1. Abrupt Changes in Arctic Sea Ice Marika Holland NCAR

2. Outline Why sea ice? Present-day observed conditions and change An example of simulated abrupt transitions Future climate projections Application to paleo-climate conditions? Considerations when using models to study Arctic change

3. Why sea ice? Influence of sea ice on climate Modifies surface energy budget Albedo effects Ice/snow albedo of 0.6-0.8 compared to ocean albedo of ~0.1 Insulates atmosphere from ocean Modifies heat and water transfer Affects ocean freshwater distribution Salinity rejected during ice growth Freshwater released during ice melt Transport of sea ice redistributes water

4. Role of sea ice as an “amplifier” Surface albedo feedback amplifies climate perturbations Ice/snow albedo of 0.6-0.8 vs ocean albedo of ~0.1 (From Hall, 2004) VA=variable albedo FA=fixed albedo (DJF SAT)

5. Role of sea ice as an “amplifier” From Li et al., 2005 Insulating effect of sea ice contributes to large atmospheric response to sea ice changes. SST LGM Reduced Ice SAT Difference

6. Processes Involving ice/ocean FW exchange In warmer climate, increased ice growth due to loss of insulating ice cover results in Increased ocean ventilation Ocean circulation changes Transient response Change in Ice growth rates at 2XCO2 Change in Ideal age at 2XCO2 From Bitz et al., 2006 Change in Ocean Circulation Yr: 40-60 Change in Ideal Age at 2XCO2 cm

7. Observed Arctic Conditions

8. Sea ice concentration (NSIDC, 2005) (Perovich, 2000) Fowler, 2003 Observed thickness Laxon et al., 2003 The observed Arctic sea ice June 6, 2005

9. Observations indicate large changes in Arctic summer sea ice cover From Stroeve et al., 2005 2002 2003 2004 1980 2000 Sept Ice Extent Trend = 7.7% per decade

10. Suggestions that ice has thinned… Rothrock et al., 1999 Ice draft change 1990s minus (1958-1976)

11. Indications that Arctic Ocean is warming Polyakov et al., 2005 19002000 Atlantic Layer Temperature “Pulse-like” warming events entering and tracked around the Arctic General warming of the Atlantic layer

12. North Atlantic Oscillation Positive Phase (From University of Reading webpage)

13. Timeseries of JFM NAO Index Maybe it is not all the NAO/AO?

14. Have led to suggestions that: “Researchers estimate that in as little as 15 years, the Arctic could be ice free in the summer” J Climate, 2005 Overpeck et al., 2005 “There is no paleoclimate evidence for a seasonally ice free Arctic during the last 800 millennia” Overpeck et al.

15. Future Projections What can models tell us?

16. Future climate scenarios Meehl et al, 2005 Wigley, 2000 Relatively gradual forcing. Relatively gradual response in global air temperature

17. September Sea Ice Conditions Observations Simulated 5-year running mean Gradual forcing results in abrupt Sept ice transitions Extent from 80 to 20% coverage in 10 years. Winter maximum shows Smaller, gradual decreases Largely due to decreases in the north atlantic/pacific “Abrupt” transition

18. Forcing of the Abrupt Change Change thermodynamically driven Dynamics plays a small stabilizing role Change in ice area over melt season Thermodynamic Dynamic Ice melt rates directly modify the ice thickness Ice thickness shows large drop associated w/abrupt event However, change is not “remarkable” March Ice Thickness

19. Processes contributing to abrupt change Increased efficiency of OW production for a given ice melt rate As ice thins, vertical melting is more efficient at producing open water Relationship with ice thickness is non-linear % OW formation per cm ice melt March Arctic Avg Ice Thickness (m)

20. Basal Melt Surface Melt Total Melt Processes contributing to abrupt change Albedo Feedback Increases in basal melt occur during transitions Driven in part by increases in solar radiation absorbed in the ocean as the ice recedes cm/day W m -2 SW Absorbed in OML 5 Year Running Mean

21. Processes contributing to abrupt change Increasing ocean heat transport to the Arctic Ocean Heat Transport to Arctic Increases in ocean heat transport occur during the abrupt transition. Contributes to increased melting and provides a “trigger” for the event.

22. Changes in Ocean Heat Transport Ocean Heat Transport to Arctic

23. Fram Strait Siberian Shelf Arctic Arctic Ocean Circulation Changes 2040-2049 Minus 1980-1999 Siberian Shelf Fram Strait Arctic

24. Processes Involving ice/ocean FW exchange In warmer climate, increased ice growth due to loss of insulating ice cover results in Increased ocean ventilation Ocean circulation changes Transient response Change in Ice growth rates at 2XCO2 Change in Ideal age at 2XCO2 From Bitz et al., 2006 Change in Ocean Circulation Yr: 40-60 Change in Ideal Age at 2XCO2 cm

25. Both trend and shorter-timescale variations in OHT appear important OHT “natural” variations partially wind driven. Correlated to an NAO-type pattern in SLP Ocean Heat Transport to Arctic

