The surface mass balance (SMB) of a glacier provides the link
between the glacier and the local climate. For this reason, it is
intensively studied and monitored. However, major efforts are required ...to
determine the point SMB at a sufficient number of locations to capture the
heterogeneity of the SMB pattern. Furthermore, because of the time-consuming
and costly nature of these measurements, detailed SMB measurements are
carried out on only a limited number of glaciers. In this study, we
investigate how to accurately determine the SMB in the ablation zone of
Vadret da Morteratsch and Vadret Pers (Engadin, Switzerland) using the
continuity equation method, based on the expression of conservation of mass
for glacier flow with constant density. An elaborate dataset (spanning the
2017–2020 period) of high-resolution data derived from unoccupied aerial
vehicle (UAV) measurements (surface elevation changes and surface
velocities) is combined with reconstructed ice thickness fields (based on
radar measurements). To determine the performance of the method, we compare
modelled SMB with measured SMB values at the position of stakes. Our results
indicate that with annual UAV surveys, it is possible to obtain SMB
estimates with a mean absolute error smaller than 0.5 m of ice
equivalent per year. Yet, our study demonstrates that to obtain these
accuracies, it is necessary to consider the ice flow over spatial scales of
several times the local ice thickness, accomplished in this study by
applying an exponential decay filter. Furthermore, our study highlights the
crucial importance of the ice thickness, which must be sufficiently well
known in order to accurately apply the method. The latter currently seems to
complicate the application of the continuity equation method to derive
detailed SMB patterns on regional to global scales.
On multi-million-year timescales, fully coupled ice sheet–climate simulations are hampered by computational limitations, even at
coarser resolutions and when using asynchronous coupling schemes. In ...this
study, a novel coupling method CLISEMv1.0 (CLimate–Ice Sheet EMulator
version 1.0) is presented, where a Gaussian process emulator is applied to
the climate model HadSM3 and coupled to the ice sheet model AISMPALEO. The
temperature and precipitation fields from HadSM3 are emulated to feed the
mass balance model in AISMPALEO. The sensitivity of the evolution of the ice sheet over time is tested with respect to the number of predefined ice sheet
geometries that the emulator is calibrated on. Additionally, the model
performance is evaluated in terms of the formulation of the ice sheet parameter
(being ice sheet volume, ice sheet area or both) and the
coupling time. Sensitivity experiments are conducted to explore the
uncertainty introduced by the emulator. In addition, different lapse rate
adjustments are used between the relatively coarse climate model and the
much finer ice sheet model topography. It is shown that the ice sheet
evolution over a million-year timescale is strongly sensitive to the
definition of the ice sheet parameter and to the number of predefined ice
sheet geometries. With the new coupling procedure, we provide a
computationally efficient framework for simulating ice sheet–climate
interactions on a multi-million-year timescale that allows for a large
number of sensitivity tests.
The emphasis for informing policy makers on future
sea-level rise has been on projections by the end of the 21st century.
However, due to the long lifetime of atmospheric CO2, the thermal
inertia of ...the climate system and the slow equilibration of the ice sheets,
global sea level will continue to rise on a multi-millennial timescale even
when anthropogenic CO2 emissions cease completely during the coming
decades to centuries. Here we present global sea-level change projections
due to the melting of land ice combined with steric sea effects during the next
10 000 years calculated in a fully interactive way with the Earth system
model of intermediate complexity LOVECLIMv1.3. The greenhouse forcing is
based on the Extended Concentration Pathways defined until 2300 CE with no
carbon dioxide emissions thereafter, equivalent to a cumulative CO2
release of between 460 and 5300 GtC. We performed one additional experiment
for the highest-forcing scenario with the inclusion of a methane emission
feedback where methane is slowly released due to a strong increase in
surface and oceanic temperatures. After 10 000 years, the sea-level change
rate drops below 0.05 m per century and a semi-equilibrated state is
reached. The Greenland ice sheet is found to nearly disappear for all
forcing scenarios. The Antarctic ice sheet contributes only about 1.6 m to
sea level for the lowest forcing scenario with a limited retreat of the
grounding line in West Antarctica. For the higher-forcing scenarios, the
marine basins of the East Antarctic Ice Sheet also become ice free,
resulting in a sea-level rise of up to 27 m. The global mean sea-level
change after 10 000 years ranges from 9.2 to more than 37 m. For the
highest-forcing scenario, the model uncertainty does not exclude the complete
melting of the Antarctic ice sheet during the next 10 000 years.
Ice sheet numerical modeling is an important tool to estimate the dynamic contribution of the Antarctic ice sheet to sea level rise over the coming centuries. The influence of initial conditions on ...ice sheet model simulations, however, is still unclear. To better understand this influence, an initial state intercomparison exercise (initMIP) has been developed to compare, evaluate, and improve initialization procedures and estimate their impact on century-scale simulations. initMIP is the first set of experiments of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), which is the primary Coupled Model Intercomparison Project Phase 6 (CMIP6) activity focusing on the Greenland and Antarctic ice sheets. Following initMIP-Greenland, initMIP-Antarctica has been designed to explore uncertainties associated with model initialization and spin-up and to evaluate the impact of changes in external forcings. Starting from the state of the Antarctic ice sheet at the end of the initialization procedure, three forward experiments are each run for 100 years: a control run, a run with a surface mass balance anomaly, and a run with a basal melting anomaly beneath floating ice. This study presents the results of initMIP-Antarctica from 25 simulations performed by 16 international modeling groups. The submitted results use different initial conditions and initialization methods, as well as ice flow model parameters and reference external forcings. We find a good agreement among model responses to the surface mass balance anomaly but large variations in responses to the basal melting anomaly. These variations can be attributed to differences in the extent of ice shelves and their upstream tributaries, the numerical treatment of grounding line, and the initial ocean conditions applied, suggesting that ongoing efforts to better represent ice shelves in continental-scale models should continue.
