The Greenland Ice Sheet (GrIS) has transitioned from a modest contributor to global sea-level rise in the 20th century to a major contributor in recent decades. Between 2005 and 2017, it caused approximately 0.76 ± 0.1 mm year⁻¹ of global mean sea-level rise, a significant portion attributed to increased surface melting and enhanced solid ice discharge. Several factors drive this increased melt production, including rising near-surface temperatures, reduced surface albedo, snowline migration, increased melt area, and cloud-radiative effects. Global climate models project accelerated Arctic warming, particularly in summer months, making understanding GrIS mass loss crucial for future sea-level rise projections. Numerous studies have quantified GrIS mass loss using various satellite observations, revealing an average loss of −148 ± 13 Gt year⁻¹ from 1992 to 2018. GRACE and GRACE Follow-On (GRACE-FO) provide invaluable insights into the ice sheet's response to meteorological forcing at sub-annual time scales. Prior studies show a correlation between enhanced melt production and more frequent anticyclonic circulation anomalies, leading to Greenland blocking events. These high-pressure systems advect warm air from mid-latitudes, amplifying melt rates through melt-albedo feedbacks. The 2010 and 2012 mass losses, exceptionally high at −462 ± 60 Gt year⁻¹ and −464 ± 62 Gt year⁻¹ respectively, exemplify the impact of such events. The GRACE mission ended in June 2017, creating uncertainty in mass balance estimates for that period. The launch of GRACE-FO in May 2018 provided a continuation of these crucial measurements. Both GRACE and GRACE-FO utilize a similar concept: two satellites measure variations in their along-track distance due to Earth's gravitational attraction, detecting mass redistributions like water and ice changes. GRACE-FO's added laser interferometer potentially enhances accuracy and resolution. The combined GRACE/GRACE-FO data provide a comprehensive view of recent GrIS mass changes.

Existing literature extensively documents the accelerating mass loss of the Greenland Ice Sheet (GrIS) since the mid-1990s, highlighting its significant contribution to global sea-level rise. Studies utilizing satellite-based Earth observations, such as GRACE and altimetry, consistently indicate a substantial increase in ice mass loss over this period. The consensus points to a combination of increased surface melting and accelerated glacier discharge as the primary drivers. Research has further explored the underlying climatic mechanisms, emphasizing the role of rising near-surface air temperatures, reduced surface albedo due to darkening of the ice surface, and changes in atmospheric circulation patterns. The impact of atmospheric blocking events, characterized by persistent high-pressure systems over Greenland, has been highlighted as a key factor in amplifying summer melt events. These findings are supported by both in-situ measurements and model simulations, using regional climate models to quantify surface mass balance and dynamic ice discharge. However, the year-to-year variability in GrIS mass balance has prompted further investigation into the short-term fluctuations and the influence of anomalous meteorological conditions.

This study analyzes data from the GRACE (2002-2017) and GRACE-FO (2018-2019) satellite missions to quantify changes in Greenland Ice Sheet (GrIS) mass. The researchers use Level-2 data from three GRACE/GRACE-FO Science Data System (SDS) teams (GFZ, CSR, and JPL), applying common corrections including insertion of degree-1 coefficients and replacement of uncertain C20 coefficients with more accurate Satellite Laser Ranging (SLR) estimates. They opt not to replace C30 coefficients due to ongoing discussion regarding potential discontinuities introduced by this procedure. Glacial-isostatic adjustment (GIA) is corrected using the GGG1.D model, accounting for long-term mass trends due to Earth's viscoelastic relaxation. To reduce noise and mitigate outliers, they create a combined GRACE/GRACE-FO monthly solution (AV RL06) by weighting Stokes potential coefficients based on calibrated uncertainties derived from the noise level of each solution. The combination is performed on detrended data to prevent artificial variability from differing processing choices by the SDS centers. Gravimetric inversion is applied to retrieve regional-scale mass anomalies, dividing the ice sheet into seven drainage basins plus Ellesmere Island. The inversion scheme uses low-frequency damping and a non-uniform a priori mass distribution. Uncertainties in GrIS mass changes are estimated by analyzing the residual mass variability after removing deterministic components from the time series. The noise level is determined from the degree power of residual GRACE/GRACE-FO coefficients in the spectral range j = 40–60, adjusted for latitude-dependent ground-track density. Surface mass balance (SMB) data are obtained from two regional climate models: MARv3.10 (20 km resolution, forced by NCEP-NCARv1 reanalysis) and RACMO2.3p2 (1 km resolution downscaled from 5.5 km, forced by ECMWF reanalysis). Solid ice discharge (D) data are taken from a temporally extended version of monthly-resolved measurements by King et al. (2018), derived from feature tracking of optical and radar imagery. A linear trend correction is applied to reconcile SMB-D with GRACE/GRACE-FO data to account for potential biases. Time series analysis involves calculating the cumulative sum of mass changes, subsampling at GRACE measurement epochs, and calculating mass fluxes using a central-difference scheme. Biennial mass balances are determined by fitting piecewise linear functions to the mass time series, with breakpoints centered on summer peak losses. Climatological means are obtained by averaging interpolated GRACE data, excluding gaps. Analysis includes comparing GRACE/GRACE-FO time series with monthly mass budgets from SMB and D, examining atmospheric circulation patterns using reanalysis data to understand the influence on melt rates, and finally, analyzing annual mass balances to identify significant changes over time.

