ice stream's speed in 2003 and 2004. These data show that the ice stream is continuing to decelerate at rates of about 0.6%/yr2, with faster rates near the grounding line. Our data also indicate that the deceleration extends over the full width of the ice plain. Extrapolation of the deceleration trend suggests the ice stream could stagnate sometime between the middle of the 21st and 22nd Centuries.

Ice-sheet and sea-level changes

Alley, R.B., P.U. Clark, P. Huybrechts, and I. Joughin, "Ice-sheet and sea-level changes," Science, 310, 456-460, 2005

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21 Oct 2005

Future sea-level rise is an important issue related to the continuing buildup of atmospheric greenhouse gas concentrations. The Greenland and Antarctic ice sheets, with the potential to raise sea level ~70 meters if completely melted, dominate uncertainties in projected sea-level change. Freshwater fluxes from these ice sheets also may affect oceanic circulation, contributing to climate change. Observational and modeling advances have reduced many uncertainties related to ice-sheet behavior, but recently detected, rapid ice-marginal changes contributing to sea-level rise may indicate greater ice-sheet sensitivity to warming than previously considered.

Marine ice modification of fringing ice shelf flow

Hulbe, C.L., R. Johnston, I. Joughin, and T. Scambos, "Marine ice modification of fringing ice shelf flow," Arctic Antarctic Alp. Res., 37, 323-330, 2005

30 Aug 2005

Subglacial conditions during and after stoppage of an Antarctic Ice Stream: Is reactivation imminent?

Vogel, S.W., S. Tulaczyk, B. Kamb, H. Engelhardt, F.E. Carsey, A.E. Behar, A.L. Lane, and I. Joughin, "Subglacial conditions during and after stoppage of an Antarctic Ice Stream: Is reactivation imminent?" Geophys. Res. Lett., 32, 10.1029/2005GL022563, 2005

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20 Jul 2005

Borehole observations from the base of the West-Antarctic Ice Sheet (WAIS) reveal the presence of a 10 to 15 m thick accretionary basal ice layer in the upstream area of Kamb Ice Stream (KIS). This ice layer has formed over a time of several thousand years by freeze-on of subglacial water to the ice base and has recorded during this time basal conditions upstream of its current location. Analysis of samples and videos sequences from boreholes drilled to the bottom of KIS confirms that KIS-stoppage was due to basal freeze-on and that relubrication of the ice stream is well underway. These results further suggest that ice stream cyclicity may be shorter than expected (1000s of years) and that a restart of KIS may be imminent within decades to centuries.

Rheology of the Ronne Ice Shelf, Antarctica, inferred from satellite radar interferometry data using an inverse control method

Larour, E., E. Rignot, I. Joughin, and D. Aubry, "Rheology of the Ronne Ice Shelf, Antarctica, inferred from satellite radar interferometry data using an inverse control method," Geophys. Res. Lett., 32, 10.1029/2004GL021693, 2005.

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8 Mar 2005

The Antarctic Ice Sheet is surrounded by large floating ice shelves that spread under their own weight into the ocean. Ice shelf rigidity depends on ice temperature and fabrics, and is influenced by ice flow and the delicate balance between bottom and surface accumulation. Here, we use an inverse control method to infer the rigidity of the Ronne Ice Shelf that best matches observations of ice velocity from satellite radar interferometry. Ice rigidity, or flow law parameter B, is shown to vary between 300 and 900 kPa a1/3. Ice is softer along the side margins due to frictional heating, and harder along the outflow of large glaciers, which advect cold continental ice. Melting at the bottom surface of the ice shelf increases its rigidity, while freezing decreases it. Accurate numerical modelling of ice shelf flow must account for this spatial variability in mechanical characteristics.

Evidence for subglacial water transport in the West Antarctic ice sheet through three-dimensional satellite radar interferometry

Gray, L., I. Joughin, S. Tulaczyk, V.B. Spikes, R. Bindschadler, and K.C. Jezek, "Evidence for subglacial water transport in the West Antarctic ice sheet through three-dimensional satellite radar interferometry," Geophys. Res. Lett., 32, doi:10.1029/2004GL021387, 2005.

