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.

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 <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^–1, 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^–1) for front position changes between 1972 and 1985, and 90 m (~9 m a^–1) 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^–1, and never more than+/-10 m a^–1 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, Journal of Glaciology, 56(198), 601–613.

Moon, T., and I. Joughin (2008), Changes in ice front position on Greenland's outlet glaciers from 1992 to 2007, J Geophys Res-Earth, 113(F2), F02022, doi:10.1029/2007JF000927.

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.