Glaciers in the Tien Shan mountains contribute considerably to the fresh water used for irrigation, households and energy supply in the dry lowland areas of Kyrgyzstan and its neighbouring countries. ...To date, reconstructions of the current ice volume and ice thickness distribution remain scarce, and accurate data are largely lacking at the local scale. Here, we present a detailed ice thickness distribution of Ashu-Tor, Bordu, Golubin and Kara-Batkak glaciers derived from radio-echo sounding measurements and modelling. All the ice thickness measurements are used to calibrate three individual models to estimate the ice thickness in inaccessible areas. A cross-validation between modelled and measured ice thickness for a subset of the data is performed to attribute a weight to every model and to assemble a final composite ice thickness distribution for every glacier. Results reveal the thickest ice on Ashu-Tor glacier with values up to 201 ± 12 m. The ice thickness measurements and distributions are also compared with estimates composed without the use of in situ data. These estimates approach the total ice volume well, but local ice thicknesses vary substantially.
•Application of the emulator CLISEMv1.0 to simulate the Antarctic ice sheet growth during the late Eocene up to the Oligocene.•Model evidence for short-lived continental-scale ice sheets prior to the ...Eocene-Oligocene transition.•Use of highly detailed CO2 forcing close to the glaciation threshold.•Large dependency of the CO2 threshold to glaciation on the bedrock topography.
It is generally believed that a large scale Antarctic ice sheet formed at the Eocene-Oligocene transition (34.44-33.65 Ma). However, oxygen isotope excursions during the late Eocene (38-34 Ma) and geomorphic evidence of glacial erosion suggest that there were ephemeral continental scale glaciations before the Oi-1 event. Here, we investigate the Antarctic ice sheet evolution over a multi-million year timescale during the late Eocene up to the early Oligocene with the most recent estimates of carbon dioxide evolution over this time period and different bedrock elevation reconstructions. A novel ice sheet-climate modelling approach is applied where the Antarctic ice sheet model VUB-AISMPALEO is coupled to the emulated climate from HadSM3 using the coupler CLISEMv1.0. Our modelling results show that short-lived continental scale Antarctic glaciation might have occurred during the late Eocene when austral summer insolation reached a minimum in a narrow range of carbon dioxide concentrations. The Antarctic ice sheet first reached the coast in Prydz Bay and later in the Weddell Sea region, supporting the glaciomarine sediments dated prior to the EOT.
Ice flow models of the Antarctic ice sheet are commonly used to simulate its future evolution in response to different climate scenarios and assess the mass loss that would contribute to future sea ...level rise. However, there is currently no consensus on estimates of the future mass balance of the ice sheet, primarily because of differences in the representation of physical processes, forcings employed and initial states of ice sheet models. This study presents results from ice flow model simulations from 13 international groups focusing on the evolution of the Antarctic ice sheet during the period 2015–2100 as part of the Ice Sheet Model Intercomparison for CMIP6 (ISMIP6). They are forced with outputs from a subset of models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), representative of the spread in climate model results. Simulations of the Antarctic ice sheet contribution to sea level rise in response to increased warming during this period varies between -7:8 and 30.0 cm of sea level equivalent (SLE) under Representative Concentration Pathway (RCP) 8.5 scenario forcing. These numbers are relative to a control experiment with constant climate conditions and should therefore be added to the mass loss contribution under climate conditions similar to present-day conditions over the same period. The simulated evolution of the West Antarctic ice sheet varies widely among models, with an overall mass loss, up to 18.0 cm SLE, in response to changes in oceanic conditions. East Antarctica ass change varies between -6.1 and 8.3 cm SLE in the simulations, with a significant increase in surface mass balance outweighing the increased ice discharge under most RCP 8.5 scenario forcings. The inclusion of ice shelf collapse, here assumed to be caused by large amounts of liquid water ponding at the surface of ice shelves, yields an additional simulated mass loss of 28mm compared to simulations without ice shelf collapse. The largest sources of uncertainty come from the climate forcing, the ocean-induced melt rates, the calibration of these melt rates based on oceanic conditions taken outside of ice shelf cavities and the ice sheet dynamic response to these oceanic changes. Results under RCP 2.6 scenario based on two CMIP5 climate models show an additional mass loss of 0 and 3 cm of SLE on average compared to simulations done under present-day conditions for the two CMIP5 forcings used and display limited mass gain in East Antarctica.
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 7.4 mm of
global sea level rise, with a standard deviation of 3.7 mm (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.
The land ice contribution to global mean sea level rise has not yet been predicted 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 models, but primarily used previous-generation scenarios and climate models, and could not fully explore known uncertainties. Here we estimate probability distributions for these projections under the new scenarios using statistical emulation of the ice sheet and glacier models. In this work, 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.
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