Rapid Expansion of Greenland’s Low-Permeability Ice Slabs in a Warming Climate

Recent increases in Greenland&rsquo;s glacial melt have accelerated runoff and become Greenland&rsquo;s dominant mechanism of ice loss. More meltwater is being generated in the ice sheet&rsquo;s lower accumulation zone, which has begun to anneal ice lenses found within the porous firn and form continuous low-permeability ice slabs (LPISs). LPISs are layers of ice meters thick that inhibit water percolating beneath them, extend horizontally for tens of kilometers, and can cause runoff from regions where water previously refroze. LPISs form on decadal timescales and have the potential to quickly increase the extent of Greenland&rsquo;s runoff zone. I present multiple lines of evidence that show LPISs have already increased runoff in recent above-average melt summers, including the record-breaking 2012 summer in Greenland. I use NASA&rsquo;s Operation IceBridge radar to map LPISs across Greenland&rsquo;s ice sheet and peripheral glaciers and show that LPISs already cover approximately 5% of Greenland&rsquo;s total glaciated area. I combine radar observations with regional climate models to show that Greenland&rsquo;s LPISs will likely be 130-850% more extensive by 2100 depending upon 21st century CO2 emissions scenarios. LPISs under a high emissions future span more than a 250% greater area in 2100 than under moderate emissions, suggesting that ongoing emissions this century play a vital role in controlling melt and determining the size of Greenland&rsquo;s runoff zone.</p

Rapid Expansion of Greenland’s Low-Permeability Ice Slabs in a Warming Climate

Recent increases in Greenland&rsquo;s glacial melt have accelerated runoff and become Greenland&rsquo;s dominant mechanism of ice loss. More meltwater is being generated in the ice sheet&rsquo;s lower accumulation zone, which has begun to anneal ice lenses found within the porous firn and form continuous low-permeability ice slabs (LPISs). LPISs are layers of ice meters thick that inhibit water percolating beneath them, extend horizontally for tens of kilometers, and can cause runoff from regions where water previously refroze. LPISs form on decadal timescales and have the potential to quickly increase the extent of Greenland&rsquo;s runoff zone. I present multiple lines of evidence that show LPISs have already increased runoff in recent above-average melt summers, including the record-breaking 2012 summer in Greenland. I use NASA&rsquo;s Operation IceBridge radar to map LPISs across Greenland&rsquo;s ice sheet and peripheral glaciers and show that LPISs already cover approximately 5% of Greenland&rsquo;s total glaciated area. I combine radar observations with regional climate models to show that Greenland&rsquo;s LPISs will likely be 130-850% more extensive by 2100 depending upon 21st century CO₂ emissions scenarios. LPISs under a high emissions future span more than a 250% greater area in 2100 than under moderate emissions, suggesting that ongoing emissions this century play a vital role in controlling melt and determining the size of Greenland&rsquo;s runoff zone.</p

Turtle Geometry on the Sphere: The Turtle Finally Escapes the Plane

Turtle geometry has for decades played an engaging role as a constructivist tool for teaching Euclidean mathematical concepts and introductory computer programming to school-aged children at nearly all educational levels. Recent advances in computing power and display technologies have enabled the full implementation of a turtle geometry world on a spherical surface, whose geometric properties are inherently different than in the Euclidian world. Until now, spherical geometry has remained only shallowly explored using turtle geometry, largely because no easily accessible surface existed upon which to implement such a system. This paper describes the concept, implementation and working examples of a spherical turtle geometry system named Geometry on a Sphere (GOS) that makes use of any personal computer and the National Oceanic and Atmospheric Administration (NOAA)\u27s Science on a Sphere (SOS) display technology, or any similar spherical rendering device. The GOS system is a functioning prototype interactive system, designed to finally \u22bring the turtle off the plane.\u2

Brief Communication: Update on the GPS reflection technique for measuring snow accumulation in Greenland

GPS interferometric reflectometry (GPS-IR) is a technique that can be used to measure snow accumulation on ice sheets. The footprint of the method (&sim;1000&thinsp;m2) is larger than that of many other in situ methods. A long-term comparison with hand measurements yielded an accuracy assessment of 2&thinsp;cm. Depending on the placement of the GPS antenna, these data are also sensitive to firn density. The purpose of this short note is to make public GPS-IR measurements of snow accumulation for four sites in Greenland, compare these records with in situ sensors, and make available open-source GPS-IR software to the cryosphere community. </div

Differences in Seasonal Melt in Greenland for Summer 2016 and 2017 - upGPR to determine liquid water percolation, retention and accumulation over the last two melt seasons

