DOI: 10.5061/dryad.vhhmgqnq6
title: Terminus data for: Multi-decadal retreat of Greenland’s marine-terminating glaciers

description: Many marine-terminating glaciers draining the Greenland ice sheet have
 retreated over the past decade, yet the extent and magnitude of retreat
 relative to past variability is unknown. We measure changes in front
 positions of 210 marine-terminating glaciers using Landsat imagery
 spanning nearly four decades and compare decadal-scale rates of change
 with earlier observations. We find that 90% of the observed glaciers
 retreated between 2000 and 2010, approaching 100% in the northwest, with
 rapid retreat observed in all sectors of the ice sheet. The current
 retreat is accelerating and likely began between 1992 and 2000, coincident
 with the onset of warming, following glacier stability and minor advance
 during a mid-century cooling period. While it is clear an extensive
 retreat occurred in the early 20th century, a period of increasing air
 temperatures, a comparison of our results with historical observations
 provides evidence that the current retreat is more widespread. The onset
 of rapid retreat with warming relative to the slow and lagged advance with
 cooling suggests an asymmetry in the response of marine fronts to external
 forcing.  
description: The details are included in the manuscript and elements of the
 'Ice-front change mapping' section are copied here, with minor
 edits: We mapped the change in ice-front position
 between image pairs using the ‘box method’ of Moon and Joughin (2008) and
 Howat and others (2010). On each image, we manually digitized (with a
 computer mouse) the outline of a polygon bounded on the down-glacier edge
 by the ice front, on each lateral side by parallel lines approximating the
 glacier margins, and on the upstream side by an arbitrary straight line
 placed inland of the minimum observed front position. This polygon was
 overlain on the second image, and the ice-front border of the polygon was
 adjusted to the new front position. The difference in the area of the
 polygon between successive images is the area change of the front, and the
 average retreat distance is obtained by dividing the area of retreat by
 the polygon width. This procedure yields a less arbitrary measure of front
 position change than the change along a center line, and captures
 spatially asymmetric retreat and advance. The primary
 sources of error in this measurement are (1) errors in manual selection of
 the front position, (2) image co-registration error due to terrain and
 other factors and (3) uncertainty in feature locations due to pixel
 resolution. The first source is difficult to quantify, but is likely
 substantially reduced by the averaging inherent in the ‘box method’, since
 the front position is sampled in many locations along the front, rather
 than at a single point (Moon and Joughin, 2008). In some cases, the front
 was difficult to locate unambiguously due to the presence of semi-detached
 icebergs. In these cases, separate maximum and minimum positions (i.e.
 including and excluding the icebergs) were mapped to provide a range in
 estimates, which typically resulted in a difference of &lt;0.1 km a–1
 for a few glaciers. One exception, however, was Zachariæ Isstrøm on the
 northwest coast, which underwent a break-up of its poorly defined front
 between 2000 and 2005 as noted by Moon and Joughin (2008). The change in
 front position due to this break-up can vary by 1.5–2.5 km, giving a rate
 uncertainty of 150–250ma<sup>–1</sup>, depending on where the
 front is placed. Since the total retreat was ~1.8 km, however, this
 uncertainty is only 8–14% of the rate of change. Additionally, this single
 retreat does not have an appreciable effect on the uncertainty of the
 average changes in the northeast subsample for 2000–10, due to the large
 number of observations (N = 35). The second and third
 sources of uncertainty are quantified by, first, measuring the offset of
 stationary features near sea level to determine the co-registration error.
 Second, the square of this error is summed with the pixel resolution and
 multiplied by the number of pixels on the perimeter of the polygon
 representing the front area difference between successive images. The RMS
 co-registration error is 94 m for warp-registered MSS imagery, 65m for TM
 imagery and 36 m for ETM+ imagery. This yields a standard error of 300 m
 (or ~23 m a<sup>–1</sup>) for front position changes between
 1972 and 1985, and 90 m (~9 m a<sup>–1</sup>) for position
 changes between 2000 and 2010, with other time combinations falling
 between these. Since the error for individual position change measurements
 is likely to be spatially and temporally random, uncertainties of
 typically +/-2–3 m a<sup>–1</sup>, and never more than+/-10 m
 a<sup>–1</sup> for all mean and median change
 estimates. References: Howat, I. M.,
 J. E. Box, Y. Ahn, A. Herrington, and E. M. McFadden (2010), Seasonal
 variability in the dynamics of marine-terminating outlet glaciers in
 Greenland, <i>Journal of Glaciology</i>,
 <i>56</i>(198), 601–613. Moon, T., and I.
 Joughin (2008), Changes in ice front position on Greenland's outlet
 glaciers from 1992 to 2007, <i>J Geophys Res-Earth</i>,
 <i>113</i>(F2), F02022, doi:10.1029/2007JF000927.
description: Details for the data are included in the manuscript. All observed
 values are included but time series for individual glaciers may differ
 based on image availability and quality. Datasets
 included are 1) the shapefiles for digitized glacier fronts and 2) a
 matlab data file.