T. Moon1, I. Joughin2
1Department of Geological Sciences, University of Oregon, Eugene, OR, USA
2Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, WA, USA
Ice loss from the Greenland Ice Sheet (see Fig. 3.4 in the essay on the Greenland Ice Sheet) is a principal source of sea level rise. During 2009-2012, the Greenland Ice Sheet lost ~380 Gt of ice per year, contributing ~1.05 mm yr-1 to sea level rise (Enderlin et al. 2014), compared with a global mean sea level rise of ~3.2 mm yr-1 during 1993-2010 (IPCC 2013). Ice loss occurs through two primary processes: (1) surface melt and runoff from across the ice sheet, and (2) calving of icebergs into the ocean from marine-terminating outlet glaciers. The rate and magnitude of discharge of icebergs is determined by glacier ice thickness and velocity. Here, we review the most current results on annual ice surface velocities for fast-flowing Greenland glaciers, and highlight several additional new and updated datasets that provide velocity measurements at higher resolution time scales (roughly seasonal) or provide supporting data valuable for studying ice sheet mass changes. All datasets discussed are currently or will be available shortly through the NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) project or the Greenland Mapping Project (GIMP), hosted at the National Snow and Ice Data Center (NSIDC).
For this review of marine-terminating glacier velocity observations (see also the essay on the Greenland Ice Sheet), we focus on glaciers on the west and southeast coasts of Greenland. These regions have shown significant, and sometimes rapid, changes in ice velocity and associated mass loss in the past (Moon et al. 2012). We use interferometric synthetic aperture radar (InSAR) and speckle tracking techniques to measure ice sheet surface velocity from SAR data, acquired via the RADARSAT-1, ALOS and TerraSAR-X satellites (Joughin et al. 2010). With these measurements we have created annual ice-sheet-wide winter velocity maps for 2000-2001 and 2005-2006 through 2009-2010. In 2009, improved satellite coverage began to support increased sampling rates (weekly to seasonal), allowing study of seasonal as well as annual and multi-year velocity changes. We use winter (November through February) data to review the long-term and most recent annual velocity variations. While seasonal changes in glacier velocity do occur across Greenland (Moon et al. 2014), regular sampling during winter is considered a good indicator of the longer-term velocity trend.
Starting with short-term changes, Fig. 12.1 shows the velocity difference between winter 2013-2014 and winter 2012-2013 for 65 Greenland glaciers. During the year, 23 glaciers slowed, but only 14 slowed more than 50 m yr-1, with 3 slowing more than 200 m yr-1. In contrast, the speed of 42 glaciers increased, with 26 speeding up more than 50 m yr-1. The increase in speed of 12 glaciers exceeded 200 m yr-1, with 4 speeding up more than 750 m yr-1 during this single year. Despite a few large changes, the speed of most glaciers increased or slowed down by less than 250 m yr-1, roughly similar in magnitude to seasonal velocity changes, with substantial local variability. High local variability in glacier behavior is well documented across Greenland (e.g., Moon et al., 2012), so it is helpful to examine longer-term records as well.
Fig. 12.1. Velocity difference (m yr-1) between winter 2012-2013 and winter 2013-2014 measurements for marine-terminating outlet glaciers along the western and southeastern coasts of the Greenland Ice Sheet. The background image is a 20-m resolution RADARSAT-1 SAR mosaic.
Figure 12.2 uses the full time span of velocity data posted through the NASA MEaSUREs project to show winter velocity differences between 2000-2001 and 2013-2014 for 63 outlet glaciers. Over the 14-year period, 13 glaciers slowed but only 8 slowed more than 100 m yr-1. Increasing speed is much more common, with 50 glaciers speeding up, 45 speeding up >100 m yr-1, and 17 reaching speeds in 2013-2014 that are at least 1,000 m yr-1 faster than 2000-2001.
