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The land ice contribution to global mean sea level rise has not yet been predicted1 using ice sheet and glacier models for the latest set of socio-economic scenarios, nor using coordinated exploration of uncertainties arising from the various computer models involved. Two recent international projects generated a large suite of projections using multiple models2-8, but primarily used previous-generation scenarios9 and climate models10, and could not fully explore known uncertainties. Here we estimate probability distributions for these projections under the new scenarios11,12 using statistical emulation of the ice sheet and glacier models. We find that limiting global warming to 1.5 degrees Celsius would halve the land ice contribution to twenty-first-century sea level rise, relative to current emissions pledges. The median decreases from 25 to 13 centimetres sea level equivalent (SLE) by 2100, with glaciers responsible for half the sea level contribution. The projected Antarctic contribution does not show a clear response to the emissions scenario, owing to uncertainties in the competing processes of increasing ice loss and snowfall accumulation in a warming climate. However, under risk-averse (pessimistic) assumptions, Antarctic ice loss could be five times higher, increasing the median land ice contribution to 42 centimetres SLE under current policies and pledges, with the 95th percentile projection exceeding half a metre even under 1.5 degrees Celsius warming. This would severely limit the possibility of mitigating future coastal flooding. Given this large range (between 13 centimetres SLE using the main projections under 1.5 degrees Celsius warming and 42 centimetres SLE using risk-averse projections under current pledges), adaptation planning for twenty-first-century sea level rise must account for a factor-of-three uncertainty in the land ice contribution until climate policies and the Antarctic response are further constrained.

The sea level contribution of the Antarctic ice sheet constitutes a large uncertainty in future sea level projections. Here we apply a linear response theory approach to 16 state-of-the-art ice sheet models to estimate the Antarctic ice sheet contribution from basal ice shelf melting within the 21st century. The purpose of this computation is to estimate the uncertainty of Antarctica's future contribution to global sea level rise that arises from large uncertainty in the oceanic forcing and the associated ice shelf melting. Ice shelf melting is considered to be a major if not the largest perturbation of the ice sheet's flow into the ocean. However, by computing only the sea level contribution in response to ice shelf melting, our study is neglecting a number of processes such as surface-mass-balance-related contributions. In assuming linear response theory, we are able to capture complex temporal responses of the ice sheets, but we neglect any self-dampening or self-amplifying processes. This is particularly relevant in situations in which an instability is dominating the ice loss. The results obtained here are thus relevant, in particular wherever the ice loss is dominated by the forcing as opposed to an internal instability, for example in strong ocean warming scenarios. In order to allow for comparison the methodology was chosen to be exactly the same as in an earlier study (Levermann et al., 2014) but with 16 instead of 5 ice sheet models. We include uncertainty in the atmospheric warming response to carbon emissions (full range of CMIP5 climate model sensitivities), uncertainty in the oceanic transport to the Southern Ocean (obtained from the time-delayed and scaled oceanic subsurface warming in CMIP5 models in relation to the global mean surface warming), and the observed range of responses of basal ice shelf melting to oceanic warming outside the ice shelf cavity. This uncertainty in basal ice shelf melting is then convoluted with the linear response functions of each of the 16 ice sheet models to obtain the ice flow response to the individual global warming path. The model median for the observational period from 1992 to 2017 of the ice loss due to basal ice shelf melting is 10.2 mm, with a likely range between 5.2 and 21.3 mm. For the same period the Antarctic ice sheet lost mass equivalent to 7.4mm of global sea level rise, with a standard deviation of 3.7mm (Shepherd et al., 2018) including all processes, especially surface-mass-balance changes. For the unabated warming path, Representative Concentration Pathway 8.5 (RCP8.5), we obtain a median contribution of the Antarctic ice sheet to global mean sea level rise from basal ice shelf melting within the 21st century of 17 cm, with a likely range (66th percentile around the mean) between 9 and 36 cm and a very likely range (90th percentile around the mean) between 6 and 58 cm. For the RCP2.6 warming path, which will keep the global mean temperature below 2 °C of global warming and is thus consistent with the Paris Climate Agreement, the procedure yields a median of 13 cm of global mean sea level contribution. The likely range for the RCP2.6 scenario is between 7 and 24 cm, and the very likely range is between 4 and 37 cm. The structural uncertainties in the method do not allow for an interpretation of any higher uncertainty percentiles.We provide projections for the five Antarctic regions and for each model and each scenario separately. The rate of sea level contribution is highest under the RCP8.5 scenario. The maximum within the 21st century of the median value is 4 cm per decade, with a likely range between 2 and 9 cm per decade and a very likely range between 1 and 14 cm per decade.

