Greenland ice sheet mass balance from 1840 through next week

The mass of the Greenland ice sheet is declining as mass gain from snow accumulation is exceeded by mass loss from surface meltwater runoff, marine-terminating glacier calving and submarine melting, and basal melting. Here we use the input–output (IO) method to estimate mass change from 1840 through next week. Surface mass balance (SMB) gains and losses come from a semi-empirical SMB model from 1840 through 1985 and three regional climate models (RCMs; HIRHAM/HARMONIE, Modèle Atmosphérique Régional – MAR, and RACMO – Regional Atmospheric Climate MOdel) from 1986 through next week. Additional non-SMB losses come from a marine-terminating glacier ice discharge product and a basal mass balance model. From these products we provide an annual estimate of Greenland ice sheet mass balance from 1840 through 1985 and a daily estimate at sector and region scale from 1986 through next week. This product updates daily and is the first IO product to include the basal mass balance which is a source of an additional ∼24 Gt yr−1 of mass loss. Our results demonstrate an accelerating ice-sheet-scale mass loss and general agreement (coefficient of determination, r2, ranges from 0.62 to 0.94) among six other products, including gravitational, volume, and other IO mass balance estimates. Results from this study are available at https://doi.org/10.22008/FK2/OHI23Z (Mankoff et al., 2021).

Geometric Controls of Tidewater Glacier Dynamics

Retreat of marine outlet glaciers often initiates depletion of inland ice through dynamic adjustments of the upstream glacier. The local topography of a fjord may promote or inhibit such retreat, and therefore fjord geometry constitutes a critical control on ice sheet mass balance. To quantify the processes of ice-topography interactions and enhance the understanding of the dynamics involved, we analyze a multitude of topographic fjord settings and scenarios using the Ice-sheet and Sea-level System Model (ISSM). We systematically study glacier retreat through a variety of artificial fjord geometries and quantify the modeled dynamics directly in relation to topographic features. We find that retreat in an upstream widening or deepening fjord does not necessarily promote retreat, but conversely, may stabilize a glacier because converging ice flow towards a constriction enhances lateral shear. An upstream narrowing or shoaling fjord, in turn, may promote retreat since fjord walls or bed provide little stability to the glacier where ice flow diverges. Furthermore, we identify distinct quantitative relationships directly linking grounding line discharge and retreat rate to fjord topography, and transfer these results to a long-term study of the retreat of Jakobshavn Isbræ. These findings offer new perspectives on ice-topography interactions, and give guidance to an ad-hoc assessment of future topographically induced ice loss based on knowledge of the upstream fjord geometry.

Repeat Subglacial Lake Drainage and Filling Beneath Thwaites Glacier

<p>Active subglacial lakes have been identified throughout Antarctica, offering a window into subglacial environments and their impact on ice sheet mass balance. We use high-resolution altimetry measurements over the Thwaites Glacier to show that a lake system underwent a second episode of drainage activity in 2017, only four years after another substantial drainage event. Our observations suggest significant modifications of the drainage system between the two events, with 2017 experiencing greater upstream discharge, faster lake-to-lake connectivity, and the transfer of water within a closed system. Measured rates of lake recharge during the inter-drainage period are significantly larger than modelled estimates, suggesting processes which drive subglacial melt production are currently underestimated. Our study highlights new methods of exploring subglacial environments through the application of altimetry, with potential applications for studying subglacial lakes across Antarctica</p>

Trends in ice sheet mass balance

<p><span>The Ice Sheet Mass Balance Inter-Comparison Exercise (IMBIE) is a community effort supported by ESA and NASA that aims to provide a consensus estimate of ice sheet mass balance. In its first phase, IMBIE showed that estimates of ice sheet mass balance derived from satellite gravimetry, altimetry and the mass budget method could be reconciled within their respective uncertainties. In its second phase, IMBIE showed that rates of ice loss from Antarctica and Greenland have increased by a factor 6 during the satellite era and are tracking the high-end (worst-case) projections reported in the IPCC’s fifth assessment report (AR5). The project now involves 96 individual participants based in 50 institutes from 13 nations and includes 26 satellite estimates of ice sheet mass balance, 11 models of glacial isostatic adjustment, and 10 models of surface mass balance. IMBIE has now begun its third phase, and the objectives are to (i) include measurements from new satellite missions, (ii) to report annual assessments, (iii) to partition changes in mass due to ice dynamics and surface mass balance, (iv) to produce regional assessments in areas of imbalance, and to (v) explore remaining biases between the various geodetic techniques involved. Participation is open to the full community, and the quality and consistency of submissions is regulated through a series of data standards and documentation requirements. This paper will introduce the objectives of IMBIE-3 and present the latest assessment of ice sheet mass balance. which has been updated for the IPCC's sixth assessment report.</span></p>