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8 Feb 2005

RADARSAT data from the 1997 Antarctic Mapping Mission are used interferometrically to solve for the 3-dimensional surface ice motion in the interior of the West Antarctic Ice Sheet (WAIS). An area of ~125 km2 in a tributary of the Kamb Ice Stream slumped vertically downwards by up to ~50 cm between September 26 and October 18, 1997. Areas in the Bindschadler Ice Stream also exhibited comparable upward and downward surface displacements. As the uplift and subsidence features correspond to sites at which the basal water apparently experiences a hydraulic potential well, we suggest transient movement of pockets of subglacial water as the most likely cause for the vertical surface displacements. These results, and related lidar observations, imply that imaging the change in ice surface elevation can help reveal the key role of water in the difficult-to-observe subglacial environment, and its important influence on ice dynamics.

Calving of large tabular icebergs from ice shelf rift systems

Joughin, I., and D.R. MacAyeal, "Calving of large tabular icebergs from ice shelf rift systems," Geophys. Res. Lett., 32, 10.1029/2004GL020978, 2005.

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21 Jan 2005

We used Interferometric Synthetic Aperture Radar to study the detachment process that allowed two large icebergs to calve from the Ross Ice Shelf, Antarctica. Time series of rift geometries indicate that rift widths increased steadily, whereas rift lengths increased episodically through several discrete rift-tip propagation events. We also conducted modeling experiments constrained by the observed rift geometry. Both the observations and model suggest that rift opening, and, thus, tabular-iceberg calving, are largely driven by "glaciological" stresses — stress introduced by the effect of gravity on the ice shelf — rather than by stress introduced by the ocean and atmosphere, e.g., tides and storms. This style of rift propagation is expected to determine the steady, background calving rate of ice shelves and, thus, differs significantly from styles that led to the recent disintegration of ice shelves in response to climate warming, e.g., the Larsen B Ice Shelf on the Antarctic Peninsula.

Large fluctuations in speed on Greenland's Jakobshavn Isbrae glacier

Joughin, I., W. Abdalati, and M. Fahnestock, "Large fluctuations in speed on Greenland's Jakobshavn Isbrae glacier," Nature, 432, 608-610, 2004.

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2 Dec 2004

It is important to understand recent changes in the velocity of Greenland glaciers because the mass balance of the Greenland Ice Sheet is partly determined by the flow rates of these outlets. Jakobshavn Isbrae is Greenland's largest outlet glacier, draining about 6.5 per cent of the ice-sheet area, and it has been surveyed repeatedly since 1991. Here we use remote sensing data to measure the velocity of Jakobshavn Isbrae between 1992 and 2003. We detect large variability of the velocity over time, including a slowing down from 6,700 m yr-1 in 1985 to 5,700 m yr-1 in 1992, and a subsequent speeding up to 9,400 m yr-1 by 2000 and 12,600 m yr-1 in 2003. These changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter. Our observations indicate that fast-flowing glaciers can significantly alter ice discharge at sub-decadal timescales, with at least a potential to respond rapidly to a changing climate.

Basal shear stress of the Ross ice streams from control method inversions

Joughin, I., D.R. MacAyeal, and S. Tulaczyk, "Basal shear stress of the Ross ice streams from control method inversions," J. Geophys. Res., 109, B09405, 10.1029/2003JB002960, 2004.

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24 Sep 2004

We used control method inversions to determine the basal shear stress beneath the Ross ice streams where new high-resolution velocity data sets have recently become available. The inversion algorithm was adapted from an earlier viscous bed algorithm to allow solution for the basal shear stress corresponding to a weak plastic bed. We performed several experiments using synthetic data to determine the quality of the inversions. These experiments indicate that with high-quality surface elevation data (e.g., errors <5 m), the inversions are relatively robust with respect to errors in ice flow velocity and bed topography. The inversions are consistent with seismic and borehole observations and indicate that the Ross ice streams lie atop a bed that is nearly everywhere weak. In contrast, the tributaries feeding these ice streams overlie alternating patches of strong and weak bed.

Ice flow direction change in interior West Antarctica

Seegert, M.J., B. Welch, D. Morse, A. Vieli, D.D. Blankenship, I. Joughin, E.C. King, G. Vieli, A.J. Payne, and R. Jacobel, "Ice flow direction change in interior West Antarctica," Science, 305, 1948-1951, 2004.

