Search

Glaciers and ice caps (GICs) are important contributors of meltwater runoff and to global sea level rise. However, knowledge of GIC mass changes is largely restricted to the last few decades. Here we show the extent of 5327 Greenland GICs during Little Ice Age (LIA) termination (1900) and reveal that they have fragmented into 5467 glaciers in 2001, losing at least 587 km3 from their ablation areas, equating to 499 Gt at a rate of 4.34 Gt yr−1. We estimate that the long‐term mean mass balance in glacier ablation areas has been at least −0.18 to −0.22 m w.e. yr−1 and note the rate between 2000 and 2019 has been three times that. Glaciers with ice‐marginal lakes formed since the LIA termination have had the fastest changing mass balance. Considerable spatial variability in glacier changes suggest compounding regional and local factors present challenges for understanding glacier evolution. Plain Language Summary: Glaciers and ice caps of Greenland peripheral to the ice sheet are important contributors of meltwater to the oceans and to global sea‐level rise. In this study we map the extent of 5467 glaciers during the Little Ice Age (LIA) termination c. 1900 and calculate that they have lost at least 587 km3. The rate of mass change of these glaciers between 2000 and 2019 was three times more negative than the long‐term average (of 4.34 Gt yr−1) since the LIA. Lake‐terminating glaciers now lose mass the fastest compared with land‐ or marine‐terminating glaciers. Considerable spatial variability in glacier responses suggests local factors are important and makes glacier evolution complex. Key Points: Total volume loss of at least 587 km3 since the Little Ice Age (LIA) termination, equating to 499 Gt and to 1.38 mm sea level equivalentGlacier mass balance from 2000 to 2019 is three times more negative than since the LIA but five times more negative in the North regionLake‐terminating glaciers have experienced the greatest change in rate of mass loss [ABSTRACT FROM AUTHOR]

We present a novel method to estimate dynamic ice loss of Greenland's three largest outlet glaciers: Jakobshavn Isbræ, Kangerlussuaq Glacier, and Helheim Glacier. We use Global Navigation Satellite System (GNSS) stations attached to bedrock to measure elastic displacements of the solid Earth caused by dynamic thinning near the glacier terminus. When we compare our results with discharge, we find a time lag between glacier speedup/slowdown and onset of dynamic thinning/thickening. Our results show that dynamic thinning/thickening on Jakobshavn Isbræ occurs 0.87 ± 0.07 years before speedup/slowdown. This implies that using GNSS time series we are able to predict speedup/slowdown of Jakobshavn Isbræ by up to 10.4 months. For Kangerlussuaq Glacier the lag between thinning/thickening and speedup/slowdown is 0.37 ± 0.17 years (4.4 months). Our methodology and results could be important for studies that attempt to model and understand mechanisms controlling short‐term dynamic fluctuations of outlet glaciers in Greenland. Plain Language Summary: A wide range of sensors and methods have been used to study the changes of the Greenland Ice Sheet, including satellite gravimetry, altimetry, and the input‐output method. Here, we present a novel fourth method to estimate dynamic ice loss of Greenland's three largest outlet glaciers: Jakobshavn Isbræ, Kangerlussuaq Glacier, and Helheim Glacier. We use Global Navigation Satellite System (GNSS) stations attached to bedrock to measure rise of land masses caused by ongoing ice mass loss near the glacier terminus. When we compare our results with ice discharge, we find a time lag between glacier speedup/slowdown and onset of dynamic induced thinning/thickening. Our results show that dynamic thinning/thickening on Jakobshavn Isbræ occurs 0.87 ± 0.07 years before speedup/slowdown. This implies that using GNSS uplift time series we are able to predict ice flow speedup/slowdown of Jakobshavn Isbræ by up to 10 months. For Kangerlussuaq Glacier and Helheim Glacier the lag between thinning/thickening and speedup/slowdown is 0.37 ± 0.17 years (4.4 months) and 0.03 ± 0.16 years, respectively. Our methodology and results could be important for studies that attempt to model and understand mechanisms controlling short‐term dynamic fluctuations of outlet glaciers in Greenland. Key Points: A novel method to estimate dynamic ice loss of Greenland's three largest outlet glaciers, Jakobshavn, Kangerlussuaq, and Helheim glacierDynamic thinning/thickening occurs 0.87 ± 0.07 years before speedup/slowdown at Jakobshavn IsbræA similar time lag between change in uplift rate and flow speed change allows us to predict future ice discharge from past uplift [ABSTRACT FROM AUTHOR]

Cosmogenic exposure dating is one of the most widely used methods to constrain the deglaciation history of former glaciated areas. In Greenland, more than 1000 cosmogenic 10Be exposure ages (10Be ages) have been published within the last two decades. However, a recurring problem is that many of these studies have reported variable amounts of nuclide inheritance making the 10Be ages too old and difficult to assess without large datasets or independent age control. In this study, we test the accuracy of 10Be dating of Holocene moraines using independent age constraints from threshold lake records. In Kangerlussuaq, West Greenland, the 10Be ages of the Ørkendalen moraine system are highly clustered with a mean age of 6.8 ± 0.3 ka (no outliers). In contrast, the nearby Little Ice Age (LIA) moraine yields scattered 10Be ages ranging from 2.5 to 0.1 ka but with a mean of 0.18 ± 0.06 ka after excluding outliers which coincides with independent age constraints from threshold lakes and boulder kill dates. At Gletscherlukket, Southeast Greenland, the 10Be ages of the LIA moraine range from 10.2 to 1.6 ka with a mean of 1.9 ± 0.2 ka after excluding outliers. This is ~1.7 ka older than recorded in the proglacial threshold lakes and suggests that all samples from this site contain a significant amount of nuclide inheritance. Our results are consistent with other reports of skewed 10Be age distributions in LIA re-advance moraines and it probably reflects nuclide inheritance from exposure during the Holocene Thermal Maximum when the glaciers in Greenland were inside the LIA extent. In contrast, there is no evidence of nuclide inheritance in the Ørkendalen moraines, most likely because the glacial erosion was more intense prior to the formation of the moraines i.e. sometime between the advance phase during Last Glacial Maximum position and the subsequent lateglacial and Holocene deglaciation. Our results highlight a potential pitfall related to dating re-advance moraines using cosmogenic exposure dating and we recommend using a multi-method dating approach. [ABSTRACT FROM AUTHOR]