"Ice Sheets and Sea Level Rise: How Should IPCC Handle Deep Uncertainty?" Michael Oppenheimer For Inside the IPCC Princeton University 1 April 2008

Transcription

1 "Ice Sheets and Sea Level Rise: How Should IPCC Handle Deep Uncertainty?" Michael Oppenheimer For Inside the IPCC Princeton University 1 April 2008

2 This Talk is about: IPCCs (controversial) assessment of future sea level rise Why predicting ice sheet behavior is problematic What might IPCC do differently in the future What should the glaciological community do differently to improve the situation

3 Keep in Mind IPCC assessments have been behind the curve on the ice sheet/sea level problem for 20 years But AR4 saw some significant advances (e.g., by including paleo-climate perspective) Overall, AR4 was 95% terrific in my view!

4 Global mean sea level has been rising (faster than previous millennial average) GEO339 Class 9 4

5 Recent trend accelerating Beckley et al GRL 2007 GEO339 Class 9 5

6 Unanticipated ice sheet contribution may be the reason although questions remain about recent glacier contribution and changes in storage 6 AR4 TS

7 and regional variation is large

8 Projection of 21 st Century Sea Level Rise Range: m vs m in TAR

9 Problems with IPCC assessment of SLR Incommensurate (and opaque) assessment of uncertainty range vs. TAR (in addition to lower sensitivity of sea level) Emphasis on numerical results from invalid ice sheet models (insufficient attention to adjusting for this limitation) Flattened estimates of future rise >>>rejection of empirical methods >>>ignoring structured elicitation >>>hesitancy on linking models and paleo-climate inferences

10 A semi-empirical projection of these accelerations yields a larger 21 st century SLR Rahmstorf, Science 2007 What about beyond the 21 st century?

11 Source of uncertainty and potentially large future contribution ~5m ~60m ~7m

12 What 6-12m of sea level rise would look like National Snow and Ice Data Center 2004

13 Terrain <6m above sea level

14 WAIS + GIS + other sources---> ~ 50 feet

15 Imagine what would happen to Bangladesh!

16 Key Issues How much are ice sheets changing? What is the role of the buildup of greenhouse gases in these changes? Why are ice sheets so difficult to model? >>> when may we have a model for ice sheets that can help us predict sea level rise? Other ways to forecast polar ice and sea level in a warmer world: how much and how fast might polar ice melt?

17 What is an ice sheet? Mass accumulates in the cold center of continent Ice piles up in center, flows under influence of gravity toward warmer edges (near ocean, lower altitude) where ice may go afloat, and loss occurs due to melting and iceberg formation Ice can be wet at base, too

18 In other words: Ice sheets both melt and move We think we have skill at modeling melting We may be OK at modeling slow ice movement We have no skill at all at modeling rapid ice movement (I ll discuss why later)

19 Observational evidence on ice-sheet model failure: Mass balances negative for both ice sheets: >>Ice loss twice as fast for Greenland as ice-sheet models indicate due to unexpected dynamical response >>Antarctica apparently shrinking rather than growing (as models project), due to fast dynamics in WAIS, lack of added accumulation overall

20 Antarctic Mass Balance from SAR Rignot et al Nature 2008

21 Short-term trend in surface melting, Greenland But surprisingly, dynamical loss is equally important (recently)

22 So, profile of Greenland is changing Greenland Ice Elevation Changes: versus loss rate ~ 0.13mm/yr Other values: GRACE (2002-4) ~ 0.2mm/yr (2002-5) ~ 0.5mm/yr Satellite radar (1996)~0.2 (2000)~0.5 SCIENCE VOL July 2000 Kabrill et al. pg 429

23 Recent changes (trends?) in Greenland and Antarctic ice sheets from multiple techniques Casenave, Sciencexpress, 19 October 2006

24 But it s not simple Two Greenland glaciers slowed down: Is it all noise? Howat et al Science 2007

25 Different forcing at each location Surface melting Mass balance (gains losses) from GRACE Chen et al, GRL 33, L11502, doi: /2006gl026369, 2006 Warm water reaching under ice shelves in Amundsen sea

26 Role of Greenhouse Warming? Short time series of ice observations Sparse temperature records with variable history For Antarctica, ocean sub-surface temperatures may be critical and we don t know this history Best Estimate: Greenhouse warming likely contributing to recent losses at Greenland, Antarctic Peninsula. Amundsen Sea area unclear.

27 Mechanisms: Why rapid dynamics occurs and how warming could accelerate it Antarctica without the ice: Possibly an earlier incursion of the sea left a bed of slippery sediment: some indications that this happens periodically

28 Slippery base allows ice to stream

29 Oppenheimer, Michael, Global warming and the stability of the West Antarctic Ice Sheet, pg 326; Nature. Vol. 393; 28 May 1998 The Loss of Ice from the West Antarctic Ice Sheet is Governed by Ice Streams which models don t capture

30 Bamber, Jonathan L. et al.: Widespread Complex Flow in the Interior of the Antarctic Ice Sheet, pg 1248; Science Vol February 2000 Ice streams are tied closely to ice dynamics far inland Figure 1. (A) Balance velocities for part of the Siple Coast covering parts of Ice Streams B, C, D, and E. The area to the left of the black line marks the limit of the ERS-1 satellite-derived elevation data. (B) Surface velocities derived from feature tracking (16) and RADARSAT SAR images with the use of a combination of interferometric and "speckle tracking" techniques (8). m a 1, m year 1.