Earlier large-scale Greenland ice sheet sea-level projections (e.g., those run during the ice2sea and SeaRISE initiatives) have shown that ice sheet initial conditions have a large effect on the projections and give rise to important uncertainties. The goal of the initMIP-Greenland intercomparison exercise is to compare, evaluate and improve the initialisation techniques used in the ice sheet modelling community and to estimate the associated uncertainties in modelled mass changes. initMIP-Greenland is the first in a series of ice sheet model intercomparison activities within ISMIP6 (the Ice Sheet Model Intercomparison Project for CMIP6), which is the primary activity within the Coupled Model Intercomparison Project - phase 6 (CMIP6) focusing on the ice sheets. Two experiments for the large-scale Greenland ice sheet have been designed to allow intercomparison between participating models of 1) the initial present-day state of the ice sheet and 2) the response in two idealised forward experiments. The forward experiments serve to evaluate the initialisation in terms of model drift (forward run without additional forcing) and in response to a large perturbation (prescribed surface mass balance anomaly), and should not be interpreted as sea-level projections. We present and discuss results that highlight the diversity of data sets, boundary conditions and initialisation techniques used in the community to generate initial states of the Greenland ice sheet. We find good agreement across the ensemble for the dynamic response to surface mass balance changes in areas where the simulated ice sheets overlap, but differences arising from the initial size of the ice sheet. The model drift in the control experiment is reduced for models that participated in earlier intercomparison exercises.

We use a spatially distributed and physically based energy and mass balance model to derive the Østrem curve, which expresses the supraglacial debris-related relative melt alteration versus the debris thickness, for the Djankuat Glacier, Caucasus, Russian Federation. The model is driven by meteorological data from two on-glacier weather stations and ERA-5 Land reanalysis data. A direct pixel-by-pixel comparison of the melt rates obtained from both a clean ice and debris-covered ice mass balance model results in the quantification of debris-related relative melt-modification ratios, capturing the degree of melt enhancement or suppression as a function of the debris thickness. The main results show that the distinct surface features and different surface temperature/moisture and near-surface wind regimes that persist over debris-covered ice significantly alter the pattern of the energy and mass fluxes when compared to clean ice. Consequently, a maximum relative melt enhancement of 1.36 is modeled on the glacier for thin/patchy debris with a thickness of 0.03 m. However, insulating effects suppress sub-debris melt under debris layers thicker than a critical debris thickness of 0.09 m. Sensitivity experiments show that especially within-debris properties, such as the thermal conductivity and the vertical debris porosity gradient, highly impact the magnitude of the sub-debris melt rates. Our results also highlight the scale-dependency as well as the dynamic nature of the debris thickness-melt relationship for changing climatic conditions, which may have significant implications for the climate change response of debris-covered glaciers. Plain Language Summary The presence of rocks, boulders and sediments on top of glacier ice can highly modify the behavior of mountain glaciers. As such, compared to a clean ice surface, a debris-covered ice surface is subject to a modified melting regime. In our study, we quantify this melt-modification effect for the Djankuat Glacier, a well-studied glacier situated in the Caucasus. The results are expressed by a so-called Østrem curve that quantifies the debris-related melt-modification effect and compares it to the corresponding debris thickness. Here, we present the first attempt to construct such a glacier-specific Østrem curve through sophisticated 2D glacier-wide energy and mass balance modeling. Our results show that the energy and mass balance at the glacier surface are greatly modified due to the debris, resulting in different melting regimes over both surface types. Hence, ice melt is enhanced for thin and patchy debris layers, whereas melt is increasingly suppressed for thick and continuous debris layers due to an insulating effect. The degree of melt modification and the shape of the Østrem curve are found to depend on the debris properties, the spatio-temporal distribution of the debris, and the local climatic conditions. Quantifying such melt-modification effects is important to more accurately understand and assess the behavior of (partly) debris-covered glaciers under a future warming climate. [ABSTRACT FROM AUTHOR]

