XIAO Cunde & ZHANG Tong

State Key Laboratory of Earth Surface Processes and Resources Ecology,Beijing Normal University,Beijing 100871,China

Keywords Greenland Ice Sheet (GrIS),global sea level rise,dynamics,ablation,ice discharge

Greenland and Antarctic ice sheets are the largest potential contributors to global sea level rise (GSLR),amounting to more than 64 m of sea level equivalence (SLE).Between the two,Greenland Ice Sheet (GrIS) alone comprises about 7 m SLE,with a much faster speed of ablation than the Antarctic Ice Sheet.The contribution of GrIS to GSLR has increased from 5% in the early 1990s to about 25% in the recent decade (Chen et al.,2017),with an accelerating rate of approximately 0.65 mm·a-1(Frederikse et al.,2020).

The ablation of Greenland is largely occurring over the marginal areas of the ice sheet,mainly through increasing surface melting and ice discharge from outlet glaciers.It is estimated that the total mass loss of tidewater and outlet glaciers of GrIS increased by 18% during 1971-2018 (Mouginot et al.,2019).Along the margins of GrIS,about 8% of the total ice front is below sea level,yet it contributes to up to 88% of the total ice discharge (Morlighem et al.,2014).Basal melting and ice calving are major controls on ice dynamics.There are clear evidences of accelerated discharge of many ice streams,such as Jakobshavn Isbr?,Zachari? Isstr?m,Kangerlussuaq and Helheim Glacier,which have contributed largely to ice loss since the 21st century (Khan et al.,2015).

Surface melting is also a key factor influencing ice sheet dynamics,and thus determining the instability of GrIS.However,there are difficulties in quantifying the role of surface melting in ice sheet changes,especially in parameterizing the meltwater within the ice sheet models.For instance,based on over 30 years of observation,Solgaard et al.(2020) indicated that the shape of glacier and the existence of supraglacial and intraglacial water determine largely the behavior of the surging glacier,Hagen Br?,in northern Greenland.Yang et al.(2020) carried out comprehensive research on surface melting by remote sensing and numerical simulations.They concluded that there are two major difficulties in integrating meltwater into the ice sheet models.Firstly,there is a scarcity of intra-glacial and basal hydrology observations,resulting in a lack of critical hydrological parameters.Secondly,ice sheet hydrology is in its early stage of development,leading to significant challenges of coupling between hydrology and ice dynamics.

Here we launch a special issue of the journalAdvances in Polar Science(APS) entitled “Observations and simulations of Greenland Ice Sheet melting”.There are many ablation processes that can lead to either a stable mass loss or an abrupt change in the ice sheet under future climate change scenarios.We call for the manuscripts on the following topics,but not limited to them: (1) Recent monitoring system on GrIS with advanced techniques;(2) Interactions of GrIS with atmosphere and ocean (including sea ice);(3) Observation,reconstruction and modelling of surface mass balance;(4) Ice flow dynamics,ice discharge estimation and global sea level contribution;(5) GrIS hydrology and other processes that may trigger ice sheet instabilities.The above papers are aimed at partly addressing the following key questions.(1) How fast is Greenland’s melting accelerating,and why? (2) What modelling needs are there to uncover any hidden mechanisms that capture critical processes? (3) What are the emergent factors that accelerate melting,such as rainfall events and darkening ice sheet surfaces? (4) What are the hard-to-detect processes,such as the interaction between tidewater glaciers and the ocean,water percolation into the ice sheet and bedrock profiles?

However,we did not collect enough submissions covering all the aspects mentioned above.Only six papers have been published in this issue,and one solicited paper will be published in the subsequent issue after its acceptance.We still encourage scientists to pay attention to this important topic in the future.These six papers mainly focus on the following topics.

(1) Overview ofChina Polar Climate Change Annual Report (2022)by Luo and Ding (this issue) provided the comprehensive information about climate changes,including temperature,sea ice,major greenhouse gases,and total ozone (hole) in the Antarctic and Arctic regions in 2020 with some interesting findings,illustrating Arctic warming influence the accelerated melting of Greenland Ice Sheet.