31 Controls on ice stream flow: basal friction (or lack thereof, due to slippery, wet sedimentary base) lateral friction with static ice Ice shelf buttressing

32 Oppenheimer, Michael, Global warming and the stability of the West Antarctic Ice Sheet, pg 327; Nature. Vol. 393; 28 May 1998 A mechanism for rapid sea level rise: Ice shelves: may partially control flow rate of ice streams. Key issue: If ice shelves were removed, would land-based ice accelerate on the slippery sediment?

33 What continental ice-sheet models do (and don t do) Assume dynamics described by empirical flow law (Glen s Law): OK for ice flow limited by basal friction, not for slippery base Shallow ice approximation : dynamics local No hydrology Such approaches can t possibly generate ice streams or ice shelf buttressing that may yield rapid responses to changes at margins (where warming matters)

34 Multiple flow regimes make modeling difficult Slow-moving ice Ice streams flowing over variable base, influenced by sub-grid scale processes ----Grounding line problem resists solution: o Why is there a stable boundary at all? o Why doesn t a marine ice sheet just flow into the sea? ----Need to know mechanical properties of ice, too OBSERVATION-POOR SITUATION!

35 Consider the very long term (beyond 21 st Century) Shortcomings of models are most obvious for WAIS because models don t even reproduce major mass loss mechanism ( ice streams )! Such fast processes may be key to timescale and temperature dependence of ice loss WAIS, as opposed to Greenland, is a marine ice sheet: >>Does it want to slide into the sea? >>Held in place (buttressed) by floating ice shelves?

36 Vaughn, David G. et al.: Devil in the Detail pg. 1778; Science, Vol. 293,September 7, 2001 Larsen A Larsen B Larsen C Wilkins Natural Experiment on Role of Buttressing

37 31 January 2002 NSIDC 2

38 5 March 2002 NSIDC 2

39 Larsen ice shelf, after collapse

40 Wide glaciers accelerated where ice shelf lost, not elsewhere C.Rapley, BAS

41 Amundsen Sea: Accelerating ice streams, thinning of grounded ice due to loss of buttressing? Too late to prevent 1.5m SLR? NASA/Goddard Space Flight Center Scientific Visualization Studio

42 Ice shelves control ice flow in parts of Greenland, too How much of Greenland could move fast?

43 Jakobshavn Isbrae glacier, Greenland, 2004 (Dietrich et al JGR E 2007)

44 Lubrication at base important in Greenland, too?

45 Future behavior: What about Ross and Filchner-Ronne ice Surface melting: shelves? ----Already affecting Antarctic Peninsula but requires a warming of ~5 0 C on these major ice shelves Basal melting due to warm circumpolar waters: ---- is destroying Amundsen Sea ice shelves already ----??? warming to destroy large ice shelves

46 What do models project: Millennial-scale change Greenland; No steady state for C global warming but very slow (~millennia) loss WAIS: Suggest total loss (over millennia) for global warming > 5 0 C; gain in mass for smaller warming Given limitations of models, are timescales credible? If not, century-scale ice-sheet loss, even if only partial, would certainly affect projection of 21 st C sea level

47 Nature/Vol. 419/12 September 2002, pg. 189 What else is available aside from models? Poles were C warmer What were ice sheets and sea level doing?

48 Paleo-climate Perspective Greenland ice extent was probably smaller by 2-4m SLR-equivalent during Last InterGlacial (Eemian) Antarctica may or may not have been smaller Global mean temperature was not much different but poles were significantly warmer: C (summer) vs present in Greenland. Sea level was (plus or minus) +4 to +6m higher (or maybe 0- to +6m) Possible rates of SLR >1m/century even in approach to final sea level stand

49 Greenland during LIG Source: IPCC AR4 WGI Fig 6.6

50 Inferences on SLR rates from Red Sea foram 18 O and coral stands Rohling et al Nature 2007

51 Uncertainty Interpreting LIG: Greenland models and Eemian information ROUGHLY consistent on temperature but not rate: large sea level rise for modest polar warming But Greenland alone may leave a gap: WAIS responsible for 0-4m SLR? Uncertainties in chronologies, rates, etc, muddy conclusions LIG warming could be just the beginning

52 Putting observations, model-based and paleo evidence together: There is medium confidence that at least partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic ice sheet, would occur over a period of time ranging from centuries to millennia for a global average temperature increase of 1-4 C (relative to ), causing a contribution to sea level rise of 4-6 m or more. WGII >>If centuries, adapting to such rates would be very difficult and costly

53 Back to the beginning: Given: limitations of ice-sheet models current contribution of ice sheets to sea level rise potential magnitude of future contribution on a (potentially) short timescale, Should there be so much emphasis on numerical model outcomes?

54 Key numerical values should reflect combined judgment So, what should IPCC do differently over the next few years and in the next assessment? Do an assessment of 21 st C and long term SLR that is not deferential to models: confront structural error Integrate paleo-climate assessment fully Elevate observational information in assessment of future Avoid premature consensus by taking advantage of availability of parallel, formalized assessment techniques (expert elicitation at least two are now published)

55 Structural proposals Don t assume modeling problem will have been solved Integrate working group activities from t=0 Interim workshops on ice/slr starting soon Greater transparency