Predictions of marine ice-sheet behaviour require models able to simulate grounding-line migration. We present results of an intercomparison experiment for plan-view marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no buttressing effects from lateral drag). Perturbation experiments specifying spatial variation in basal sliding parameters permitted the evolution of curved grounding lines, generating buttressing effects. The experiments showed regions of compression and extensional flow across the grounding line, thereby invalidating the boundary layer theory. Steady-state grounding-line positions were found to be dependent on the level of physical model approximation. Resolving grounding lines requires inclusion of membrane stresses, a sufficiently small grid size (

We present a reconstruction of the Greenland Ice Sheet surface mass balance (SMB) from 1870 to 2010, based on merged Twentieth Century Reanalysis (20CR) and European Centre for Medium-Range Weather Forecasts (ECMWF) meteorological reanalyses, and we compare our new SMB series with global and regional climate and atmospheric circulation indices during this period. We demonstrate good agreement between SMB annual series constructed from 20CR and ECMWF reanalyses for the common period of overlap and show statistically significant agreement of long-term modeled snowfall with ice-core-based accumulation data. We analyze variations in SMB for the last 140 years and highlight the periods with significantly increased runoff and decreased SMB since 1870, which have both been enhanced in the period since 1990, as well as interannual variations in SMB linked to Greenland climate fluctuations. We show very good agreement of our SMB series variations with existing, independently derived SMB series (RACMO2) variations for the past few decades of overlap but also a significant disparity of up to ∼200 km3 yr-1 in absolute SMB values due to poorly constrained modeled accumulation reflecting a lack of adequate validation data in southeast Greenland. There is no significant correlation between our SMB time series and a widely referenced time series of North Atlantic icebergs emanating from Greenland for the past century, which may reflect the complex nature of the relationship between SMB and ice dynamical changes. Finally, we discuss how our analysis sheds light on the sensitivity and response of the Greenland Ice Sheet to ongoing and future global climate change, and its contribution to global sea level rise. Copyright 2011 by the American Geophysical Union.

Glaciers in the Tien Shan are vital for freshwater supply, emphasising the importance of modelling their future evolution. While detailed 3D models are suitable for well-studied glaciers, regional and global assessments rely on simplified approaches. However, their accuracy remains understudied. Here, we compare the evolution of six glaciers in the Tien Shan using (i) a 3D higher-order ice flow model and (ii) a global glacier model (GloGEMflow). Additionally, we explore the impact of using in-situ measurements of mass balance and ice thickness, as opposed to relying on globally available data. Our findings reveal that the choice of mass balance model complexity and calibration has a minimal impact on aggregated volume projections, with less than 3% variation by 2050 and less than 1% thereafter. The use of a detailed versus a simplified ice flow model results in some noticeable discrepancies in the first half of the century, with an 8% variation in aggregated volume change by 2050. These disparities primarily stem from calibration, while the glacier evolution pattern remains consistent, showing good agreement between the detailed and simplified model. In general, our results demonstrate that the initial ice thickness estimation has the largest effect on the future remaining ice volume, potentially resulting in 2 to 3, and even up to 4 times, more ice mass remaining. Our findings thus suggest that when modelling small to medium-sized glaciers the emphasis should be on having a reliable reconstruction of the glacier geometry rather than focusing on a detailed representation of ice flow and mass balance processes. [ABSTRACT FROM AUTHOR]

An accurate ice thickness distribution is crucial for correct projections of the future state of an ice mass. However, measuring the ice thickness with an in-situ system is time-consuming and not scalable. Therefore, models have been developed that estimate the ice thickness without direct measurements. In this study, we reconstruct the ice thickness of the Grigoriev ice cap, Kyrgyzstan, from in-situ observations and the yield stress method. We compare the results with data from 6 global ice thickness datasets composed without the use of our local measurements. The results highlight shortcomings of these generic datasets and demonstrate the importance of local observations for accurate representations of the ice thickness. [ABSTRACT FROM AUTHOR]

