Abrupt Changes in Arctic Sea Ice

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” VA=variable albedo FA=fixed albedo (From Hall, 2004) (DJF SAT) • Surface albedo feedback amplifies climate perturbations • Ice/snow albedo of 0.6-0.8 vs ocean albedo of ~0.1

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

6. Change in Ice growth rates at 2XCO2 • 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 cm Change in Ocean Circulation Change in Ideal Age at 2XCO2 Change in Ideal age at 2XCO2 Yr: 40-60 From Bitz et al., 2006

7. Observed Arctic Conditions

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

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

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

11. Atlantic Layer Temperature 1900 2000 Indications that Arctic Ocean is warming • “Pulse-like” warming events entering and tracked around the Arctic • General warming of the Atlantic layer Polyakov et al., 2005

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

15. Future ProjectionsWhat can models tell us?

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

17. September Sea Ice Conditions • 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 Observations Simulated 5-year running mean

18. 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 Forcing of the Abrupt Change Dynamic • Change thermodynamically driven • Dynamics plays a small stabilizing role Thermodynamic Change in ice area over melt season

19. Processes contributing to abrupt change % OW formation per cm ice melt March Arctic Avg Ice Thickness (m) 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

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

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

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

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

24. Change in Ice growth rates at 2XCO2 • 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 cm Change in Ocean Circulation Change in Ideal Age at 2XCO2 Change in Ideal age at 2XCO2 Yr: 40-60 From Bitz et al., 2006

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

26. Mechanisms Driving Abrupt Transition • Transition of ice to a more vulnerable state • thinning of the ice • A Trigger - rapid increases in ocean heat transport. • Other “natural” variations could potentially play the same “triggering” role? • 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 2040-2049 minus 1990-1999 • Precipitation generally increases over the 20th-21st centuries • Rate of increase is largest during the abrupt sea ice transition

30. Ice Extent 106 km2 Permafrost (CCSM) Sept. sea-ice (CCSM) Sept. sea-ice (Observed) Projections of Near-surface Permafrost Courtesy of Dave Lawrence, NCAR (Lawrence and Slater, 2005)

31. September Ice Extent Obs How common are abrupt transitions? Simulated 5yr running mean “Abrupt” transition Transitions defined as years when ice loss exceeds 0.5 million km2 in a year

32. How common are forcing mechanisms?

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

34. 8.2 k event Dansgaard/Oeschger oscillations Younger Dryas Heinrich events Role of sea ice for abrupt transitions in a paleoclimate context? GISP2, Greenland -30 Temperature (C) -40 d18O (per mil SMOW) -50 -60 x1000 years ago (slide courtesy of Carrie Morrill)

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

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

37. ITD Influence on Albedo Feedback ITD (5 cat) 1 cat. 1cat tuned “Strength” of albedo feedback in climate change runs (Holland et al., 2006) • 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

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 (From Holland and Bitz, 2003) • 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

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

42. Aspects of the Model’s Internal Variability

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

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

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

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 DOHT Both control state and change in OHT are correlated to polar amplification (From Holland and Bitz, 2003)

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