Fig. 12.2. Velocity difference (m yr-1) between winter 2000-2001 and winter 2013-2014 measurements for marine-terminating outlet glaciers along the Greenland Ice Sheet western and southeastern coasts. The background image is a composite velocity map derived from InSAR and speckle tracking measurements using the datasets discussed here.
Both the long- and short-term trends in velocity, showing an overall increase in speed across the ice sheet, are consistent with modeling studies examining the influence of the warming ocean and air temperatures on the Greenland Ice Sheet (Nick et al. 2013). The new glacier terminus position dataset (details about these datasets are included in Table 12.1) shows that glacier retreat is widespread (also see the essay on the Greenland Ice Sheet here and in previous Arctic Report Cards), and the increase in speed is consistent with glacier retreat into deeper waters. We emphasize, however, that for some glaciers, particularly those that retreat into shallower water, a decrease in speed is an expected response to retreat and may not indicate stabilization, especially if the glacier flows fast enough to discharge mass.
Along with the velocity data highlighted in the results in Figs. 12.1 and 12. 2, several related data products are now available or were updated during 2015; they are summarized in Table 12.1 (dataset also available through NASA MEaSUREs). Updates to these data continue and are posted at NSIDC as soon as they are available.
Table 12.1. New and updated NASA MEaSUREs datasets for the Greenland Ice Sheet.
||Winters: 2000-2001, 2005-2006 to 2008-09, 2012-2013
||Synthetic aperture radar (SAR) image mosaics of the full Greenland Ice Sheet
|Glacier terminus positions
||Winters: 2000-2001, 2005-2006 to 2009-2010, 2012-2013
||Digitized glacier terminus positions created from SAR image mosaics
|Annual velocity data
||Winters: 2000-2001, 2005-2006 to 2009-2010
||Updated ice surface velocity maps derived from SAR data, with near-complete coverage of the Greenland Ice Sheet. This includes the first release of an ALOS velocity map (for 2009-2010).
|Weekly to seasonal velocity data
||Velocity maps for Greenland outlet glacier areas derived from TerraSAR-X image pairs.
||2007 (including data from 2003-2009)
||The Greenland Mapping Project (GIMP) Digital Elevation Model (DEM).
The spatial coverage of the Greenland Ice Sheet MEaSUREs data, along with their extended annual and growing weekly to monthly temporal coverage, provide an unprecedented record of ice sheet motion and its evolution over the last decade and a half. Additional data (elevation, terminus position, image mosaics) deliver critical complimentary records that also support analysis of ice sheet change. Together, these datasets have enabled local characterization of glacier behavior in concert with ice-sheet-wide analysis, supported the first multi-region assessments of seasonal to multiyear velocity change, and provided observational data for full ice sheet modeling. With the data freely available to the scientific community, we can expect their research use and value to continue to grow.
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IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. T. F. Stocker et al., Eds. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1552 pp.
Joughin, I., B. E. Smith, I. M. Howat, T. A. Scambos, and T. Moon, 2010: Greenland flow variability from ice-sheet-wide velocity mapping. J. Glaciol., 56, 415-430.
Moon, T., I. Joughin, B. Smith, and I. Howat, 2012: 21st-Century Evolution of Greenland Outlet Glacier Velocities. Science, 336, 576-578, doi:10.1126/science.1219985.
Moon, T., I. Joughin, B. Smith, M. R. van den Broeke, W. J. van de Berg, B. Noël, and M. Usher, 2014: Distinct patterns of seasonal Greenland glacier velocity. Geophys. Res. Lett., 41, 7209-7216, doi:10.1002/2014GL061836.
Nick, F. M., A. Vieli, M. L. Andersen, I. Joughin, A. Payne, T. L. Edwards, F. Pattyn, and R. S. W. van de Wal, 2013: Future sea-level rise from Greenland's main outlet glaciers in a warming climate. Nature, 497, 235-238, doi:10.1038/nature12068.
November 17, 2015