The Arctic is changing faster than anticipated in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) [Intergovernmental Panel on Climate Change (IPCC), 2007; Serreze et al., 2007; Stroeve et al., 2007]. By September 2008 Arctic sea ice reached a new milestone: two sequential years of extreme summer minimum sea ice coverage during the satellite era. Monthly mean September Sea ice extent, which is the area with sea ice concentration more than 15%, was 4.7 M km2 in 2008 following the record minimum of 4.3 M km2 set in 2007 according to the National Snow and Ice Data Center (NSIDC) (http://nsidc.org/arcticseaicenews/) [Comiso et al., 2008; Stroeve et al., 2008]. The average of the two minima is 37% below the climatology of sea ice extent for the period of 1980–1999. Such sea ice minima open up more than 60% of the Arctic Ocean to increased solar and longwave absorption near the end of summer. The September 2007/8 sea ice extent estimates from the Hadley Centre sea ice concentration analysis (HadISST) at a 1° Latitude/Longitude resolution were slightly higher at 4.6/5.2 M km2 (http://www.metoffice.gov.uk/hadobs/hadisst/). We use the "observed" value of 4.6 M km2 (34% below climatology) based on HadISST throughout this article for comparisons to model hindcasts. This new record of Arctic sea ice minimum at the end of summer melt season is occurring at least 30 years earlier than the expected value for the timing of sea ice loss derived from climate models provided for the IPCC AR4 [Zhang and Walsh, 2006; Stroeve et al., 2007; Overland and Wang, 2007]. Due to the recent loss of sea ice, the 2005–2008 autumn (October–November) central Arctic surface air temperatures were greater than 5° C above their climatology, a magnitude similar to the projected autumn temperature increase for 2070 by IPCC AR4 [Chapman and Walsh, 2007; Overland et al., 2008; Schweiger et al., 2008a].
Climate models provide multiple simulations (referred to as ensemble members) to distinguish uncertainties due to natural variability, caused by internal instabilities in the climate system, from long term trends due to external anthropogenic forcing caused by increases in greenhouse gases. For example, when IPCC AR4 models (now phase 3 of the Coupled Model Intercomparison Project, CMIP3) were initialized with slightly different initial conditions, the details of the time trajectories of each ensemble member were substantially different [Wang et al., 2007]. Among many possible trajectories, the real world will manifest as only one realization drawn from a range of possible future climate scenarios. Projections from CMIP3 models were also based upon different future emissions scenarios; these were labeled as "low" (B1), "medium" (A1B) and "high" (A2) [Nakićenović and Swart, 2000].
A difficulty with the AR4 is there are too few ensemble members from quality climate models to obtain a frequency distribution of possible future extreme events. Compared to the range of CMIP3 model projections, the one climate realization that we are living through appears to be a fast track for September sea ice loss. The recent reduction of sea ice extent is consistent with a combination of an emerging greenhouse gas contribution, wind-driven variability in sea ice circulation, a sequence of warm years beginning in the late 1990s, and non-linear feedbacks from decreased albedo and anomalously warm sea temperatures in regions of low sea ice concentration [Rigor and Wallace, 2004; Serreze and Francis, 2006; Nghiem et al., 2007; Ogi and Wallace, 2007; Maslanik et al., 2007; Gascard et al., 2008; Schweiger et al., 2008b].
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