Ice Sheet Modeling: Current Challenges and Perspectives. Eric Larour (Jet Propulsion Laboratory) Eric Rignot (University of California Irvine/JPL) NASA Sea Level Rise Workshop 2009.
Introduction. • Conclusions of IPCC AR4. • Recent achievements. • Challenges ahead. • Conclusions and recommendations. NASA Sea Level Rise Workshop 2009.
Introduction • The Ice sheets in Antarctica and Greenland will account for a large share of future Sea Level Rise. • Projecting future mass balance (sensitivity, range of probable outcomes) of these ice sheets is challenging but necessary. • Ice sheet evolution is controlled by a multitude of factors poorly constrained by (basic) observations. • There is a pressing need for both more complete models and more comprehensive observations. • In July 2008, in St Petersburg, SCAR summarized current views on ice sheet modeling and prediction. NASA Sea Level Rise Workshop 2009.
1 IPCC AR4. IPCC (Intergovernmental Panel on Climate Change) AR4 (Assessment Report 4): key uncertainties. • “Future changes in the Greenland and Antarctic ice sheet mass, particularly due to changes in ice flow, are a major source of uncertainty that could increase sea level rise projections. The uncertainty in the penetration of the heat into the oceans also contributes to the future sea level rise uncertainty. “ • “Large-scale ocean circulation changes beyond the 21st century cannot be reliably assessed because of uncertainties in the meltwater supply from the Greenland ice sheet and model response to the warming.” • As a result of these shortcomings, IPCC AR4 did not include ice sheet dynamics in its prediction of future sea level change. NASA Sea Level Rise Workshop 2009.
What is SIA? • Models used in IPCC assessments are for the most part based on the Shallow Ice Approximation (Hutter 1982) Ice is controlled (locally) by driving stress (gravity) and basal stress (bedrock friction) in equilibrium. Stresses cannot be transmitted over long distances instantaneously; changes are slow. NASA Sea Level Rise Workshop 2009.
What is SIA good at? • Slow moving grounded ice on frozen bedrock. • “Long” transitions in ice dynamics. - Simulations over centuries to millions of years for paleo reconstructions. • Recent improvements: Coupling of SIA and ice-shelf models with grounding line coupling from Schoof (2007). Pollard and DeConto 2009: Movies\nature07809-s2.mov NASA Sea Level Rise Workshop 2009.
Huybrechts 1999. Limitations of SIA: • Horizontal and part of vertical shear stresses are neglected -> dynamical effects are not well captured. • Ice streams are not captured. • Longitudinal coupling is simplified or ignored. • Simplified buttressing of ice shelves on ice streams -> weak ice/ocean feedback. NASA Sea Level Rise Workshop 2009.
Can IPCC-type models explain recent ice sheet changes? Transition time for dynamic processes: • SIA cannot explain short-time (months) accelerations of Greenland glaciers and Antarctica WAIS. • SIA cannot explain the transmission of perturbation at the ice front 10kms inland instantaneously. Ad-hoc treatment of ice/ocean interactions: • SIA cannot include rapid ice/ocean feedback (months to years). • Ad-hoc treatment of grounding line migration. NASA Sea Level Rise Workshop 2009. Howatt et al., 2007.
2. Recent achievements with simple models • 1D flowline models have been successful at giving practical and tractable explanations for recent changes. • They assume a fixed lateral shear, include GL migration and longitudinal coupling. • They correctly predict the evolution of Pine Island Glacier and Jakobshavn Isbrae! Thomas et al. 2005 “Continued ice-plain thinning at the current rate of about 2 m/yr will result in a velocity increase by 1 km/yr within the next 11 years as the ice plain becomes totally ungrounded.” Thomas et al 2005.
Recent achievements Antarctica • 3D higher-order model revealed upstream migration of diffusive slope wave on Pine Island Glacier. • The model captures the upstream migration of acceleration observed with GPS data by Scott et al. 2009. • The model points at an oceanic origin of the acceleration, similar to the 1D model. Payne et al, 2004. NASA Sea Level Rise Workshop 2009.
Recent achievements of mixed models • SIA+ Shallow shelf 2d models with integrated dynamics of grounding line retreat. Pollard 2009, using Schoof 2007. • Better prediction of grounding line position than standard SIA models. • Reproduces current configuration of Antarctica after paleo-run of 5M yrs. • Correct spin-up of large scale ice sheet model. Pollard and DeConto 2009.
Inputs for a new generation of models: ice-atmosphere interactions • Surface mass balance reconstructions from regional climate models have recently achieved major improvements; field data only used for verification. • Ice-atmosphere interactions are well constrained by regional climate model reconstructions. (PMM5, RACMO). Van den Broeke, 2008 NASA Sea Level Rise Workshop 2009.
3. What are the challenges? Ice Sheet dynamics. • Fundamental role of ice-shelf buttressing. • Fundamental role of basal drag. There is no universal law describing the physics of sliding. • Basal drag can be inferred using satellite data assimilation and inverse control methods (MacAyeal 1989, Rommelaere 1997, Larour 2004, Vieli 2005) or by tuning of an hydrological model (Fastook 1996). • Melt pond draining (moulins, crevasses) may enhance bed lubrication through complex, poorly known pathways; but this is not a major driver of glacier speed up at present. NASA Sea Level Rise Workshop 2009.
