Using CCSM3 to investigate future abrupt Arctic sea ice change

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

2. “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.” “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)

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

4. Observed Arctic Conditions

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

6. 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

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

8. 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

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. 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:

12. Future ProjectionsWhat can models tell us?

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

14. 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

15. 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

16. 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

17. 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

18. 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.

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

20. 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

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

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. 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)

24. Some Cautions in Using Models to Examine these (and other) issues… Models provide a powerful 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 “Ethical Considerations”

25. 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

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

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” 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. Models are a useful tool to quantify these impacts.

31. 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)

32. 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

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

34. 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

35. 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

36. How common are forcing mechanisms?

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

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

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

42. 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)

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 (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 Complicates paleoclimate issues since “control state” not as well known

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;