While summer 2016 air temperatures were above long term average over the entire Greenland ice Sheet (GrIS), melt in summer 2017 was considered as significantly below average, which may lead to an even positive surface mass balance in 2017 for the GrIS. However, apart from surficial extent of melt, only very little is known about effects of melt induced changes for snow and firn such as liquid water content, percolation depth and mass fluxes. To overcome this deficit, we installed an upward-looking radar systems (upGPR) 3.5 m below the snow surface in May 2016 close to Camp Raven (66.4779N/ 46.2856W) at 2120 m a.s.l. within the deep percolation zone of the GrIS. The radar is capable to monitor quasi-continuously changes in snow and firn stratigraphy, which occur above the antennas. For summer 2016, we observed four major melt events, which routed liquid water into various depths. The last event in mid-August resulted in the deepest percolation down to about 2.5 m beneath the surface. For the subsequent summer season in 2017, liquid water percolation barely reached the previous summer horizon until 15 August. In consequence, seasonal mass flux into underlying firn was strongly different for summer 2016 and 2017 at the site. While until mid-August 2016, melt events transferred a cumulative mass of almost 60 kg m−2 from the surface into firn, in 2017, for the same time period, no mass flux beneath the previous summer horizon has been observed. Comparisons with results predicted by the regional climate model MAR are in very good agreement in terms of specific surface accumulation, while neither the temporal evolution of density, nor bulk liquid water contents nor percolation depths agree with upGPR data. Such inaccuracies bias simulations of changes in snow and firn and limit our understanding of effects of water percolation as well as water retention in firn. A multi-yearsummer monitoring with upGPR may lead to a valuable data base for melt effects in perennial firn. At the current stage, we have continuous observations for a very strong melt season and a below average melt in 2017. We are looking forward to monitor even more extreme events to provide temporally continuous in-situ data for a large variety of melt years in perennial firn within the percolation zone of the GrIS

The Greenland Firn Compaction Verification and Reconnaissance (FirnCover) dataset, 2013-2019

Assessing changes in the density of snow and firn is vital to convert volume changes into mass changes on glaciers and ice sheets. Firn models simulate this process but typically rely upon steady-state assumptions and geographically and temporally limited sets of field measurements for validation. Given rapid changes recently observed in Greenland's surface mass balance, a contemporary dataset measuring firn compaction in a range of climate zones across the Greenland ice sheet's accumulation zone is needed. To fill this need, the Firn Compaction Verification and Reconnaissance (FirnCover) dataset comprises daily measurements from 48 strainmeters installed in boreholes at eight sites on the Greenland ice sheet between 2013 and 2019. The dataset also includes daily records of 2&thinsp;m air temperature, snow height, and firn temperature from each station. The majority of the FirnCover stations were installed in close proximity to automated weather stations that measure a wider suite of meteorological measurements, allowing the user access to auxiliary datasets for model validation studies using FirnCover data. The dataset can be found here: https://doi.org/10.18739/A25X25D7M (MacFerrin et al., 2021). &nbsp;</p

Seasonal monitoring of melt and accumulation within the deep percolation zone of the Greenland Ice Sheet and comparison with simulations of regional climate modeling

Increasing melt over the Greenland Ice Sheet (GrIS) recorded over the past several years has resulted in significant changes of the percolation regime of the ice sheet. It remains unclear whether Greenland's percolation zone will act as a meltwater buffer in the near future through gradually filling all pore space or if near-surface refreezing causes the formation of impermeable layers, which provoke lateral runoff. Homogeneous ice layers within perennial firn, as well as near-surface ice layers of several meter thickness have been observed in firn cores. Because firn coring is a destructive method, deriving stratigraphic changes in firn and allocation of summer melt events is challenging. To overcome this deficit and provide continuous data for model evaluations on snow and firn density, temporal changes in liquid water content and depths of water infiltration, we installed an upward-looking radar system (upGPR) 3.4 m below the snow surface in May 2016 close to Camp Raven (66.4779 degrees N, 46.2856 degrees W) at 2120 m a.s.l. The radar is capable of quasi-continuously monitoring changes in snow and firn stratigraphy, which occur above the antennas. For summer 2016, we observed four major melt events, which routed liquid water into various depths beneath the surface. The last event in mid-August resulted in the deepest percolation down to about 2.3 m beneath the surface. Comparisons with simulations from the regional climate model MAR are in very good agreement in terms of seasonal changes in accumulation and timing of onset of melt. However, neither bulk density of near-surface layers nor the amounts of liquid water and percolation depths predicted by MAR correspond with upGPR data. Radar data and records of a nearby thermistor string, in contrast, matched very well for both timing and depth of temperature changes and observed water percolations. All four melt events transferred a cumulative mass of 56 kg m(-2) into firn beneath the summer surface of 2015. We find that continuous observations of liquid water content, percolation depths and rates for the seasonal mass fluxes are sufficiently accurate to provide valuable information for validation of model approaches and help to develop a better understanding of liquid water retention and percolation in perennial firn

Firn data compilation reveals widespread decrease of firn air content in western Greenland

The perennial snow, or firn, on the Greenland ice sheet each summer stores part of the meltwater formed at the surface, buffering the ice sheet’s contribution to sea level. We gathered observations of firn air content, indicative of the space available in the firn to retain meltwater, and find that this air content remained stable in cold regions of the firn over the last 65 years but recently decreased significantly in western Greenland