The two sequential years of extreme low sea ice extent at the end of summer in 2007 and 2008 are indeed evidence that the Arctic may be on a fast track for increased September sea ice reduction over the next 30 years. A consequence will be increased autumn air temperatures due to heat released by the ocean, which is absorbed during the summer as a result of reduced sea ice coverage. Reduction of September sea ice extent during single year events appears to be an important feature in both the 2007 observation and in certain ensemble members from CMIP3 models (Figure 1). Ensemble members for simulations without anthropogenic forcing (control runs) indicate that anthropogenic forcing is a necessary condition for future major sea ice loss to occur. However, the influence of natural variability in the form of both recent warm years and wind-driven sea ice drift supports the conclusion that the observed reduction of September 2007 sea ice occurred many decades earlier than the expected timing of sea ice reduction under the influences of greenhouse gases forcing alone, i.e., the mean of all model ensembles projections (Figure S2).
The first quartile of the distribution of time intervals for sea ice extent to drop from 4.6 to 1.0 M km2 implies that a sea ice free Arctic in September may occur as early as the late 2020s, based on projections by six CMIP3 models selected for their ability to simulate the current conditions of sea ice extent. To reach these conclusions it is important to apply the observational constraints that the models produce a reasonable mean and seasonality of sea ice extent. Further confidence is based on three of the selected models having sophisticated sea ice physics packages. Sea ice thickness, while crudely represented in most CMIP3 models, show reductions in summer (September) and winter (March) over the next 30 years. The uncertainty in timing of a summer sea ice free Arctic is largely due to both within-model contributions from natural variability and between-model differences.
We appreciate the support of NOAA Arctic Research of the Climate Program Office and discussions on this topic with many Arctic colleagues. This publication is partially funded by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement NA17RJ1232, contribution 1605. PMEL contribution 3261.We also like to thank Qiang Fu for his comments and suggestions on the manuscript. We acknowledge the modeling groups, the Program for Climate Model Diagnosis and Intercomparison (PCMDI) and the World Climate Research Programme (WCRP)'s Working Group on Coupled Modelling (WGCM) for their roles in making available the WCRP CMIP3 multi-model dataset. We would like to thank the two anonymous reviewers and the editor for their valuable comments. Support of this dataset is provided by the Office of Science, U.S. Department of Energy.
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