1. The Dilemma of RESOLUTION: How Good is Good Enough: A Case Study from Greenland James L Fastook University of Maine We thank the NSF, which has supported the development of this model over many many years through several different grants, and especially CReSIS (ANT-0424589) for the excellent data product used here. James L Fastook University of Maine We thank the NSF, which has supported the development of this model over many many years through several different grants, and especially CReSIS (ANT-0424589) for the excellent data product used here.

2. SeaRISE ● Whole-icesheet modelers are predicting the response of the Greenland and Antarctic Ice Sheets to projected climate change. ● With a database provided by Jesse Johnson – http://websrv.cs.umt.edu/isis/index.php/SeaRISE_Assessment http://websrv.cs.umt.edu/isis/index.php/SeaRISE_Assessment ● These datasets include surface elevation, thickness, bedrock elevation, and other boundary conditions necessary for the models. ● Both Greenland and Antarctica are provided at 5 km resolution. ● Whole-icesheet modelers are predicting the response of the Greenland and Antarctic Ice Sheets to projected climate change. ● With a database provided by Jesse Johnson – http://websrv.cs.umt.edu/isis/index.php/SeaRISE_Assessment http://websrv.cs.umt.edu/isis/index.php/SeaRISE_Assessment ● These datasets include surface elevation, thickness, bedrock elevation, and other boundary conditions necessary for the models. ● Both Greenland and Antarctica are provided at 5 km resolution.

3. An Accurate Representation ● We have seen from previous work that an accurate representation of the bed is essential if models are to produce reasonable results. ● As we look at the response of the ice sheets to projected climate change, we wonder how good our resolution must be, especially in the fast-flow areas of the ice streams, many of which are topographically controlled.

4. A High-Resolution Dataset ● CRESIS has provided us with a very high- resolution bed for the Jakobshavn catchment. ● Joel Plummer, PhD student, Dept. Geography, U. Kansas ● 1294 X 438 rectangular grid – 566,772 points, – 565,041 quadrilaterals – Average spacing 100 X 300 m. ● Covering 16,000 square kilometers – equal to 4 “Rhode Islands” or 1.1 “Connecticuts” – one-and-a-half B-15 icebergs

5. CReSIS Flightlines

6. 1993

7. 1995

8. 1997

9. 1998

10. 1999

11. 2001

12. 2002

13. 2003

14. 2005

15. 2007

16. 2008

17. ALL

19. A High-Resolution Dataset ● We show the results of – Using this dataset at a degraded resolution comparable to the whole-icesheet dataset provided by Jesse Johnson – Using progressively higher resolutions to more accurately capture the deep channel in which the Jakobshavn Ice Stream flows. – Doing this using the embedded-model feature of UMISM, whereby a higher-resolution, small-domain model (the ice stream) is run inside a lower- resolution, broader-domain model (the whole icesheet).

20. Ice Shelf Physics ● Weertman (1957), the spreading of an ice shelf in a parallel-walled channel is proportional to h 3. – Thinning therefore is proportional to h 4.

21. Thinning Rate ●

22. Ice Shelf Parameter ● Add Weertman thinning rate times a parameter between 0 and 1. – 0: fully buttressed, no thinning. – 1: no buttressing, maximum thinning rate. ● To the accumulation rate (source or sink of mass) in the right-hand side of the continuity equation in the Shallow Ice Approximation Formulation.

23. The Canyon ● The most remarkable feature is the ice stream channel – Approximately 100 km long – Incised as much as 1750 m into the surrounding terrain – More than 10 km wide in places ● It is on the same scale as the Grand Canyon – 446 km long – 1500 - 1800 m deep – 16-20 km wide

24. The Canyon

35. Spinup ● A very abbreviated spinup of just 5000 years – with climate held constant at climate knob +1

36. Spinup

37. Velocities after spinup ● A Logarithmic scale – 1 m/a = 0 (10 0 ) – 10 m/a = 1 (10 1 ) – 100 m/a = 2 (10 2 ) – 1000 m/a = 3 (10 3 ) – 10000 m/a = 4 (10 4 )

44. Perturbation scenario ● With captured spunup configurations as the starting point of the perturbation scenario – Hold steady for 100 years – Increase temperature knob by 4 C sinusoidally over 100 years – Hold steady for further 100 years

45. Response: Area

46. Response: Volume

47. Response: Area

48. 350 km^2 lost in under 5 years

67. Conclusions: The representation of ice stream “Canyons” is essential, even at the 5 km resolution of whole ice sheet models. At 2 km resolution, even a simple mass-balance climate-initiated retreat (simply warming temperature) produces different results. The abrupt retreat episode occurring in the Jakobshavn Canyon clears 350 km^2 in under 5 years, with velocities approaching 10 km/yr. an event commensurate with the observed retreat in the last decade. The representation of ice stream “Canyons” is essential, even at the 5 km resolution of whole ice sheet models. At 2 km resolution, even a simple mass-balance climate-initiated retreat (simply warming temperature) produces different results. The abrupt retreat episode occurring in the Jakobshavn Canyon clears 350 km^2 in under 5 years, with velocities approaching 10 km/yr. an event commensurate with the observed retreat in the last decade.