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24 Sep 2004

Upstream of Byrd Station (West Antarctica), ice-penetrating radar data reveal a distinctive fold structure within the ice, in which isochronous layers are unusually deep. The fold has an axis more than 50 kilometers long, which is aligned up to 45° to the ice flow direction. Although explanations for the fold's formation under the present flow are problematic, it can be explained if flow was parallel to the fold axis ~1500 years ago. This flow change may be associated with ice stream alterations nearer the margin. If this is true, central West Antarctica may respond to future alterations more than previously thought.

In The News

Hidden lakes drain below West Antarctica's Thwaites Glacier

UW News and Information, Hannah Hickey

Thwaites Glacier on the edge of West Antarctica is one of the planet’s fastest-moving glaciers. Research shows that it is sliding unstoppably into the ocean, mainly due to warmer seawater lapping at its underside.

8 Feb 2017

Satellite system tracks glaciers' flow in real time

Nature News, Jeff Tollefson

The Global Land Ice Velocity Extraction project (GoLIVE) is the first to provide scientists with regular, semi-automated measurements of ice movement across the entire world. The Landsat 8 satellite covers the planet every 16 days.

16 Dec 2016

RIft in Pine Island glacier points to a coming, broader collapse

Mashable, Maria Gallucci

Scientists say they discovered the reason why a massive iceberg splintered off one of West Antarctica's largest glaciers last year. Ian Joughin comments that the new findings are "something to be concerned about, but it's too soon to tell whether this might be a process that could alter the already substantial pace of retreat" on Pine Island.

28 Nov 2016

More News Items

UW has 29 faculty on list of ‘highly cited researchers’ for 2016

UW News and Information

Twenty-nine University of Washington faculty members are among a list of the year’s most highly cited researchers in the natural and social sciences.

22 Nov 2016

John Kerry just visited the most stunning example of our changing climate

The Washington Post, Chris Mooney

John Kerry visited perhaps the starkest indicator of the problem in our hemisphere: the enormous Jakobshavn glacier of Greenland. The article quotes Ian Joughin's recent studies on the glacier's increasing flow speed, retreat rate, and contribution to global sea level rise.

19 Jun 2016

We've been getting these key details about Greenland’s melting all wrong

Washington Post, Chelsea Harvey

Ice melt in Greenland is one of the biggest preoccupations for climate scientists today, mostly because of the ice sheet's potential contributions to sea-level rise. But there’s still a lot we don’t know about the processes that contribute to melting. Ian Joughin comments that scientists knew the prior bathymetric maps were wrong, and this paper shows just how much.

1 Apr 2016

With collapsing West Antarctica, sea level rise may be twice as high as we thought

Mashable, Andrew Freedman

A study published in the journal Nature on Wednesday, indicates that this ice sheet is far more sensitive to changes in the amount of greenhouse gases in the atmosphere as well as air and ocean temperatures than previously thought. APL-UW glaciologist Ian Joughin notes the resolution of the model used in the study, and urges caution with interpretation.

31 Mar 2016

The secrets in Greenland's Ice Sheet

New York Times Magazine, Jon Gertner

By studying the largest glaciers on earth, scientists hope to determine whether we%u2019ll have time to respond to climate change or whether it%u2019s already too late. Glaciologist Ian Joughin contributes expertise on in the Antarctic and Greenland ice sheets.

12 Nov 2015

Is Antarctica losing ice or gaining it?

Scientific American, Malavika Vyawahare

A recent study reporting that the Antarctic ice sheet is expanding because accumulated snowfall is outpacing melting glaciers has drawn sharp criticism from many climate scientists. Ian Joughin, from the APL-UW Polar Science Center, weighs in.

5 Nov 2015

What Antarctica's incredible "growing" icepack really means

National Geographic, Brian Clark Howard

Are the Antarctic's ice sheets shrinking or growing? And what does that mean for global sea-level rise? Ben Smith, who was not involved with the study, notes that the technology used to collect surface elevation measurements might not be up to the task of distinguishing snowpack volume based on differences of one or two centimeters.

