The cryosphere and the processes that force its evolution have profound and permanent effects on the global climate. In particular, Arctic amplification leads to extreme mid-latitude weather and glacier and ice sheet retreat is increasing global mean sea level causing the ocean to encroach onto coastal communities. Despite potentially devastating impacts, accurate predictions of future dynamics and rigorous characterizations of the associated uncertainty remain elusive. Misunderstood physics and computational limitations require complex physical processes to be parameterized and calibrated using noisy data that is sparse in both space and time. However, collecting data in remote polar regions is difficult, dangerous, and expensive. Therefore, we must leverage remote sensing techniques and wisely allocate limited resources. Finally, predictive uncertainties must be quantified to give meaningful error bounds on quantities of interest, such as future mean sea level. This session discusses recent advancements trying to understand the dynamic processes governing the cryosphere given observations and/or models as well as techniques to obtain and analyze data.
14:00
SMBMIP: Intercomparison of modelled 1980-2012 surface mass balance over the Greenland Ice sheet
Xavier Fettweis | University of Liège | Belgium
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Xavier Fettweis | University of Liège | Belgium
The Greenland Ice Sheet (GrIS) mass loss has been accelerating with a rate of about 20 +/- 10 Gt/yr2 and around 60% of this mass decrease can be directly attributed to an increase of the surface meltwater runoff. However, in the climate community different approaches exist on how to model the different surface mass balance (SMB) components; i) from physical based climate models, ii) intermediary complexity energy balance models to iii) positive degree day models. Additionally, many of these models compute the SMB components based on different spatial and temporal resolutions, with different forcing fields, different ice sheet extent, rendering inter-comparison difficult or sometimes unfeasible. In this GrIS SMB model intercomparison project (SMBMIP), the SMB outputs from 13 different models are then compared on a common 1-km grid, ice sheet mask and period (1980-2012) over the GrIS to (1) SMB estimates using gravimetric remote sensing data from the GRACE mission and ice discharge data, (2) ice cores, snow pits and in-situ SMB observations and (3) remote sensing data of the GrIS bare ice extent using the MODIS sensor. Our results show that the mean GrIS SMB of all 13 models has been positive between 1980 and 2012 with an average of 340 +/- 112 GT/yr, but has decreased at an average rate of -6.7 Gt/yr2, mainly driven by an increase of +7.4 Gt/yr2 in meltwater runoff.
14:30
Projecting Antarctica’s contribution to future sea level rise from basal ice-shelf melt using linear response functions of 16 ice sheet models (LARMIP-2)
Torsten Albrecht | Potsdam Institute for Climate Impact Research | Germany
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Anders Levermann | Potsdam Institute for Climate Impact Research | Germany
Torsten Albrecht | Potsdam Institute for Climate Impact Research | Germany
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 that arises from large uncertainty in the external forcing that future warming may exert onto the ice sheet. While ice shelf melting is considered to be a major if not the largest perturbation of the ice sheet’s flow into the ocean, the approach is neglecting a number of processes such as surface mass balance related contributions and mechanisms. In assuming linear response theory, we are able to capture complex temporal responses of the ice sheets, but we neglect any dampening or self-amplifying processes. We include uncertainty in the atmospheric warming response to carbon emissions, uncertainty in the oceanic transport to the Southern Ocean and the observed range of responses of basal ice shelf melting to oceanic warming outside the ice shelf cavity. 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 RCP-8.5 scenario. The maximum within the 21th century of the median value is 4cm/decade with a likely range between 2 cm/dec and 9 cm/dec and a very likely range between 1 cm/dec and 14 cm/dec.
15:00
GIA Model Statistics for GRACE Hydrology, Cryosphere, and Ocean Science
Lambert Caron | Jet Propulsion Laboratory | United States
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Lambert Caron | Jet Propulsion Laboratory | United States
We provide a new analysis of glacial isostatic adjustment (GIA) with the goal of assembling the model uncertainty statistics required for rigorously extracting trends in surface mass from the Gravity Recovery and Climate Experiment (GRACE) mission. Such statistics are essential for deciphering sea level, ocean mass, and hydrological changes because the latter signals can be relatively small (≤2 mm/yr water height equivalent) over very large regions, but also because solid-earth deformation and sea-level change influence the stability of marine-terminating ice sheets at the grounding line. With abundant new >7 year continuous measurements of vertical land motion (VLM) reported by Global Positioning System stations on bedrock and new relative sea level records, our new statistical evaluation of GIA uncertainties incorporates Bayesian methodologies. A unique aspect of the method is that both the ice history and 1‐D Earth structure vary through a total of 128,000 forward models. We find that best fit models poorly capture the statistical inferences needed to correctly invert for lower mantle viscosity and that GIA uncertainty exceeds the uncertainty ascribed to trends from 14 years of GRACE data in polar regions.
15:30
Quantification of Surface forcing Requirements for a Greenland Ice Sheet Model using Uncertainty Analyses
Nicole-Jeanne Schlegel | Jet Propulsion Laboratory | United States
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Nicole-Jeanne Schlegel | Jet Propulsion Laboratory | United States
The Greenland Ice Sheet is a substantial reservoir of almost 7 m of sea-level equivalent, and on average, climate dictates 60% of its sea-level contribution. Changes in ice discharge, driven by perturbations in outlet glacier ice dynamics, constitute the rest. Climate also affects ice discharge, since the flow of interior ice feeding the outlet glaciers evolves in response to surface changes over time. Here, using an ice sheet model and uncertainty quantification, we explore ice flow sensitivity to climate-driven changes in ice surface topography on multi-decadal timescales. We find that changes in surface forcing near large outlet glaciers can influence region-wide ice flow. Improvements to climate products should be prioritized in these areas, especially in the Central West and Southeast. Results also suggest that over most of Greenland, surface forcing should be supplied at a spatial resolution of 21 km or finer to accurately simulate ice response to climate change.