26. Mechanisms Driving Abrupt Transition 1.Transition of ice to a more vulnerable state thinning of the ice 2.A Trigger - rapid increases in ocean heat transport. Other “natural” variations could potentially play the same “triggering” role? 3.Positive feedbacks that accelerate the retreat Surface albedo feedback OHT feedbacks associated with changing ice conditions

27. Impacts of Abrupt Ice Transitions on Other Aspects of the Climate System Using the model to assess

28. Associated atmospheric conditions Winter air temperature increases rapidly during abrupt ice change Arctic region warms ~5C in 10 years in December Changes are particularly large along the Eurasian coast

29. Precipitation Changes Precipitation generally increases over the 20th-21st centuries Rate of increase is largest during the abrupt sea ice transition 2040-2049 minus 1990-1999

30. Projections of Near-surface Permafrost Courtesy of Dave Lawrence, NCAR (Lawrence and Slater, 2005) Ice Extent 10 6 km 2 Permafrost (CCSM) Sept. sea-ice (CCSM) Sept. sea-ice (Observed)

31. How common are abrupt transitions? Transitions defined as years when ice loss exceeds 0.5 million km 2 in a year Obs Simulated 5yr running mean September Ice Extent “Abrupt” transition

32. How common are forcing mechanisms?

33. How common are effects? Lagged composites relative to initiation of abrupt sea-ice retreat event Courtesy of David Lawrence, NCAR Arctic Land Area

34. x1000 years ago  18 O (per mil SMOW) Heinrich events Dansgaard/Oeschger oscillations Younger Dryas 8.2 k event -30 -40 -50 -60 Temperature (  C) GISP2, Greenland Role of sea ice for abrupt transitions in a paleoclimate context? (slide courtesy of Carrie Morrill)

35. Simulated abrupt transitions in sea ice abrupt forcing (freshwater hosing) can result in abrupt ice changes Sea ice changes amplify climate response Global teleconnections can result Longevity of these changes are an issue Sea ice change SAT Change (From Vellinga and Wood, 2002; Vellinga et al, 2002)

36. Some Cautions in Using Models to Examine these (and other) issues… Biases in simulated control state can affect feedback strength Uncertainties in model physics/response Acknowledgement that model physics matters for simulated feedbacks Models provide a wonderful tool for examining climate feedbacks, mechanisms, etc BUT…

37. ITD Influence on Albedo Feedback Model physics influences simulated feedbacks Getting the processes by which sea ice amplifies a climate signal “right” can be important for our ability to simulate abrupt change ITD (5 cat) 1 cat. 1cat tuned “Strength” of albedo feedback in climate change runs (Holland et al., 2006)

38. Feedbacks contribute to Arctic amplification But, that amplification varies considerably among models (Holland and Bitz, 2003)

39. Sea ice in fully coupled GCMs IPCC AR4 1980-1999 ice thickness Red line marks observed extent

40. Importance of sea ice state for the magnitude of polar amplification Magnitude of polar amplification is related to initial ice thickness With thinner initial ice, melting translates more directly into open water formation and consequent albedo changes (From Holland and Bitz, 2003)

41. SAT Change at end of 21st century From A1B scenario

42. Aspects of the Model’s Internal Variability Model Standard Deviation Model 11.93 Model 21.90 Model 31.72 Model 41.68 Model 50.42

43. Summary Sea ice plays an important role in the climate system and is an effective amplifier of climate perturbations: due to surface albedo changes due to ice/ocean/atm exchange processes, OHT changes Observations indicate recent changes in the Arctic system Climate models indicate continued change into the foreseeable future and suggest abrupt reductions in the Arctic ice cover These studies have possible implications for paleo-climate transitions Climate models are a useful tool for exploring the mechanisms that may contribute to rapid climate transitions, but need to be used with some caution

44. Importance of sea ice state for location of warming Models with more extensive ice cover obtain warming at lower latitudes The location of warming can modify the influence of changes on remote locations

45. 20th Century 21st Century Increased Arctic Ocean heat transport occurs even while the Atlantic MOC weakens

46. Do other models have abrupt transitions? Some do… Data from IPCC AR4 Archive at PCMDI

47. Processes involving ice/ocean FW exchange Change in poleward ocean heat transport at 2XCO2 conditions Yr 40-60 V’T VT’ (Bitz et al., 2006) Change in Arctic OHT

48. By end of 21st century SAT consistently warms SLP changes are evident Meehl et al, 2005

49. OHT and polar amplification Change in poleward ocean heat transport at 2XCO2 conditions Both control state and change in OHT are correlated to polar amplification  OHT (From Holland and Bitz, 2003)

50. Sea ice representation in GCMs Motionless Slab of uniform thickness Slab of uniform thickness in motion A thickness distribution of slabs in motion, ridging/rafting parameterized SW Basal heat flux

51. Changes in sea ice model representation over last 5 Years %