(2) Analysis of the record-breaking August 2021 rainfall over the Greenland Ice Sheet by Dou et al.(this issue) described the striking event that heavy rainfall occurred over the summit of the ice sheet.The study revealed the basic mechanism behind the event: A strong southward intrusion of the polar vortex favored the maintenance of a deep cyclone over Baffin Island,along with an amplification of anticyclonic circulation over the southeastern ice sheet.This circulation pattern suggested that such events may occur more frequently,mainly due to the decreased temperature contrast between the Arctic and the mid-latitude regions,which drived highly amplified jet streams.

(3) Assessment of Greenland surface melt algorithms based on DMSP and SMOS data by Li et al.(this issue) retrieved data acquired by microwave radiometers onboard the F17 satellite of the United States of America Defense Meteorological Satellite Program (DMSP) and the Soil Moisture and Ocean Salinity (SMOS) satellite of the European Space Agency.Surface melt was simulated using the DMSP dataset as input for a brightness temperature threshold algorithm,the Microwave Emission Model of Layered Snowpacks (MEMLS2),and the SMOS dataset as input for the L-band Specific MEMLS (LS-MEMLS).An accuracy evaluation was performed to verify the suitability of these products for surface melt measurement.

(4) Performance of surface radiation products of Greenland Ice Sheet using in-situ measurements by Che et al.(this issue) evaluates five reanalysis datasets: ERA5,ERA-Interim,JRA55,NCEP2 and MERRA2,for the period of 1997-2022,using observations from 26 PROMICE automatic weather stations (AWSs) and 3 K-transect AWSs on GrIS.They concluded that ERA5 had the best performances both downward and upward shortwave radiation and longwave radiation.

(5) Marginal weakening promoted rift propagation of the Petermann Ice Shelf in northwestern Greenland from 2016 to 2022 by Li et al.(this issue) investigated the effect of margin strength weakening on rift propagation of the Petermann Glacier Ice Shelf,one of the largest in Greenland.The results illustrated that crevasses widened and deepened from 2016 to 2022.Therefore,they concluded that the weakening of the margin due to strength failure promoted rift propagation and increased the potential risk of iceberg calving in the Petermann Ice Shelf.

(6) Vibrocorer for undisturbed sampling of firn aquifer layers in Greenland: general concept by Talalay et al.(this issue) proposed the use of the vibrocoring technique for undisturbed sampling of dry firn and firn aquifer layers.The remotely controlled vibrocorer is designed to obtain 1 m long cores with a diameter of 100 mm.Both traditional mechanical drilling and electric thermal drilling are poorly adapted for effective and minimally disruptive sampling in firn aquifers.This new drill has the potential to enable the sampling of firn aquifers beneath the GrIS,thereby aiding in the understanding of their role in accurately calculating the mass balance of the ice sheet.

In the last 5 years,Dr.Cunde Xiao led the National Key Research and Development Program of China titled “A Study of the Monitoring，Simulation and Climate Impact of Greenland Ice Sheet”.The project conducted monitoring and simulation studies on the key processes of instability in the “ice sheet-outlet glacier-sea ice” system.It established a satellite-airborne-ground integrated observation system,supporting the numerical simulation and impact research of the ice sheet and its surrounding sea ice.The program aims to reduce the uncertainty of sea level change projections by improving the ice sheet dynamic model,which is informed by ice core records.Additionally,it seeks to reveal the driving mechanisms behind sea ice changes around the ice sheet.In the future,we look forward to more in-depth research that will contribute to a comprehensive understanding of the ablation of the GrIS.

AcknowledgmentsWe thank Co-Editors-in-Chief,Dr.Huigen Yang and Dr.Ad H.L.Huiskes for reviewing this manuscript.This work was supported by National Key Research and Development Program of China (Grant no.2018YFC1406100) and International Cooperation Project by Geography Faculty of Beijing Normal University (Grant no.2022-GJTD-01).