3 Nov 2015

Antarctic melting irreversible in 60 years

Discovery News, Eric Niiler

In 2014, climate researchers at the University of Washington reported in the journal Science that the melting of two glaciers — Thwaite's and Pine Island —- in the West Antarctic, was underway and would lead to the destabilization of the larger ice sheet from which they arise. A new paper published by other researchers reports that the destabilization of the entire ice sheet would be irreversible if current conditions don't change by 2075.

2 Nov 2015

Scientists declare an 'urgent' mission — study West Antarctica, and fast

Washington Post, Chris Mooney

Scientists who have been raising alarms about the endangered ice sheet of West Antarctica say they've identified a key glacier that could pose the single most immediate threat to the world's coastlines — and are pushing for an urgent new effort to study it.

29 Sep 2015

Two UW researchers elected AGU fellows

UW News and Information, Hannah Hickey

Two University of Washington scientists have been elected as new fellows of the American Geophysical Union. The Earth sciences group recognizes only one in 1,000 members each year for major scientific work and sustained impact. The UW honorees are among 60 new 2015 fellows from U.S. and international institutions. They will both be honored in December at the union%u2019s annual meeting in San Francisco.

29 Jul 2015

Prediction of rapid sea level rise won't change global climate talks

National Geographic, Brian Clark Howard

A bombshell climate study due out this week warns that sea levels may rise a catastrophic 10 feet by the end of this century. Ian Joughin responds the study's doubling in the rate of ice melt in Greenland seems unlikely given past trends and what the scientific community know about the processes.

21 Jul 2015

Greenland's glaciers are accelerating so fast, they have stretch marks

Slate, Eric Holthaus

NASA's Operation IceBridge is an initiative to map the planet%u2019s dwindling ice sheets by aircraft in three dimensions. Now in its eighth year, Operation IceBridge is starting to produce some stunning results. As glaciers accelerate, they should stretch more and produce more crevasses.

14 May 2015

The big melt: Antarctica's retreating ice may re-shape Earth


Water is eating away at the Antarctic ice, melting it where it hits the oceans. As the ice sheets slowly thaw, water pours into the sea.

27 Feb 2015

Collapse or catastrophe?

The Economist

The West Antarctic ice sheet looks doomed — eventually. Glaciologists use the word "collapse" to describe a shift towards an irretrievable loss of an ice sheet. There is, reckons Dr Joughin, probably nothing that can now be done to save the Thwaites glacier.

17 May 2014

Antarctic ice sheet slipping into the sea

National Public Radio: Science Friday, Ira Flatow

Two studies out this week confirm that the glaciers of West Antarctica have begun to slip into the Amundsen Sea. Scientists estimate it could take 200 years for the ice sheet to completely melt. Glaciologist Ian Joughin tells us what this could mean for rising sea levels.

16 May 2014

Antarctic glaciers melting 'passed point of no return'

USA Today, Traci Watson

The vast glaciers of western Antarctica are rapidly melting and losing ice to the sea and almost certainly have 'passed the point of no return,' according to new work by two separate teams of scientists.

13 May 2014

New study: Glacial collapse in Antarctica 'unstoppable'

KUOW Radio, Steve Scher, Ashley Ahearn, and Posey Gruener

New research from the University of Washington and other institutions provides detailed predictions for the collapse of an ice shelf in West Antarctica. When the Thwaites Glacier melts, it could trigger even more extreme sea level rise than scientists previously thought.

13 May 2014

UW researchers: Polar ice sheet doomed, but how soon?

The Seattle Times, Craig Welch

It%u2019s too late to halt the collapse of the West Antarctic ice sheet into the sea, triggering several feet of sea-level rise, scientists have found. But UW researchers say the speed of that collapse depends on our response to climate change.

13 May 2014

Antarctic glacier loss is 'unstoppable,' study says

Time, Bryan Walsh

The Intergovernmental Panel on Climate Change had projected that sea level will rise by about 35.5 in (98 cm) at most by 2100, but that prediction will likely need to be revisited in the wake of these new studies.

12 May 2014

Irreversible collapse of Antarctic glaciers has begun, studies say

Los Angeles Times, Scott Gold

A slow-motion and irreversible collapse of a massive cluster of glaciers in Antarctica has begun, and could cause sea levels to rise across the planet by another 4 feet within 200 years, scientists concluded in two studies released Monday.

