Ian Joughin

Senior Principal Engineer

PSC Department


Affiliate Professor, Earth and Space Sciences



Benjamin Smith

Principal Physicist

PSC Department


Affiliate Associate Professor, Earth and Space Sciences




Clocking Greenland's Glaciers

Ice-Sheet-Wide Velocity Mapping

Glaciology has traditionally been a data-limited science.

To really say waht the ice sheet is doing, you have to watch glaciers continuously. You have to se what it's doing year by year and month by month to get the picture for each glacier.

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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.

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.

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.

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.

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.

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.

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