Ice shelf rigidity Ice shelf dynamics. • Ice-shelf rigidity is controlled by temperature; temperature is controlled by ice-ocean melting/freezing. This requires a complete modeling of ice-ocean interactions. • Constitutive nature of ice is non-linear, hence requires inversion methods and data assimilation (MacAyeal 1989, Rommelaere 1997, Larour 2004, Vieli 2006). Larour, 2004 NASA Sea Level Rise Workshop 2009.
Ice shelf rifting • Hydrological fracturing, rift propagation, iceberg calving are important processes of ice shelf disintegration that are poorly observed and understood. • Existing models have no calving laws, no ice shelf rifting, no hydrological fracturing. Larour, 2004 NASA Sea Level Rise Workshop 2009.
Grounding line Grounding line instability: • Grounding lines are inherently unstable when the bed slopes down inland (Thomas, 1974) • WAIS instability may yield 3.6m SLR (Bamber 2009). • Grounding line stability criterion (Schoof 2007) is dynamic in nature. Exact course of retreat can be an unstable phenomenon, has a strong dependence on initial conditions, and a strong sensitivity to dynamical parameters ( ice-shelf back-pressure; ice viscosity; bed slipperiness), and bed topography. • This is not developed for higher order 3D models. • Nowicki and Wingham (2008) suggest there is no simple link between ice flux and ice thickness. A full treatment of Navier Stokes equations is necessary at the grounding line. • Dominant forcing from ice-ocean interactions. NASA Sea Level Rise Workshop 2009.
Thermal forcing from the ocean Ocean forcing and sub-ice shelf circulation. • Ice pump mechanism (Lewis and Perkin, 1988) on ice shelves (Antarctica and northern Greenland). • Amundsen Sea Embayment: • wind-driven circulation forces warm Circumpolar Deep Water onto the continental shelf, resulting in high basal melting near GL. - Ice-shelf submarine melting increases 10 m/yr per degree C increase in ocean temperature. • Ice-ocean interaction modeling: • 1D-2D plume models overpredict (Jenkins 1991, Holland and Feltham 2006) • 2D overturning models (Hellmer and Olbers 1989, Hellmer and Olbers 1991, Hellmer and Jacobs 1992, 1995, Hellmer et al. 1998) do better. • 3D models, with rotational effects (Determann and Gerdes 1994, Grosfeld et al 1997, Holland and Jenkins 2001) are best. • Ice-ocean interactions included in MIT/GCM ECCO2(Schodlock 2005) • Other processes e.g. sub-glacial discharge, submarine melting of tidewater glaciers are important (Holland et al., 2008; Rignot et al., 2009) but not yet included. NASA Sea Level Rise Workshop 2009.
How do we get there? Technology: • Higher-order modeling. • Embed higher order physics into lower order. • Full Stokes solutions at the grounding line (and at the ice divide). • Pattyn (2003) solution inland. • MacAyeal (1989) solution in fast flowing ice streams. NASA Sea Level Rise Workshop 2009.
Large scale modelling at high spatial resolution (to include ice streams). • Scalable software. • Solvers (Petsc) (Brown 2005) • Meshing (anisotropy, adaptative grid refinement). • Computational power. • MIT GCM-ECCO2: Pleiades and Columbia clusters (1000 CPUS). • PISM (500 CPUS- Ronne Ice Shelf 100 years evolution). • ISSM (128 CPUS Cosmos cluster) NASA Sea Level Rise Workshop 2009.
Required observations • Glacier thickness: • Need ~1km or better resolution for all fast flowing ice in Antarctica and Greenland. • 10 km or better resolution for all Antarctica and Greeland. • Examples of progress: Jakobshavn Isbrae, Pine Island Glacier. 2001 2009 NASA Sea Level Rise Workshop 2009.
Surface elevation and temporal changes in elevation • Need ~1km resolution, meter-scale vertical precision elevation. • Need meter-scale changes in surface elevation. • Basal friction and Surface Velocities: • Requires the assimilation of surface velocity data. • Velocities are needed at ~1km resolution over entire ice sheets. • Time series are essential to validate ice flow models over periods of several decades. • Essential to validate basal friction and hydrology models. • Sub-ice-shelf cavities: • Most are unmapped, which makes ice-ocean interactions modeling difficult. NASA Sea Level Rise Workshop 2009.
International efforts and inter-comparison exercises • Model intercomparison: • 13 years history. • EISMINT (Europen Ice Sheet Modeling Initiative). . • ISMIP-HOM (Ice Sheet Model Intercomparison Project for Higher-Order ice sheet Models) • MISMIP: Marine ice sheet model intercomparison project (Schoof, Hindmarsh, Pattyn 2008). • Model frameworks: • ice2sea: science program funded by EU FP7. Ice2sea has 24 partners, European and international. Ice2sea will improve projections of the contribution of ice to future sea-level rise. • seaRISE (sea-level Response to Ice Sheet Evolution) is a community organized effort to estimate the upper bound of ice sheet contributions to sea level in the next 100--200 years. NASA Sea Level Rise Workshop 2009.
Conclusions. • New models have been developed to capture higher-order physics and include grounding line migration. • Ice/ocean coupling is essential but not there yet. • Current datasets are insufficient to constrain ice flow models at the glacier scale, especially ice thickness; but progresses are being made. • Observational time series with no gap are essential to validate and constrain ice flow transient models. But we are losing capabilities to observe some aspects of ice sheets (e.g. no InSAR observations of Greenland glaciers after 2008). • Modeling frameworks are in place to validate models, compare and improve. • Major improvements are possible and will likely take place in the near future but reliable projections are still a long way down the road. NASA Sea Level Rise Workshop 2009.
THANKS ! NASA Sea Level Rise Workshop 2009.