Glacial Isostatic Adjustment (GIA) in Greenland: A Review

Using the most recently published regional and global deglaciation histories we provide updated estimates of the Glacial Isostatic Adjustment (GIA) component of present day uplift at a suite of GPS sites in Greenland. The GIA of the solid Earth beneath Greenland contributes -6 to +10 Gt/yr to the present day mass trends observed by the Gravity Recovery and Climate Experiment (GRACE), representing <5% contribution to the observed mass trends over the last decade. Although the contribution of GIA to GRACE estimates of mass imbalance is insignificant for Greenland as a whole, differences between deglacial models reviewed here and their assumed viscoelastic Earth structures result in significantly different estimates of regional patterns and magnitudes of GIA. This means that for some areas of Greenland (e.g. the north-west, south- and north-east) the use of GNSS to estimate elastic uplift patterns is more affected by the choice of GIA correction applied

Antarctic ice sheet paleo-constraint database

We present a database of observational constraints on past Antarctic Ice Sheet changes during the last glacial cycle intended to consolidate the observations that represent our understanding of past Antarctic changes, for state-space estimation, and paleo-model calibrations. The database is a major expansion of the initial work of Briggs and Tarasov (2013). It includes new data types and multi-tier data quality assessment. The updated constraint database “AntICE2” consists of observations of past grounded and floating ice sheet extent, past ice thickness, past relative sea level, borehole temperature profiles, and present-day bedrock displacement rates. In addition to paleo-observations, the present-day ice sheet geometry and surface ice velocities are incorporated to constrain the present-day ice sheet configuration. The method by which the data is curated using explicitly defined criteria is detailed. Moreover, the observational uncertainties are specified. The methodology by which the constraint database can be applied to evaluate a given ice sheet reconstruction is discussed. The implementation of the “AntICE2” database for Antarctic Ice Sheet model calibrations will improve Antarctic Ice Sheet predictions during past warm and cold periods and yield more robust paleo model spin ups for forecasting future ice sheet changes

Greenland Temperature Response to Climate Forcing during the Last Deglaciation

Greenland ice core water isotopic composition (δ¹⁸O) provides detailed evidence for abrupt climate changes, but is by itself insufficient for quantitative reconstruction of past temperatures and their spatial patterns. We investigate Greenland temperature evolution during the last deglaciation using independent reconstructions from three ice cores and simulations with a coupled ocean-atmosphere climate model. Contrary to the traditional δ¹⁸O interpretation, the Younger Dryas period was 4.5±2°C warmer than the Oldest Dryas, due to increased CO₂ forcing and summer insolation. The magnitude of abrupt temperature changes is larger in central Greenland (9-14°C) than in the northwest (5-9°C), fingerprinting a North-Atlantic origin. Simulated changes in temperature seasonality closely track changes in the Atlantic overturning strength, and support the hypothesis that abrupt climate change is mostly a winter phenomenon

Mass balance of the Greenland Ice Sheet from 1992 to 2018

In recent decades, the Greenland Ice Sheet has been a major contributor to global sea-level rise1,2, and it is expected to be so in the future3. Although increases in glacier flow4–6 and surface melting7–9 have been driven by oceanic10–12 and atmospheric13,14 warming, the degree and trajectory of today’s imbalance remain uncertain. Here we compare and combine 26 individual satellite measurements of changes in the ice sheet’s volume, flow and gravitational potential to produce a reconciled estimate of its mass balance. Although the ice sheet was close to a state of balance in the 1990s, annual losses have risen since then, peaking at 335 ± 62 billion tonnes per year in 2011. In all, Greenland lost 3,800 ± 339 billion tonnes of ice between 1992 and 2018, causing the mean sea level to rise by 10.6 ± 0.9 millimetres. Using three regional climate models, we show that reduced surface mass balance has driven 1,971 ± 555 billion tonnes (52%) of the ice loss owing to increased meltwater runoff. The remaining 1,827 ± 538 billion tonnes (48%) of ice loss was due to increased glacier discharge, which rose from 41 ± 37 billion tonnes per year in the 1990s to 87 ± 25 billion tonnes per year since then. Between 2013 and 2017, the total rate of ice loss slowed to 217 ± 32 billion tonnes per year, on average, as atmospheric circulation favoured cooler conditions15 and as ocean temperatures fell at the terminus of Jakobshavn Isbræ16. Cumulative ice losses from Greenland as a whole have been close to the IPCC’s predicted rates for their high-end climate warming scenario17, which forecast an additional 50 to 120 millimetres of global sea-level rise by 2100 when compared to their central estimate