12 May 2014

Melting of Antarctic ice sheet might be unstoppable

National Public Radio, Scott Neuman

Scientists have long worried about climate change-induced melting of the huge West Antarctic Ice Sheet. Now they say that not only is the disintegration of the ice already underway, but that it's likely unstoppable.

12 May 2014

NASA spots worrisome Antarctic ice sheet melt

The Washington Post

The huge West Antarctic ice sheet is starting a glacially slow collapse in an unstoppable way, two new studies show. Alarmed scientists say that means even more sea level rise than they figured.

12 May 2014

Scientists warm of melting ice sheet, rising sea level

The Wall Street Journal, Robert Lee Hotz

Six rapidly melting glaciers in Antarctica are destabilizing one of the world's largest ice sheets, a process which, if unchecked, could release enough water to raise sea levels world-wide significantly in centuries to come.

12 May 2014

Scientists warn of rising oceans from polar melt

The New York Times, Justin Gillis and Kenneth Chang

A large section of the mighty West Antarctica ice sheet has begun falling apart and its continued melting now appears to be unstoppable, two groups of scientists reported.

12 May 2014

Studies: Seas to rise up to 10 feet from 'unstoppable' glacier melt

U.S. News and World Report, Alan Neuhauser

A huge swath of the West Antarctic ice sheet has "passed the point of no return," a pair of new science reports say, melting far faster than expected in an "irreversible decline" that will raise the world%u2019s oceans by anywhere from 4 to 10 feet.

12 May 2014

West Antarctic glacier loss: 'We have passed the point of no return'

Christian Science Monitor, Pete Spotts

Two studies released Monday signal that five glaciers in West Antarctica are undergoing irreversible decline over the next several hundred years, signaling sea level-rise of nearly four feet.

12 May 2014

West Antarctic ice sheet collapse is under way

UW News and Information, Hannah Hickey

University of Washington researchers used detailed topography maps and computer modeling to show that the collapse of the West Antarctic ice sheet appears to have already begun. The fast-moving Thwaites Glacier will likely disappear in a matter of centuries, researchers say, raising sea level by nearly 2 feet.

12 May 2014

West Antarctic ice sheet's collapse triggers sea level warning

NBC News, Alan Boyle

Two teams of scientists say the long-feared collapse of the West Antarctic Ice Sheet has begun, kicking off what they say will be a centuries-long, "unstoppable" process that could raise sea levels by as much as 15 feet.

12 May 2014

Western Antarctic ice sheet collapse has already begun, scientists warn

The Guardian, Suzanne Goldenberg

Two separate studies confirm loss of ice sheet is inevitable, and will cause up to 4m of additional sea-level rise. This collapse will change the coastline of the whole world.

12 May 2014

Antarctic ice may be held in place by small plugs — for now

National Geographic Daily News, Larry O'Hanlon

Centuries of warmer waters would be needed before Wilkes Basin could contribute to sea level rise. Ian Joughin comments that there are warmer waters offshore and that changing winds can pull warm water to the ice shelves. But those waters would require a few thousand years to eat away enough East Antarctic ice to endanger the ice plugs and the entire ice shelves.

4 May 2014

Greenland ice sheet loses its last grip

CBS News, Becky Oskin

Greenland is losing the battle against rising air and ocean temperatures, a new study finds. The last bulwark to fall is the northeast corner of the Greenland Ice Sheet, which started shrinking rapidly in 2003.

17 Mar 2014

Glacier that sank the Titanic is really on the move, say scientists

The Christian Science Monitor, Sudeshna Chowdhury

Jakobshavn Glacier has bagged the tile of Greenland's Fastest Glacier in Greenland: In the summer of 2012 it reached a record speed of over 150 feet per day.

4 Feb 2014

Greenland glacier hits speed record

BBC News, Paul Rincon

A river of ice in Greenland has become the fastest-flowing glacier currently known in the world, a study suggests. Ian Joughin and Ben Smith of the University of Washington's Polar Science Center in Seattle analysed pictures from the German TerraSAR-X satellites to measure the speed of the glacier.

