1. Arctic Sea Ice – Now and in the Future

2. J. Stroeve National Snow and Ice Data Center (NSIDC), Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado (CU) With contributions from S. Drobot, M. Holland, R. Kwok, A. Mahoney, J. Maslanik, W. Meier, M. Serreze, J. Zwally

3. Arctic Sea Ice – Now and in the Future The Poster Child of Climate Change: Arctic Sea Ice Variability of September Sea Ice Cover (1979-2007)

4. Arctic Sea Ice – Now and in the Future 2005 Median September Ice Extent 1979-2000 Median September Ice Extent 1953-2000 Median September Ice Extent Ice Loss in 2007 Extended Further Into Central Arctic

5. Arctic Sea Ice – Now and in the Future Transition Towards Younger Ice Ice age tracking algorithm from C. Fowler and J. Maslanik show ice 5 years or older now only makes up 10% of the perennial ice pack. Spring 1986Spring 1990Spring 2007 Maslanik et al., 2007

6. Arctic Sea Ice – Now and in the Future Younger Ice is Thinner Ice Comparison between ice age and ice thickness from 4 years of spring ICESat GLAS-derived thickness fields from J. Zwally and D. Yi. Results suggest a decrease in mean thickness of 2.6 m in March 1987 to 2.0 m in March 2007 Maslanik et al., 2007

7. Arctic Sea Ice – Now and in the Future Conditions this Spring: Ice Extent/Concentration Some recovery in winter extent, but still well-below normal Source: nsidc.org/seaice_index

8. Arctic Sea Ice – Now and in the Future Conditions for Spring 2008: Ice Age First-year ice now covers 73% of the Arctic Basin, whereas in 2007 it covered 58%. Data from C. Fowler and J. Maslanik

9. Arctic Sea Ice – Now and in the Future Conditions for Spring 2008: Freeboard Comparison between 2007 and 2008 suggests the seasonal ice cover is 5-10 cm thinner and covers more area in 2008 Image courtesy of R. Kwok

10. Arctic Sea Ice – Now and in the Future Age as Proxy for Thickness and Volume 1987 1990 1995 1997 2000 2006 2007 2004 2008

11. Arctic Sea Ice – Now and in the Future This Spring is Warm April 2008 Air Temperature Anomalies (relative to 1979-2007)

12. Arctic Sea Ice – Now and in the Future Survival of Winter Formed First Year Ice

13. Arctic Sea Ice – Now and in the Future Estimated 2008 Minimum Sea Ice Extent Based on survival rates from previous years (Drobot et al., GRL, submitted) 2007 Record minimum extent: 4.13 mil. sq-km Average estimate: 3.48 mil. sq-km

14. Arctic Sea Ice – Now and in the Future Does 2007 Represent a Tipping Point? Tipping Point – what is it?  A threshold at which the sea ice cover rapidly transitions to a new stable (seasonally ice-free) equilibrium state. Abrupt climate change is often defined in the context of this type of tipping point behavior (e.g. National Research Council, 2002). Some climate models suggest that a transition towards seasonally ice-free conditions may result from “tipping point” behavior (e.g. Winton, 2006). Other models suggest abrupt change may happen, but may not constitute a threshold response (e.g. CCSM3).

15. Arctic Sea Ice – Now and in the Future Abrupt Ice Loss Climate models suggest that once the ice thins sufficiently, it becomes vulnerable to natural variability such that abrupt ice losses may occur (Holland et al., 2006). NCAR CCSM3 model simulation Observations Model drop 1.8 million sq km, 2024–2025 Observed drop 1.6 million sq km, 2006–2007 September Sea Ice Extent

16. Arctic Sea Ice – Now and in the Future The Set up Looks Right Mean thickness (70-90N) in CCSM3 before abrupt change: 1.71 m Mean thickness (70-90N) from ICESat in Spring 2007: 1.75 m (data from D. Yi and J. Zwally)

17. Arctic Sea Ice – Now and in the Future Natural Variability Remains Important! NCAR CCSM3 Mean Ice Thickness vs. Std. Dev. Ice Extent Natural variability increases with a thinning ice pack until the ice is gone

18. Arctic Sea Ice – Now and in the Future Thus, Ice May Recover this Summer If we have a strong cyclonic pattern with attendant cold temperatures as in 1996, we may not set a new record in summer 2008. CCSM3 Run 6 Observations

19. Arctic Sea Ice – Now and in the Future Does the Thin Ice Imply a Tipping Point? If the simulated abrupt transitions are the manifestation of a “tipping point”, then it is likely they would be preceded by a similar critical state From Holland et al. in press Solid line ice extent preceding the event, dashed line, extent after the event Run 1: Year 2024Run 2: Year 2025Run 3: Year 2030 Run 4: Year 2027Run 5: Year 2029

20. Arctic Sea Ice – Now and in the Future However, Thin Ice Remains Important Although there is not a “critical” ice thickness (or ice concentration) that precedes rapid ice loss, it remains clear that an abrupt ice loss event is only possible if the ice has thinned adequately. Other GCMs also show thin ice is needed for large regions of the ice pack to melt completely out. Annual Average Ice Thickness (m) September Ice Extent (10 6 km 2 ) From Holland et al., in press

21. Arctic Sea Ice – Now and in the Future Concluding Statements Current models tend to underestimate the observed rate of ice loss (Stroeve et al., 2007) and the large ice loss seen in 2007 typically doesn’t happen until mid-century. NCAR CCSM3 model simulations show 3 factors that contribute to abrupt transitions:  Thinning sea ice  Pulse-like increases in ocean heat transport to the Arctic  Positive surface albedo feedback. Ice loss in 2007 is similar in character to abrupt ice loss events simulated by CCSM3 later in this century.

22. Arctic Sea Ice – Now and in the Future Concluding Statements (cont) Outstanding question remains whether the abrupt events occur due to a threshold or “tipping point”. Abrupt ice loss in NCAR CCSM3 seems to represent the interaction of large intrinsic Arctic variability with increasing forced change from rising GHGs. However, the result of an abruptly changing ice cover would be the same, with considerable impacts on the socio-economics, climate and biological systems in the Arctic. Summer 2008 is poised for yet another dramatic retreat of the sea ice cover.