1. Using CCSM3 to investigate future abrupt Arctic sea ice change Marika Holland NCAR

2. “A mechanism that might lead to abrupt climate change would need to have the following characteristics: A trigger or, alternatively, a chaotic perturbation, with either one causing a threshold crossing (something that initiates the event). An amplifier and globalizer to intensify and spread the influence of small or local changes. A source of persistence, allowing the altered climate state to last...” From Abrupt Climate Change: Inevitable Surprises (2002) “an abrupt climate change occurs when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate … faster than the cause.”

3. Role of sea ice as an “amplifier” Surface albedo feedback amplifies climate perturbations Models have been used to explore/quantify these effects. (From Hall, 2004) VA=variable albedo FA=fixed albedo (DJF SAT)

4. Observed Arctic Conditions

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

6. 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

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

8. 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

9. North Atlantic Oscillation Positive Phase From University of Reading webpage JFM NAO Index 1950-1992

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

11. 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.

12. Future Projections What can models tell us?

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

14. 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

15. 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

16. 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)

17. 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

18. 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.

19. Fram Strait Siberian Shelf Arctic Arctic Ocean Circulation Changes 2040-2049 Minus 1980-1999 Siberian Shelf Fram Strait Arctic Evidence that ocean circulation changes are related to changing ice/ocean freshwater exchange (Bitz et al., 2006)

20. 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

21. Mechanisms Driving Abrupt Transition 1.Transition to a more vulnerable state thinning of the ice cover 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

22. Effects of transition on atmospheric conditions Winter air temperature increases rapidly during abrupt ice change, with a >5C warming in 10 yrs Precipitation shows general increasing trend with largest rate of change over abrupt ice event

23. 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)

24. 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 powerful tool for examining climate feedbacks, mechanisms, etc but… “Ethical Considerations”

25. 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)

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

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

28. 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

29. Summary Sea ice is an effective amplifier of climate perturbations: due to surface albedo changes due to ice/ocean/atm exchange processes CCSM3 simulates abrupt transitions in the future ice cover preconditioning (thinning) trigger (ocean heat transport changes) positive feedbacks (surface albedo; oht changes) Models provide a useful tool for exploring the mechanisms that result in simulated rapid climate transitions Never completely trust the tool comparisons to other models; sensitivity tests; “digging” into the feedbacks, etc. can increase confidence in simulated processes

30. 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. Models are a useful tool to quantify these impacts. SST LGM Reduced Ice SAT Difference

31. 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)

32. 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)

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

34. 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

35. 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

36. How common are forcing mechanisms?

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

38. 20th Century 21st Century Increased Arctic Ocean heat transport occurs even while the Atlantic MOC weakens

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

40. Climate models as a useful tool for addressing ACC As a tool to flesh out/test hypotheses or processes How is a climate signal amplified by sea ice interactions What processes influence threshold behavior in the sea ice How does the control climate state modify the persistence of anomalies How are teleconnections between high latitudes and tropics realized

41. 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

42. 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)

43. 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

44. 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 Complicates paleoclimate issues since “control state” not as well known (From Holland and Bitz, 2003)

45. Does it matter? Sea ice is an important “amplifier” in the system When a change is made in a coupled model, often the most dramatic response is in the ice covered regions This often occurs for changes that are not polar specific - e.g. diurnal cycle stuff. Getting the processes by which sea ice amplifies a climate signal “right” can be quite important for our ability to simulate abrupt change These will likely include ice/ocean and ice/atmosphere interactions (ice growth/brine rejection - how it changes - seasonally, etc.; how changes influence the ocean; Threshold behavior of the ice cover - examples: on/off of the initial ice growth (freezing temp); perennial to seasonal ice cover; perennial to seasonal snow cover;