4 Feb 2014

Glacier blamed for berg that sank Titanic unleashes more ice

National Public Radio, Richard Harris

Greenland's Jakobshavn glacier is essentially a river of ice that floats in a narrow valley. Over the past decade, it has been flowing faster toward the sea. And that rate has increased rapidly over the past two summers, apparently because the front of the glacier is now sitting in deep water, so there's not much holding it back.

3 Feb 2014

Greenland's fastest glacier sets new speed record

UW News and Information, Hannah Hickey

The latest observations of Jakobshavn Glacier show that Greenland's largest glacier is moving ice from land into the ocean at a speed that appears to be the fastest ever recorded.

3 Feb 2014

UW researchers mapping changes in glacier ice

KING5 News, Seattle, Adam Mertz

University of Washington researchers are working with NASA to create digital maps of glaciers in Greenland.

25 Nov 2013

International study provides more solid measure of shrinking in polar ice sheets

UW News and Information, Hannah Hickey

Dozens of climate scientists have reconciled their measurements of ice sheet changes in Antarctica and Greenland during the past two decades. The results, published Nov. 29 in the journal Science, roughly halve the uncertainty and discard some conflicting observations.

29 Nov 2012

Data sheds light on speed of Greenland's glaciers

BBC News, Mark Kinver

Greenland's glaciers are not speeding up as much as previously thought, researchers have estimated. A team of US researchers based their findings on data stretching back to 2000-2001, collected from more than 200 outlet glaciers. "So far, on average, we are seeing about a 30% speed-up in 10 years," observed lead author Twila Moon.

4 May 2012

Greenland ice melt could raise seas less than feared, study says

CNN, Matt Smith

Greenland's glaciers are sliding into oceans at a faster pace than previously known, but they may contribute less to an expected rise in global sea level than feared, scientists reported Thursday.

4 May 2012

Greenland glaciers shrinking quickly, but not worst case

The Washington Post (Associated Press)

Greenland's glaciers are hemorrhaging ice at an increasingly faster rate but not at the breakneck pace that scientists once feared, a new study says.

3 May 2012

Greenland's ice melting more slowly than expected

NPR 'All Things Considered', RIchard Harris

The flow of Greenland glaciers to the sea has increased by 30 percent over the past decade. But Polar Science Center researchers report in Science that they aren't seeing a runaway meltdown of Greenland that some have feared.

3 May 2012

Increasing speed of Greenland glaciers gives new insight for rising sea level

UW Today, Vince Stricherz

Changes in the speed that ice travels in more than 200 outlet glaciers indicates that Greenland's contribution to rising sea level in the 21st century might be significantly less than the upper limits some scientists thought possible, a new study shows.

3 May 2012

Sea-level rise 'may not be as high as worst-case scenarios have predicted'

The Guardian, Damian Carrington

New research published in Science suggests that Greenland's glaciers are slipping into the sea more slowly than was previously thought. But scientists warn that ice loss still sped up by 30% and is driving rises in sea levels that endanger low-lying coasts around the world.

3 May 2012

Giant Greenland glacier cracks, piece floats off

MSNBC, Larry O'Hanlon

It appears that the record retreat of Jakonbshavn may have been helped by an exceptionally warm winter, which prevented sea ice from forming in front of the glacier, said Ian Joughin of the Polar Science Center Applied Physics Lab at the University of Washington.

13 Jul 2010

Scientists step closer to saving the ice shelf

USA Today, Michelle Kessler

APL-UW Engineer Ian Joughin, with colleagues at Penn State, UC Berkley, UC Irvine, and the Scripps Institution arrived at a mathematic solution to help glaciologists better understand where and when a glacier will calve, or send a sheet of ice crashing into the sea. Their findings are reported in the 28 November issue of Science.

28 Nov 2008

The sober science of migrating rubber duckies

Wall Street Journal, Rorbert Lee Hotz

An armada of tub toys sets sail in new research discipline, 'Flotsam Science,' and helps unravel enduring planetary mysteries.

14 Nov 2008

Good signs from Greenland's ice sheet

National Public Radio, Richard Harris

Scientists project that sea level is likely to rise a foot or two this century as a result of climate change. But there's one important asterisk here: How will the Greenland ice sheet respond to global warming? This week, scientists report a bit of reassuring news.

17 Apr 2008

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