Mass balance of the Greenland and Antarctic ice sheets from 1992 to 2020

Ice losses from the Greenland and Antarctic ice sheets have accelerated since the 1990s, accounting for a significant increase in the global mean sea level. Here, we present a new 29-year record of ice sheet mass balance from 1992 to 2020 from the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE). We compare and combine 50 independent estimates of ice sheet mass balance derived from satellite observations of temporal changes in ice sheet flow, in ice sheet volume, and in Earth's gravity field. Between 1992 and 2020, the ice sheets contributed 21.0±1.9g€¯mm to global mean sea level, with the rate of mass loss rising from 105g€¯Gtg€¯yr-1 between 1992 and 1996 to 372g€¯Gtg€¯yr-1 between 2016 and 2020. In Greenland, the rate of mass loss is 169±9g€¯Gtg€¯yr-1 between 1992 and 2020, but there are large inter-annual variations in mass balance, with mass loss ranging from 86g€¯Gtg€¯yr-1 in 2017 to 444g€¯Gtg€¯yr-1 in 2019 due to large variability in surface mass balance. In Antarctica, ice losses continue to be dominated by mass loss from West Antarctica (82±9g€¯Gtg€¯yr-1) and, to a lesser extent, from the Antarctic Peninsula (13±5g€¯Gtg€¯yr-1). East Antarctica remains close to a state of balance, with a small gain of 3±15g€¯Gtg€¯yr-1, but is the most uncertain component of Antarctica's mass balance. The dataset is publicly available at 10.5285/77B64C55-7166-4A06-9DEF-2E400398E452 (IMBIE Team, 2021)

Early Holocene Greenland-ice mass loss likely triggered earthquakes and tsunami

Due to their large mass, ice sheets induce significant stresses in the Earth's crust. Stress release during deglaciation can trigger large-magnitude earthquakes, as indicated by surface faults in northern Europe. Although glacially-induced stresses have been analyzed in northern Europe, they have not yet been analyzed for Greenland. We know that the Greenland Ice Sheet experienced a large melting period in the early Holocene, and so here, we analyze glacially-induced stresses during deglaciation for Greenland for the first time. Instability occurs in southern Greenland, where we use a combined analysis of past sea level indicators and a model of glacially-induced fault reactivation to show that deglaciation of the Greenland Ice Sheet may have caused a large magnitude earthquake or a series of smaller magnitude earthquakes around 10,600 years ago offshore south-western Greenland. The earthquake(s) may have shifted relative sea level observations by several meters. If the earthquake-induced stress release was created during a single event, it could have produced a tsunami in the North Atlantic Ocean with runup heights of up to 7.2 m in the British Isles and up to 7.8 m along Canadian coasts

LM17.3 - a globalvertical land motion model of glacial isostatic adjustment.

Additional models (W12, Whitehouse et al. 2012, and IJ05_R2, Ivins et al. 2013, for Antarctica; ANU-ICE, Lambeck et al. 2017, and NAIce, Gowan et al. 2016, for North America) were tested in the development of the model but not used in the end. Little ice age is not included nor any ice mass change during the last 100 years. The eustatic sea-level equivalent at last glacial maximum amounts to 113.8 m for all ice sheets and glaciers together. Because we use an ice model that has not been tuned to fit global constraints, it may highlight areas which cannot match commonly used GIA observations. However, we note that the earth model used in our calculations is different to the earth model used in the development of some regional ice models, e.g. HUY3, ANU-ICE, IJ04_Patagonia (see respective references), thus some differences can be related to this. The LM17.3 model was introduced in Jäggi et al. (2019), and its DDK5-filtered geoid and water heights can be found in the EGSIEM plotter (http://plot.egsiem.eu/index.php?p=timeseries). The GIA model uses material compressibility and includes time-dependent coastlines and rotational feedback. The vertical land motion can be used/tested in sea-level investigations and projections. Work towards a model that incorporates 3D earth structure, and an updated ice model, is ongoing

Oxygen isotope ratios, elevation changes and temperature record from the Agassiz ice cap and Greenland ice sheet

We present a revised and extended high Arctic air temperature reconstruction from a single proxy that spans the past ∼12,000 y (up to 2009 CE). Our reconstruction from the Agassiz ice cap (Ellesmere Island, Canada) indicates an earlier and warmer Holocene thermal maximum with early Holocene temperatures that are 4-5 °C warmer compared with a previous reconstruction, and regularly exceed contemporary values for a period of ∼3,000 y. Our results show that air temperatures in this region are now at their warmest in the past 6,800-7,800 y, and that the recent rate of temperature change is unprecedented over the entire Holocene. The warmer early Holocene inferred from the Agassiz ice core leads to an estimated ∼1 km of ice thinning in northwest Greenland during the early Holocene using the Camp Century ice core. Ice modeling results show that this large thinning is consistent with our air temperature reconstruction. The modeling results also demonstrate the broader significance of the enhanced warming, with a retreat of the northern ice margin behind its present position in the mid Holocene and a ∼25% increase in total Greenland ice sheet mass loss (∼1.4 m sea-level equivalent) during the last deglaciation, both of which have implications for interpreting geodetic measurements of land uplift and gravity changes in northern Greenland