The hysteresis behaviour of ice sheets arises because of the different thresholds for growth and decline of a continental-scale ice sheet depending on the initial conditions. In this study, the hysteresis effect of the early Cenozoic Antarctic ice sheet is investigated with an improved ice sheet-climate coupling method that accurately captures the ice-albedo feedback. It is shown that the hysteresis effect of the early Cenozoic Antarctic ice sheet is about ~180 ppmv or between 3.5 °C and 5.5 °C, depending only weakly on the bedrock elevation dataset. Excluding the solid Earth feedback decreases the hysteresis effect significantly towards ~40 ppmv, because the transition to a glacial state can occur at a higher forcing. The rapid transition from a glacial to a deglacial state and oppositely from deglacial to glacial conditions is strongly enhanced by the ice-albedo feedback, in combination with the elevation – surface mass balance feedback. Variations in the orbital parameters show that extreme values of the orbital parameters are able to exceed the threshold in summer insolation to induce a (de)glaciation. It appears that the long-term eccentricity cycle has a large influence on the ice sheet growth and decline and is able to pace the ice sheet evolution for constant CO2 concentration close to the glaciation threshold. [ABSTRACT FROM AUTHOR]

High Mountain Asia (HMA) contains the largest concentration of glaciers outside the polar regions. These glaciers play an essential role in terms of water supply for the surrounding densely populated dry lowland areas. The retreat of glaciers and ice caps in this region can consequently have a major impact on societies. However, few modelling studies exist that examine in detail how individual ice bodies in the area are responding to climate change. Further, different climatic and topographic settings ensure a heterogenous impact of climate change on ice masses in the area. In this study, we focus on the western and central part of the Tien Shan Mountain range in the northwest of HMA. We use several measurements and reconstructions of the ice thickness, surface elevation, surface mass balance and ice temperature to study in detail six different ice bodies in the Kyrgyz Tien Shan: five valley glaciers and one ice cap. The selected ice masses are located in different sub-regions of the Tien Shan with different climatic settings, and they are all characterised by detailed recent glaciological measurements. A 3-dimensional higher-order thermomechanical ice-flow model is calibrated and applied to simulate the evolution of the ice masses since the Little Ice Age and to make a prognosis of the future evolution up to 2100 under different CMIP6 SSP climate scenarios. Further, projections of the total runoff of the ice masses are calculated. The results of this study reveal a strong retreat of most of the ice masses under all climate scenarios, however with important differences. These can be related to the specific climate regime of each of the ice bodies and their geometry. It is highlighted that because the main precipitation occurs in spring and early summer, the ice masses respond to climate change with an accelerating retreat. [ABSTRACT FROM AUTHOR]

High Mountain Asia (HMA) contains the largest concentration of glaciers outside the polar regions. These glaciers play an essential role in terms of water supply for the surrounding densely populated dry lowland areas. The retreat of glaciers and ice caps in this region can consequently have a major impact on societies. However, few modelling studies exist that examine in detail how individual ice bodies in the area are responding to climate change. Further, different climatic and topographic settings ensure a heterogenous impact of climate change on ice masses in the area. In this study, we focus on the western and central part of the Tien Shan Mountain range in the northwest of HMA. We use several measurements and reconstructions of the ice thickness, surface elevation, surface mass balance and ice temperature to study in detail six different ice bodies in the Kyrgyz Tien Shan: five valley glaciers and one ice cap. The selected ice masses are located in different sub-regions of the Tien Shan with different climatic settings, and they are all characterised by detailed recent glaciological measurements. A 3-dimensional higher-order thermomechanical ice-flow model is calibrated and applied to simulate the evolution of the ice masses since the Little Ice Age and to make a prognosis of the future evolution up to 2100 under different CMIP6 SSP climate scenarios. Further, projections of the total runoff of the ice masses are calculated. The results of this study reveal a strong retreat of most of the ice masses under all climate scenarios, however with important differences. These can be related to the specific climate regime of each of the ice bodies and their geometry. It is highlighted that because the main precipitation occurs in spring and early summer, the ice masses respond to climate change with an accelerating retreat. [ABSTRACT FROM AUTHOR]