The study's analysis of GRACE and GRACE-FO data reveals a significant slowdown in GrIS mass loss during 2017-2018, approximately 58% lower than the 2003-2018 average (−98 ± 26 Gt year⁻¹). This slowdown is attributed to two anomalously cold summers in western Greenland coupled with snow-rich autumn and winter conditions in the east. Regional climate model simulations (MARv3.10 and RACMO2.3p2) support this conclusion by showing a correspondence between reduced melt and the observed colder conditions. However, 2019 witnessed an abrupt return to high melt rates, with GRACE-FO indicating a record annual mass loss of −532 ± 58 Gt year⁻¹, surpassing even the previously highest recorded loss in 2012 (−464 ± 62 Gt year⁻¹). July 2019 alone experienced a mass change of −223 ± 12 Gt month⁻¹, the second largest monthly loss on record. The shift from the low melt years of 2017-2018 to the extreme melt year of 2019 is strongly linked to changes in atmospheric circulation. In 2017-2018, a dominant low-pressure anomaly over Greenland advected cold Arctic air southward along the ice sheet’s western flank, suppressing melt. In 2019, conditions reversed, with a high-pressure anomaly advecting warm mid-latitude air to northwestern Greenland, leading to increased melt. Low snowfall in 2019 further exacerbated the mass loss. The findings are supported by both GRACE/GRACE-FO measurements and SMB-D estimates from the regional climate models. The models also highlight the importance of considering the interplay of accumulation and melt processes in the total mass balance. The study clearly shows the sensitivity of the GrIS to changes in synoptic conditions, which are likely being amplified by ongoing Arctic warming. Despite year-to-year variability, the trend of increasing mass loss since the late 1990s remains evident.

The study's findings directly address the research question concerning the recent fluctuations in Greenland Ice Sheet mass loss. The observed slowdown in 2017-2018, followed by the record loss in 2019, highlights the substantial influence of short-term meteorological variability on the ice sheet's mass balance. The significant correlation between atmospheric circulation patterns and melt rates emphasizes the importance of considering these dynamic processes in projecting future sea-level rise. The study underscores the complex interplay between atmospheric forcing, surface processes (melt and accumulation), and ice dynamics. The results have significant implications for the field, improving our understanding of GrIS mass balance and its sensitivity to climate change. The combination of satellite gravimetry and regional climate models provides a more robust and comprehensive assessment of ice sheet mass changes than either method alone, strengthening confidence in the conclusions drawn.

This study demonstrates the value of long-term, high-resolution satellite data (GRACE and GRACE-FO) in understanding short-term variations in Greenland Ice Sheet mass balance. The dramatic shift from a period of reduced ice loss in 2017-2018 to a record-breaking loss in 2019 underscores the ice sheet’s vulnerability to changing atmospheric conditions and Arctic amplification. Future research should focus on further refining regional climate models to better capture the complex interactions between atmospheric circulation, surface melt, and ice dynamics. Improved model representation of these processes is vital for accurate projections of future sea-level rise.

The study acknowledges uncertainties inherent in satellite gravimetry and regional climate models. Uncertainties in GRACE/GRACE-FO data arise from measurement errors, corrections applied, and gravimetric inversion procedures. Regional climate models also have limitations in accurately representing all aspects of surface mass balance and dynamic ice discharge. The study addresses these limitations by utilizing multiple data sources and models, providing a more robust analysis. However, improvements in both satellite measurements and model simulations will always be beneficial for future studies.