What have we learnt from ICESat on greenland ice sheet change and what to expect from current ICESat-2

Ice-sheet mass balance and ice behaviour have been effectively monitored remotely by space-borne laser ranging technology, i.e. satellite laser altimetry, and/or satellite gravimetry. ICESat mission launched in 2003 has pioneered laser altimetry providing a large amount of elevation data related to ice sheet change with high spatial and temporal resolution. ICESat-2, the successor to the ICESat mission, was launched in 2018, continuing the legacy of its predecessor. This paper presents an overview of the satellite laser altimetry and a review of Greenland ice sheet change estimated from ICESat data and compared against estimates derived from satellite gravimetry, i.e. changes of the Earth’s gravity field obtained from the GRACE data. In addition to that, it provides an insight into the characteristics and possibilities of ice sheet monitoring with renewed mission ICESat-2, which was compared against ICESat for the examination of ice height changes on the Jakobshavn glacier. ICESat comparison (2004–2008) shows that an average elevation change in different areas on Greenland varies up to ±0.60 m yr−1. Island’s coastal southern regions are most affected by ice loss, while inland areas record near-balance state. In the same period, gravity anomaly measurements showed negative annual mass balance trends in coastal regions ranging from a few cm up to -0.36 m yr-1 w.e. (water equivalent), while inland records show slightly positive trends. According to GRACE observations, in the following years (2009–2017), negative annual mass balance trends on the coast continued.

Physics-based SNOWPACK model improves representation of near-surface Antarctic snow and firn density

Estimates of snow and firn density are required for satellite-altimetry-based retrievals of ice sheet mass balance that rely on volume-to-mass conversions. Therefore, biases and errors in presently used density models confound assessments of ice sheet mass balance and by extension ice sheet contribution to sea level rise. Despite this importance, most contemporary firn densification models rely on simplified semi-empirical methods, which are partially reflected by significant modeled density errors when compared to observations. In this study, we present a new drifting-snow compaction scheme that we have implemented into SNOWPACK, a physics-based land surface snow model. We show that our new scheme improves existing versions of SNOWPACK by increasing simulated near-surface (defined as the top 10 m) density to be more in line with observations (near-surface bias reduction from −44.9 to −5.4 kg m−3). Furthermore, we demonstrate high-quality simulation of near-surface Antarctic snow and firn density at 122 observed density profiles across the Antarctic ice sheet, as indicated by reduced model biases throughout most of the near-surface firn column when compared to two semi-empirical firn densification models (SNOWPACK mean bias=-9.7 kg m−3, IMAU-FDM mean bias=-32.5 kg m−3, GSFC-FDM mean bias=15.5 kg m−3). Notably, our analysis is restricted to the near surface where firn density is most variable due to accumulation and compaction variability driven by synoptic weather and seasonal climate variability. Additionally, the GSFC-FDM exhibits lower mean density bias from 7–10 m (SNOWPACK bias=-22.5 kg m−3, GSFC-FDM bias=10.6 kg m−3) and throughout the entire near surface at high-accumulation sites (SNOWPACK bias=-31.4 kg m−3, GSFC-FDM bias=-4.7 kg m−3). However, we found that the performance of SNOWPACK did not degrade when applied to sites that were not included in the calibration of semi-empirical models. This suggests that SNOWPACK may possibly better represent firn properties in locations without extensive observations and under future climate scenarios, when firn properties are expected to diverge from their present state.

Effect of horizontal divergence on estimates of firn-air content

Ice-sheet mass-balance estimates derived from repeat satellite-altimetry observations require accurate calculation of spatiotemporal variability in firn-air content (FAC). However, firn-compaction models remain a large source of uncertainty within mass-balance estimates. In this study, we investigate one process that is neglected in FAC estimates derived from firn-compaction models: enhanced layer thinning due to horizontal divergence. We incorporate a layer-thinning scheme into the Community Firn Model. At every time step, firn layers first densify according to a firn-compaction model and then thin further due to an imposed horizontal divergence rate without additional density changes. We find that horizontal divergence on Thwaites (THW) and Pine Island Glaciers can reduce local FAC by up to 41% and 18%, respectively. We also assess the impact of temporal variability of horizontal divergence on FAC. We find a 15% decrease in FAC between 2007 and 2016 due to horizontal divergence at a location that is characteristic of lower THW. This decrease accounts for 16% of the observed surface lowering, whereas climate variability alone causes negligible changes in FAC at this location. Omitting transient horizontal divergence in estimates of FAC leads to an overestimation of ice loss via satellite-altimetry methods in regions of dynamic ice flow.