Summer Modeling School/Notes/vanderVeen Aug4.rtf
Kees van der Veen, University of Kansas
August 4, 2009
Portland Summer Modeling School
Ice Sheet Mass Balance and Sea Level (ISMASS): St. Petersburg and Beyond
notes by Kristin Poinar
Kees has been studying ice sheets / mass balance for 25 years, but just now there is focused international interest in it. Why?
IPCC Summary for Policy Makers (2007): Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to waring, increasing future sea level rise. Understanding of these processes is presently limited, and important.
Challenges for glaciologists: - Improve understanding of processes - Develop quantitative prognostic models ( = models to predict future response) - Incorporate small-scale processes into whole ice-sheet models
Focus on the important question for society: sea level rise! 18-59 cm by 2100 is the present (conservative?) estimate: mountain glaciers, thermal expansion, and the present bit of melting from Greenland / Antarctica. It does *not* include unstable rapid changes observed in Greenland / Antarctica. We should try to put an upper limit on SLR from those changes.
History of ice sheet modelling
- Mahaffy model, 1976 - the first "3D" model (not truly 3D, it was depth-averaged). The Model A. - Jenssen, 1977 - included temperature, so model is actually 3D in a sense - x,y,T. Temperature is important to dynamics - viscosity is highly temperature-dependent, and basal processes (sliding / frozen on) depend on basal temperature. The Model T. - Glimmer (Rutt, Hagdom, Hulton, Payne) 2009. The Ford Mustang - has a sleeker body but the same fundamental engine.
The engine of ice sheet models: - Laminar flow (convenient and easy to use, not necessarily truthful) - Basal sliding - Temperature calculation Available extra options: - Ice shelves - Basal hydrology - Isostasy - Calving (Kees's favorite) - myriad others
Frank Pattyn (pat-TAYNE) model 2003 in JGR - Toyota Prius
Slow physics (glacial orbital cycles) Feedback between elevation & mass balance: higher surface = more snowfall Adjustment of earth crust to ice loading
The new paradigm? Jakobshavn Isbrae since 1851... retreat & speedup
Addressing the IPCC 4AR SCAR report, November 2007: A need for more realistic ice sheet models - "mea culpa", we are aware of the problems and realize the need for better models (Scientific Committee on Antarctic Research) Easy part: make a list of things that are missing from the models Hard part: add them Workshop in July 2008 in St. Petersburg, to begin the hard part. 40-45 participants brainstorming, creating document. Document is still floating around somewhere, lacks full consensus.
Overarching questions from St. Petersburg
- What can we achieve within 5-10 years? Reasonable timeline is important to address global climate change. - Will climate change lead to irreversible (rapid nonlinear) ice sheet response? (i.e. is Jim Hanson right?) Is there a threshold / tipping point? - Do rapid changes at the margins lead to large mass changes? How coupled are the margins to the interior? - Are observed rapid ice sheet changes "natural variability", response to recent warming (ice shelf breakup), or results of a basal switch?
Integrated approach - englacial processes - surface forcing (atmos scientists) - basal conditions (hydrologists) - marine margins (oceanographers) - numerical issues (computing / applied math) Members of these different glaciological communities must talk to each other!!
A high-resolution full Stokes model is *not* enough. - Yes, we can and should - but as a long-term objective - Remain aware of limitations - It excludes some processes necessarily: shear margins (too small-scale: 3-4 km is subgrid spacing) subglacial valleys weak basal layers more Boundary conditions
What ice sheet models need (1)
- Better understanding of physical processes, and which processes are important to include subglacial lakes in Antarctica (see Byrd glacier on EAIS / Ben Smith) - are these short-term changes important in a large model? flow law (strainrate vs. stress) - can we nail it down better? should we? - On what scales should ice sheets me modeled? - Acceptable parameterizations of physics - What do you mean by physics? says Charles Jackson. In atmos sci, there is "physics", "dynamics", "chemistry" all separate. Olga say: in glaciology, physics means everything.
What happens under the ice? (super-nice-looking slide!) Subglacial morphology - channels and trenches - sediment vs. hard beds - soft sediments allow ice to move faster. Important on WAIS. - geothermal flux - northeast Greenland's fast-moving ice stream may be due to a geothermal hot spot there (Fahnestock paper) - bed topography at grounding line
Grounding line stability Helheim glacier and bedrock topography (Ian Howat) - will retreat down a downslope until it reaches an upslope, where it regains stability - Topography - Sediment deposition - Mathematics of the transition zone (Schoof) may be less important than these two issues
What ice sheet models need (2 & 3)
- Subglacial lakes water storage importance of drainage events - Interaction of till and subglacial water - Addition of supraglacial meltwater - Subglacial water budget of different hydrologic systems - Coupling to ice flow model water transport & storage - Sliding laws basal pressure water storage sediment strength
Ice shelves and other (near-) floating peripherals - Mechanisms for breakup thermal limit of viability (general) - it'd be nice if we had a more quantitative explanation / model. Ted Scambos has put forward some ideas - Role of sea ice - Subshelf circulation and melting - Restraint on interior ice - how important are they?
What ice sheet models need (4 & 5)
- Calving "law" that works for all scales of icebergs and bits one of the more important boundary conditions for your model feedback between geometry & calving it is extremely difficult with current calving laws to get a glacier to advance - be careful that your calving law is not automatically building in a retreat / instability that may not be realistic - Better mesoscale models for Greenland & Antarctica (need atmos science input)
Model validation - model inter-comparisons - data validation - capability of simulating past and current changes
What ice sheet models need (6)
- Data for calibration (knob-turning) & validation (compare model's results to other observations) what data? open data access compatible formats easy to use
What are we supposed to do here at the Summer Modeling School? - Student / teacher line should be blurred, or nonexistent - Make Dave Holland happy: come up with a number to limit SLR from dynamical processes - Ian Rutt suggests thinking about *how* to test your code every time you are writing code - Ken Jezek wants us to better incorporate a measure of confidence we have in a model, but Charles Jackson says this is very hard to know.
Bad polar bear limerick
A thin polar bear called Onassis Moaned "Why cant you get off your asses? "It's clear climate change "Is restricting my range "So you must make big cuts to bad gases!"