The document summarizes recent changes observed in the Arctic climate that exceed expectations from climate models. These changes include a record low September sea ice extent in 2007 that was 23% below the previous record, younger and thinner Arctic sea ice, and rising permafrost temperatures across the Arctic. While natural variability and factors like the North Atlantic Oscillation contributed to changes in the past, many scientists are now convinced that greenhouse gas emissions are playing a role in driving changes in the Arctic that are faster than predicted by earlier climate models.

1. Arctic Climate Change: Where Reality Exceeds Expectations Mark C. Serreze National Snow and Ice Data Center (NSIDC) Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder With thanks to the entire community.

2. Serreze et al., 2000: Observational evidence of recent change in the Northern high-latitude environment, Climatic Change , 46, 159-207. “ Taken together, these results paint a reasonably coherent picture of change, but their interpretation as signals of enhanced greenhouse warming is open to debate.” Seven years ago…

3. Why the fence-sitting? Natural variability appeared to dominate; many changes over recent decades could be directly linked to behavior of the NAO/NAM. J. Hurrell (top left), M. Visbeck (top right), 0. Johannessen (bottom left)

4. What changed my thinking? Increased confidence in global climate models In many respects, reality is exceeding expectations. Figures from the IPCC Fourth Assessment Report Persistent trajectory despite circulation changes

5. Rapid loss of Arctic sea ice September sea ice extent in 2007 compared to previous record set in 2005: A decline of 23% Mark Serreze, Julienne Stroeve, Walt Meier, Ted Scambos, Marika Holland, Jim Maslanik, Stephanie Renfrow, Matt Savoie

6. 2007 sea ice conditions in context Mark Serreze, Julienne Stroeve, Walt Meier, Ted Scambos, Marika Holland, Jim Maslanik, Stephanie Renfrow, Matt Savoie September Sea Ice Extent (1979–2007) September 2007 4.28 million km 2 2000

7. Some useful comparisons Donald Perovich, Cold Regions Research & Engineering Laboratory

8. Factors contributing to the 2007 record A very warm Arctic High pressure over central Arctic Ocean Low pressure over Siberia NCEP/NCAR Reanalysis; NOAA/ESRL Physical Sciences Division

9. Sea ice is becoming younger and thinner James Maslanik, Chuck Fowler, Julienne Stroeve Responses to behavior in North Atlantic Oscillation/ Northern Annular Mode and other patterns Oceanic influences? A generally warming Arctic

10. Observed rate of loss is faster than expected Julienne Stroeve, Mark Serreze, Walt Meier, Ted Scambos, Marika Holland

11. Moorings at Svinoy and Fram Strait I. Polyakov et. al, 2005 A puzzle: warming of Atlantic inflow

12. K. Shimada et al., 2006 Feedback associated with Pacific summer water

13. Are we near a tipping point? Ice concentration: Marika Holland, Cecilia Bitz, Bruno Tremblay, Julienne Stroeve 1990–1999 2010–2019 2040–2049 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

14. Sea ice thickness: an intriguing comparison Modeled ice thickness before abrupt ice loss, March 2024 (CCSM3) Observed satellite-derived ice thickness, March 2007 (ICESat) Marika Holland et al, 2006 (left), Julienne Stroeve (right)

15. Declining polar bear habitat George Durner et al., 2007

16. The Northwest Passage: open by late August 2007 G. Spreen, L. Kaleschke, and G. Heygster/IUP Universitat Bremen, AMSR data from National Snow and Ice Data Center (left); unknown (upper right); Dave Kavanagh (middle right); D. Thoreson (lower right) Manhattan 1968 Gjoa 1903 Cloud Nine 2007

17. As the sea ice cover continues to retreat, the Arctic will become increasingly accessible, not just to commercial shipping, but to oil extraction. Mike Keefe, 08/18/2007 Denver Post An accessible Arctic

18. Model-Projected Arctic Amplification Mark Serreze, Andy Barrett, Marika Holland Model projection of 2-meter temperature anomalies by month and year for 70–90 degrees North latitude, compared to 1979–1998 means (CCSM3) Latitude by height dependence of zonally averaged October–March temperature anomalies for 2050–2059, compared to 1979–1998 means (CCSM3)

19. Arctic Amplification appears to have emerged Corresponding anomalies in 2-meter temperature (NCEP/NCAR) Anomalies observed sea ice extent by year and month, relative to 1979-2007 means, for an Arctic Ocean domain (SSMR and SSM/I) Seasonality is as expected Andy Barrett, Mark Serreze, Matt Savoie

20. The vertical structure also looks correct Latitude (0 to 90 degrees North) by height (1,000 to 100 hectopascals) dependence of temperature anomalies, 1995–2007 minus 1979–2007 October November NCEP/NCAR, Climate Diagnostics Center

21. Surface melt over Greenland 2007 Melting Day Anomalies Melting Index Time Series Marco Tedesco, City College of New York-CUNY

22. Melt-induced ice flow and moulins Konrad Steffen, CIRES at the University of Colorado at Boulder

23. Cryospheric sea-level rise: glaciers and ice sheets Konrad Steffen, CIRES at the University of Colorado at Boulder

24. “ Greening” of the Arctic Trends in vegetation synthetic activity from1982–2005 (GIMMS-G AVHRR Vegetation indices) Significant positive trends Significant negative trends Scott Goetz, Woods Hole

25. Increases in permafrost temperature Alaska: 4 to 6 o C increase in 20 th Century, 2 to 3 o C, last 30 years Siberia: >3 o C increase, mid-1950s to 1990 Canadian Arctic: 1 to 3 o C increase, past several decades Tibetan Plateau: up to 1.0 o C increase, since1970s Alaskan permafrost temperatures, 20 meters deep Soil temperatures, active layer/upper permafrost Active layer depth +20 centimeters in Siberia Tingjun Zhang, Oliver Frauenfeld, Roger Barry, Richard Armstrong, Kevin Schaefer, Larry Hinzman, David Atkinson, Andrew Etringer, James McCreight, Mark Parsons, Andrew Slater, Ted Scambos, Stephanie Renfrow, and David Gilichinsky

26. Decadal changes in multi-model mean freshwater budget terms for the Arctic Ocean: positive anomalies indicate an increasing source (or decreasing sink) of freshwater for the Arctic Ocean Holland, Finnis, Barrett and Serreze, JGR, 2007 Anomalies are with respect to 1950–1980 means for ten models participating in the IPCC-AR4, A1B emissions scenario for the 21 st Century Not everything is rapidly changing…

27. Concluding Comments The future is today. Even our earliest climate models projected that effects of greenhouse gas loading would be seen first in the Arctic. Even seven years ago, attribution of observed changes was uncertain. Despite strong imprints of natural variability, I am now convinced of a role of greenhouse gas loading. A number of key changes are exceeding expectations from climate models. Could we lose the summer sea ice cover by 2030? Much remains to be understood—the idea of “tipping points” has emerged as a key issue. Not everything is rapidly changing (hydrologic cycle).